WO2000040343A1

WO2000040343A1 - Apparatus and method for applying flux to a substrate

Info

Publication number: WO2000040343A1
Application number: PCT/US2000/000427
Authority: WO
Inventors: Timothy J. Provencher; Harrie Woudstra
Original assignee: Provencher Timothy J; Harrie Woudstra
Priority date: 1999-01-07
Filing date: 2000-01-07
Publication date: 2000-07-13
Also published as: EP1156886A1; AU2409500A; EP1156886A4

Abstract

An apparatus and method is provided for applying a liquid solder flux in a pattern to a substrate. A stencil is utilized which comprises first apertures (16) corresponding in location to the pattern and second apertures (32) in fluid communication with the first apertures, the second apertures being disposed adjacent an upper surface of the substrate. The first apertures are disposed in a first section (26) of the stencil and the second apertures are disposed in a second section (28) of the stencil. The first section (26) and the second section (28) meet at a planar intersection substantially parallel to first and second sides of the stencil. The planar intersection has a cross section different from that of the first and second sections.

Description

TTTLE: "APPARATUS AND METHOD FOR APPLYING FLUX TO A SUBSTRATE"

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to apparatus and methods pertaining to flux application and, more specifically, to apparatus and methods for applying a liquid solder flux on substrates such as semiconductor wafers, printed circuit boards, electronic components,- devices, parts, and assemblies. Description of the Related Art The electronics industry is one of the most dynamic and important industries today. It has literally transformed the world and provides many products that affect our daily lives, for example, telephones, television, personal computers, cellular phones, pagers, video camcorders, audiovisual products, etc. One of the key technologies that helps make these products possible is electronics packaging. This field of technology can be divided into a hierarchy of levels beginning with chip level packages and proceeding to multi-chip packages, printed circuit boards, mother boards, and component cases including boards, power supplies, etc.

A key area of development in the field of electronic packaging is the area of chip level packaging and interconnections. The most common types of chip level interconnections are wire bonding, tape-automated bonding (TAB) , and solder bumping. Among these three technologies, solder bumped flip chip provides the highest level of packaging density with the least package space. The solder bumping is created by solder balls which are reflowed onto connection points or solder pads (otherwise known as landing pads, lands, or pads) on the substrate . The solder balls are arranged in arrays corresponding to the location of the solder pads on the substrate. These arrays of solder balls are known as ball grid arrays ("BGA") .

Ball grid array packaging is rapidly emerging as the technology of choice for high input/output (I/O) count integrated circuits (IC's). Ball grid arrays deliver higher density and yields than traditional packages without requiring fine-pitch processing or new assembly equipment. Driven by the increasing I/O as IC's become larger and more complex, the demand for ball grid array packages is expected to grow from fewer than 20 million units in 1995 to more than 2 billion by 1999. Another key area of development in the field of electronic packaging is the area of board level packaging involving the interconnection between electronic components and high density printed circuit boards. This is an area of great technological expansion as higher I/O densities and increased component densities on boards push the field of surface mount technology to its limits. The most common method of interconnection of surface mount chips to printed circuit boards involves wave soldering. In this case the solder pads on the printed circuit board are exposed to molten solder when the surface of the board is contacted with a "wave" of solder. The wave is usually formed by a local fountain of solder with the board being passed over the tip of the fountain or "wave" of molten solder. The molten solder adheres to the metal solder pads locally, but it does not adhere to the balance of the board. This process may be performed prior to mounting the chips to the board, or it may be performed while the chips are already in place on the board. In the former case, the chip is mounted to the board after the solder has been applied to the solder pads. With the chip leads in contact with the solder plated pads, the solder pads and contacting chip leads are heated to reflow the solder on the pad, thus soldering the lead to the pad. In the later case, the solder "wave" adheres to both the pad and the chip lead when the board is exposed to the "wave". In this case, the lead is soldered to the pad during the "wave" soldering process. As is described in more detail below, both of these processes require careful processing to get the solder to adhere to the proper locations, and not to adhere to the balance of the printed circuit board assembly. This generally requires the use of solder masks or inhibitors coated over the areas of the printed circuit board assembly which are not to be soldered and solder wetting and pad cleaning agents coated over the solder pads to which the solder is intended to adhere.

Most high technology fields of electronic assemblies have one thing in common, interconnection technology is a key limiting factor for continued product improvements and evolution. For chip level and board level interconnections, the majority of the high density interconnections are formed by solder joints of one form or another. All these solder joints require a cleaning and wetting agent to clean the connection point being soldered to assist the solder in wetting the surface of the connection point when the solder is melted or reflowed. This cleaning and wetting agent is known as a flux. Flux is available in large variety of forms and compositions. Two primary categories of flux are liquid flux, which has a low viscosity, typically on the order of centipoise to hundreds of centipoise, and flux paste, which has a high viscosity, typically on the order of tens of thousand to millions of centipoise. These two key categories of flux may be obtained in a variety of subcategories such as no clean, water soluble, or non-water soluble varieties. These descriptors are used to identify the type of clean up that is required on the substrate after the flux has been applied and the solder has been reflowed. The liquid fluxes are generally flux dissolved in a solvent with little or no other additives. The paste fluxes have a number of additives such as, for example, solder in the form of small solder balls, which can be up to 90 percent of the paste, thickening agents, and other additives. Generally speaking, liquid solder fluxes are used for BGA and solder bumping technology. The paste fluxes are used for certain specialty BGA applications such as ceramic BGA'ε and some high density surface mount applications.

In high density interconnection applications using solder joints between electronic components and packages, the solder pads are arranged in precise patterns or arrays. FoA example, in BGA technology, the solder balls must be placed onto the substrate in an array corresponding to the location of the solder pads. "Substrate" as the term is used herein refers to the item upon which the solder joints are to be created. Examples of such substrates include printed circuit boards, flux circuits, electronic components, semiconductor wafers, multipacks of electronic components such as tape strips, and ceramic panels. They may be housed in the form of individual chips or boards, or held in carriers such as strips, boats, or trays. Each substrate typically includes a plurality of solder sites or pads positioned in an array corresponding to the desired pattern of bonding sites for the substrate. The solder pads typically comprise a metal pad. In BGA applications, for example, the pads typically measure slightly smaller than the solder ball which is to be placed upon the solder pad.

Solder typically used in interconnection technology such as BGA technologies or surface mount technologies generally comprise eutectic solders such as tin-lead solders. Solder balls used in BGA technology may be pure solder or solder coated copper or other high temperature alloys. Examples of tin-lead solder compositions, such as is used in solder balls, are 63% tin - 37% lead and 62% tin - 36% lead - 2% silver, typically for use with plastic ball grid arrays, and 10% tin - 90% lead, typically for use with ceramic ball grid arrays, although other compositions are possible. Due to the demand for higher and higher interconnection densities, the size of solder pads and the spacing between adjacent pads is shrinking to exceedingly small sizes. For example, the solder balls used in current applications typically have sizes as small as 5 mils (thousandths of an inch), but they can be as large as 30 mils. Solder pad sizing and spacing for such applications is correspondingly small. Pad sizes of 0.005 inches with spacing between adjacent pads of 0.012 inches are common place. Other sizes and size ranges have been used in the past and quite well may be used in the future. In the case of BGA technology, in a given application, a single size of solder ball is commonly used, although this is not necessarily true universally. In the case of other surface mount technologies the size of solder pads often varies from component to component within a given application. This is often due to the differing requirements of the variety of electronic components which may be mounted on a substrate such as a printed circuit board assembly. A microprocessor may not have the same current and voltage requirements as a memory chip, a power transistor, or a passive component such as a capacitor or a resistor. It is generally a necessity for high density interconnection technology that flux be accurately applied to the solder pads. In vary high density applications such as solder bumping of semiconductor wafers during chip fabrication, solder pad counts can be as high as hundreds of thousands on a single eight inch diameter wafer. In such applications, the tolerance required for pad to pad location in any zone of the wafer can be oh the order of several ten thousandths of an inch. The application of flux to these pads requires similar precision. Given flux application requirements this demanding, ^'the tools used to perform this function generally require exacting precision. There are a variety of techniques for applying flux to substrates . The methods and apparatus for applying flux is dependent upon whether the flux is liquid or paste. The higher viscosity and surface tension of paste flux lends itself to simpler techniques for transferring the flux to the substrate in the required pattern. For example, simple stencils are common with through holes called apertures located in positions corresponding to the pattern of the solder pads on the substrate. These holes are typically of similar shape to the solder pad shape and their size is slightly smaller than the actual pad size. For example, a 0.005 inch diameter circular pad will typically have a corresponding stencil hole with a 0.004 inch diameter. These stencils are places over the substrate such that the stencil pattern lines up with the solder pad pattern on the substrate. The flux paste is then placed on the side of the stencil opposite the substrate side of the stencil and a squeegee blade is used to wipe the flux over the pattern of the stencil apertures much like buttering a slice of bread with a knife. This forces the flux paste through the apertures and onto the solder pads on the substrate. When the stencil is removed from the substrate, the flux paste remains on the substrate in the required pattern. Stencils of this variety are typically made of stainless steel and the aperture holes are either chemically etched, laser cut, or electro-discharge machined into the stencil. These stencils may also include cavities to clear electronic components, vias, or other protrusions which are present on the substrate which would otherwise interfere with the flux printing process.

Another apparatus used to apply flux paste to substrates is a tool consisting of what appears top be a "bed of nails". This tool has protrusion ("nails") which extend from the tool at the locations corresponding to the pattern of the solder pads on the substrate. The flux is applied to the substrate by first dipping the tips of the "nails" into the flux. Due to the high viscosity and surface tension of the flux, some flux adheres to the tips of the "nails". The tool is then placed over the substrate and the tool is contacted to the substrate so the tips of the "nails" are contacted to the substrate at the solder pads. The tool is then removed from the substrate and the requisite amount of flux adheres to the solder pads and remains on the substrate.

One limitation of known apparatus for applying flux paste is that flux paste is only used for certain solder applications.

Specifically, flux paste is not the flux of choice for most BGA applications .

Another limitation of known apparatus for applying flux paste is that the flux printing to the substrate tends to be less precise than is possible with liquid flux systems. The same properties which make it simple to apply paste type fluxes, namely high viscosity and high surface tension, also often limit their utilization. These properties make it difficult to perform high precision printing of flux paste on substrates. The "thick" nature of the flux paste tends to make precise quantity delivery of paste type flux difficult. Delivery thickness tend to vary and "stringers" tend to be formed when the flux printing tool is removed from the substrate. Additionally, the periphery of the flux paste which is printed on the solder pads tends to be "fuzzy" and less precise than the periphery of the solder pad or the tool delivering the flux. In addition, the thick nature of the paste can make it difficult to consistently force the flux to flow through the stencils. Too much pressure or speed on the squeegee blade and too much flux is delivered, too little pressure or speed, and too little or no flux may be delivered. Liquid flux improves the accuracy with which flux can be printed on more demanding applications, but the handling and delivery of the lower viscosity, lower surface tension fluid makes precise flux delivery more difficult. For example, solder paste type stencils generally do not work with liquid solder flux because the low viscosity and low surface tension of the liquid flux cause the flux to seep out of the through holes. This can result in excessive amounts of flux being delivered to the substrate.

Known apparatus for applying liquid solder flux on substrates consists of screens with a polymeric resist layer impregnated on the screen at all locations except where the flux is intended to flow. The polymeric resist layer blocks off the screen through holes at all locations except those corresponding to the pattern of the solder lands on the substrate. This screen is essentially similar to a silk screen for printing ink onto flat surfaces. This technique is relatively simple, at least in concept, but it has a variety of shortcomings.

One limitation of known apparatus for applying liquid solder fluxes is that they do not apply the flux accurately to the substrate. The screens have a tendency to stretch, leading to smearing. This problem is addressed at least in part by precision frames for the screens and careful tensioning of these frames. This improves the situation, but does not eliminate it.

Another limitation of known apparatus for applying liquid solder flux is that they do not deliver consistent quantities of flux to each solder pad on the substrate. The screen pores can be inconsistent, leading to too little or too much flux deposition and smudging at the edge of each flux pad.

Another limitation of known apparatus for applying liquid flux is that they are intended to print flux on flat substrates. They do not adequately compensate for protrusions on the substrate. If there are protrusions close to the location of the solder lands, the flux pattern is smeared on the substrate, which can result in an inaccurate flux pattern. Another limitation of known apparatus for applying liquid solder flux is that the flux is not delivered quickly with high reliability. Due to the inconsistent nature of the screens, such as pore size, screen thickness, emulsion contamination of flow areas in the screen, etc., the screens do not reliably deliver a precise pattern of flux in a precise quantity to the substrate at high rates . Objects of the Invention

Accordingly, an object of the present invention is to provide an apparatus and method for applying liquid solder flux in a pattern to a substrate wherein the application of the liquid solder flux onto the substrate can be performed accurately.

Another object of the invention is to provide an apparatus and method for applying liquid solder flux in a pattern to a substrate wherein a consistent quantity of flux can be applied to each solder pad on the substrate.

Another object of the present invention is to provide an apparatus and method for applying liquid solder flux in a pattern to a substrate wherein the flux application can be performed quickly and/or reliably.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be .learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations pointed out in the appended claims .

SUMMARY OF THE INVENTION

To achieve the foregoing Objects, and in accordance with the purposes of the invention as embodied and broadly described in this document, an apparatus is provided for applying a liquid solder flux in a pattern to a substrate. The apparatus according to one aspect of the invention comprises a first side for distribution of the liquid solder flux to the apparatus; a second side opposite the first side; and a plurality of first apertures passing through the apparatus from the first side to the second side so that the liquid solder flux passes in fluid communication from the first side through the first apertures to the substrate. The first apertures correspond in location to the pattern. Each of the first apertures has a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end. The first section and the second section meet at a planar intersection substantially parallel to the first and second sides. The planar intersection has a third cross section. The second section includes a plurality of second apertures in fluid communication with the first section and the second end. Each of the second apertures has a third end adjacent to the third cross section and a fourth end adjacent to the second cross section.

In some embodiments, each third end is located within the third cross section. Embodiments also are possible in which each second aperture is substantially cylindrical. The first cross section and the third cross section may be substantially circular. The first cross section and the third cross section also may be substantially the same size, and each first section may be substantially cylindrical. According to one aspect, the first cross section is larger than the third cross section and each first section is substantially conical.

In a variant of the invention, one in which the substrate includes at least one protrusion having at least one corresponding protrusion location, the second side may include at least one cavity corresponding in location to the at least one protrusion location, the at least one cavity of the second side receiving the at least one protrusion when the second side is adjacent to the substrate.

In accordance with another aspect of the invention, an apparatus is provided for applying a liquid solder flux in a pattern to a substrate, wherein the apparatus comprises a first side for distribution of the liquid solder flux to the apparatus; a second side opposite the first side; and a plurality of first apertures passing through the apparatus from the first side to the second side so that the liquid solder flux passes in fluid communication from the first side through the first apertures to the substrate. The first apertures correspond in location to the pattern. Each of the first apertures has a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end. The first section and the second section meet at a planar intersection substantially parallel to the first and second sides. The planar intersection has a third cross section. The second section includes a second aperture in fluid communication with the first section and the second end. Each second aperture has a third end adjacent to the third cross section and a fourth end adjacent to the second cross section .

With this apparatus, preferably but optionally, the third end is located within the third cross section and the third end is smaller than the first cross section. Similarly, the second section may be substantially cylindrical. The first cross section and the third cross section may be substantially circular. The first cross section also may be larger than the third cross section and each first section may be substantially conical. In another variant, the first cross section.and the third cross section may be substantially the same size, and each first section may be substantially cylindrical.

Where the substrate includes at least one protrusion having at least one corresponding protrusion location, the second side may include at least one cavity corresponding in location to the at least one protrusion location, the at least one cavity of the second side receiving the at least one protrusion when the second side is adjacent to the substrate.

In accordance with another aspect- of the invention, a method is provided for applying a liquid solder flux in a pattern to a substrate. The method comprises a first step of providing an apparatus having a first side and ε second side opposite the first side, and a plurality of first apertures passing through the apparatus from the first side to the second side in fluid communication from the first side through the first apertures to the second side. The first apertures correspond in location to the pattern. Each of the apertures has a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end. The first section and the second section meet at a planar intersection substantially parallel to the first and second sides. The planar intersection has a third cross section. The second section includes a plurality of second apertures in fluid communication with the first section and the second end. Each of the second apertures has a third end adjacent to the third cross section and a fourth end adjacent to the second cross section.

The method according to this aspect of the invention includes a second step of positioning the second side of the apparatus adjacent to the substrate. It also includes a third step of applying the liquid solder flux to the first side of the apparatus so the liquid solder flux flows through the first and second apertures passing from the first side of the apparatus to the substrate. A fourth step also is provided of removing the substrate from the apparatus.

Optionally, the first step may include locating each third end within each respective third cross section. In another option, the first step includes providing each second aperture with a substantially cylindrical shape. In another option, the first step includes providing the first cross section and the third cross section with a substantially circular shape. In yet another option, the first step includes providing the first cross section and the third cross section with substantially the same size and each first section is substantially cylindrical. In still another option, the first step includes providing the first cross section to be larger than the third cross section and each first section is substantially conical.

Where the substrate includes at least one protrusion having at least one corresponding protrusion location, the second step of the method may include providing the second side with at least one cavity corresponding in location to the at least one protrusion location, and the at least one cavity of the second side may receive the at least one protrusion when the second side is adjacent to the substrate. In accordance with another aspect of the invention, another method is provided for applying a liquid solder flux in a pattern to a substrate. This method comprises a first step of providing an apparatus having a first side and a second side opposite the first side, and a plurality of first apertures passing through the apparatus from the first side to the second side in fluid communication from the first side through the first apertures to the second side. The first apertures correspond in location to the pattern. Each of the first apertures has a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end. The first section and the second section_meet at a planar intersection substantially parallel to the first and second sides. The planar intersection has a third cross section, and the second section includes a second aperture in fluid communication with the first section and the second end. The second section has a third end adjacent to the third cross section and a fourth end adjacent to the second cross section.

This method includes a second step of positioning the second side of the apparatus adjacent to the substrate. It also includes a third step of applying the -liquid solder flux to the first side of the apparatus so the liquid solder flux flows through the first and second apertures passing from the first side of the apparatus to the substrate. It further includes a fourth step of removing the substrate from the apparatus. Optionally, the first step of this method may include providing the third end located within the third cross section, the third end being smaller than the first cross section. In another option, the first step may include providing the second section with a substantially cylindrical shape. In another option, the first step may include providing the first cross section and the third cross section with a substantially circular shape. In another option, the first step may include providing the first cross section to be larger than the third cross section and each first section may be substantially conical. In still another option, the first step may include providing the first cross section and the third cross section to be substantially the same size, and first section may be substantially cylindrical.

Where the substrate includes at least one protrusion having at least one corresponding protrusion location, the first step may include providing the second surface to include at least one cavity corresponding in location to the at least one protrusion location, so that the at least one cavity of the second surface receives the at least one protrusion when the second surface is placed adjacent to the substrate. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments and methods of the invention and, together with the general description given above and the detailed description of the preferred embodiments and methods given below, serve to explain the principles of the invention.

Fig. 1 is a perspective view of an apparatus for applying liquid solder flux to a substrate according to a first preferred embodiment of the invention, wherein a substrate is located below the apparatus ;

Fig. 2 is a top view of the apparatus of Fig. 1 ;

Fig. 3 is a side cutaway view of the apparatus of Fig. 1;

Fig. 4 is an enlarged view of one of the first apertures, including the second apertures, for the apparatus of Fig. 1; Fig. 5 is a top view of the second apertures of the device shown in Fig. 1;

Fig. 6 provides a view essentially identical to Fig. 5, but wherein there are six second apertures per first aperture instead of four second apertures per first aperture; Fig. 7 is an apparatus according to the first preferred embodiment similar to that shown in Fig. 1, but wherein the first section has a cylindrical shape instead of a conical shape;

Fig. 8 shows a device according to a preferred embodiment of the invention which includes a cavity in the lower or second side of the apparatus for receiving and accommodating a protrusion on the substrate;

Fig. 9 shows an apparatus according to a second preferred embodiment of the invention;

Fig. 10 shows an enlarged view of a first aperture from the apparatus of Fig. 9;

Fig. 11 shows a single second aperture for the apparatus of Fig. 9;

Fig. 12 shows an aparatus as shown in Fig. 9 but wherein the first section has a cylindrical shape; Fig. 13 shows a top view of the aparatus of Fig. 1 mounted in a frame;

Fig. 14 shows a side view of the apparatus of Fig. 13; Fig. 15 is a side cutaway view of the apparatus of Fig. 1, and which illustrates one method of application of liquid solder flux by a flooding blade to the first side of the apparatus with a substrate adjacent to the second side of the apparatus; and Fig. 16 is a side cutaway view of the apparatus of Fig. 1, and which depicts application of liquid solder flux to a substrate by forcing the flux through the apertures with a squeegee blade. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS AND METHODS

Reference will now be made in detail to the presently preferred embodiments and methods of the invention as illustrated in the accompanying drawings, in which like reference characters designate like or corresponding parts throughout the drawings. It should be noted, however, that the invention in its broader aspects is not limited to the specific details, representative devices and methods, and illustrative examples shown and described in this section in connection with the preferred embodiment and method. The invention according to its various aspects is particularly pointed out and distinctly claimed in the attached claims read in view of this specification, and appropriate equivalents.

In accordance with one aspect of the invention, an apparatus is provided for applying liquid solder flux in a pattern on a substrate. The substrate is as described above, and may take any one of a number of forms, as noted above. In these illustrative examples, the substrate may be assumed to be a semiconductor chip or printed circuit board 2 which is to become a ball grid array. Substrate 2 has on one of its surfaces 4 a pattern. In this particular example, substrate 2 includes a pattern 6 which comprises a plurality of solder lands 6.

In accordance with this aspect of the invention, the apparatus comprises a first side for distribution of the liquid solder flux to the apparatus, and a second side opposite the first side.

To illustrate this aspect of the invention, an apparatus in the form of a stencil 10 according to a first preferred embodiment of the invention is shown in Figs. 1 and 2. Stencil 10 is for use in connection with a substrate such as substrate 2 with a pattern such as, for example, pattern 6, as shown in Fig.

1. A top view of stencil 10 is depicted in Fig. 2.

Stencil 10 comprises a substantially flat plate of rigid material. It may be slightly bowed in the center to accommodate forces placed upon it, but such bowing would be no more than is commonly used in the industry at present for commercially known devices. Stencil 10 may be made of a variety of materials or material combinations. It may comprise, for example, metals such as steel, aluminum, brass, copper, stainless steel, or plastics such as teflon, nylon, polycarbonate. The preferred materials for stencil 10 in this type of example, however, are metals, and more preferably, stainless steels.

Stencil 10 includes a first side 12 for the distribution of liquid solder flux to the stencil, as will be described in greater detail below. Stencil 10 includes a second side 14 opposite first side 12. Second side 14 typically would be the side of stencil 10 which would be adjacent to, or would contact, substrate 10 during application of the liquid solder flux. Further in accordance with this aspect of the invention, the apparatus includes a plurality of first apertures passing through the apparatus from the first side to the second side so that the liquid solder flux passes in fluid communication from the first side through the first apertures to the substrate. The first apertures correspond in location to the pattern, generally with one aperture per land, so that flux passing through a given aperture is deposited on a corresponding and respective land.

As implemented in the first preferred embodiment, a plurality of first apertures 16 pass through stencil 10, from first side 12 to second side 14, so that the liquid solder flux passes in fluid communication from first side 12 through first apertures 16 to substrate 2. First apertures 16 are disposed in stencil 10 is a pattern 18 corresponding to the pattern 6 of the substrate 2, so that each first aperture is mated with a corresponding one of the solder lands 8 of substrate 2 when stencil 10 is mated with substrate 2. Each of the first apertures 16 may be assumed to be disposed about an axis (a theoretical construct for references purposes, see Fig. 3) . In the preferred embodiments, axis 20 is substantially orthogonal to first and second sides 12 and 14 of stencil 10 which, as noted, are substantially planar. Alternatively, axis 20 could be at an angle other than 90 degrees .jwith respect to first and second sides 12 and 14, provided that first apertures 16 are in fluid communication with the first and second sides 12 and 14.

Also in accordance with this aspect of the invention, each of the first apertures has a first end adjacent to first side of the stencil with a first cross section substantially parallel to the first and second sides, and a second end adjacent to the second side with a second cross section substantially parallel to the first and second sides.

Each of the first apertures also includes a first section adjacent to first end and a second section adjacent to the second end. The first section and the second section meet at a planar intersection substantially parallel to the first and second sides. The planar intersection has a third cross section.

The second section includes a plurality of second apertures in fluid communication with first section and second end. Each of the second apertures has a third end adjacent to or at the third cross section and a fourth end adjacent to or at the second cross section.

As implemented in the preferred embodiment, each of the first apertures 16 has a first end 22 adjacent to first side 12 of stencil 10 with a first cross section 22a substantially parallel to the first and second sides 12 and 14, and a second end 24 adjacent to second side 14 with a second cross section 24a substantially parallel to first and second sides 12 and 14. First end 22 is for receiving the liquid solder flux when it is applied to first side 12 of stencil 10.

Each of the first apertures 16 also includes a first section 26 adjacent to first end 22 and a second section 28 adjacent to second end 22. First section 26 and second section 28 meet at a planar intersection 30 substantially parallel to first and second sides 12 and 14. Planar intersection 30 has a third cross section, also defined by reference numeral 30. First section 26 is contained within and defined by the three-dimensional region shown two-dimensionally in Fig. 3 as bounded by points 22b, 22c, 30a, and 30b. Second section 28 is contained within and defined by the three-dimensional region shown two-dimensionally in Fig. 3 as bounded by points 30a, 30b, 24b, and 24c.

Second section 28 includes a plurality of second apertures 32 in fluid communication with first section 26 and second end 24. The number of second apertures for a given first aperture will depend upon the application, and may vary from two to many. Examples of four and six second apertures are shown in Figs. 5 and 6, respectively. Each of the second apertures 32 has a third end 34 adjacent to or at third cross section 30 and a fourth end 36 adjacent to or at second cross section 24a. Each third end 34 is located within third cross section 30. Second apertures 32 thus in a sense are a part of the first apertures 16 in that together they form a channel or flow path for liquid solder flux when the flux is applied to first side 12 of stencil 10. The shapes of first and second sections 26 and 28 may vary from application to application. Preferably, first cross section 22a and third cross section 30 are substantially circular. Even with these constraints, however, a number of first section geometries are possible, such as cylindrical sections, conic sections, and others. Also preferably but optionally, first cross section 22a will be larger than third cross section 30. One version which meets this criteria would be where the first section 22a is substantially conical, as is shown in Figs. 3 and 4. First section 22 thus may be configured so that first cross section 22a is larger than third cross section 30 (at or within first section 22) and each first section 22 is substantially conical. In somewhat more general terms, first sections 22a may have a cross sectional diameter in planes parallel to first and second sides 12 and 14 of stencil 10 which are decreasing in size as the cross section moves from first side 12 or first end 22 toward second side 14 or second end 24. Other shapes also may be suitable, depending upon the application.

Second apertures 32 of second section 28 similarly may assume a number of different shapes and sizes. In the presently preferred embodiments, second apertures 32 are substantially cylindrical in shape along axis 20 and axes parallel to axis 20 which pass through the center of a given second aperture.

In another embodiment, first cross section 22A and the first aperture cross section at or just inside first section 26 from third cross section 30 are substantially the same size, and each first section is substantially cylindrical. An example of this might be an arrangement, for example, as shown in Fig. 7.

In some applications, the substrate includes at least one protrusion having at least one corresponding protrusion location. Examples would include devices or assemblies in which a chip or similar element is located on the surface of the substrate upon which the solder lands are located. This is sometimes referred to in the integrated circuit field as a "cavity down" device. This design creates problems for the application of liquid solder flux using a stencil or like device, for example, because the protrusion impacts and interferes with the substantially flat, continuous surface of the stencil.

In accordance with another aspect of this invention, an apparatus is provided for flux to a substrate of this type, i.e., one which includes at least one protrusion. The apparatus may comprise a stencil as heretofore described, such as stencil 10, wherein the second side includes at least one cavity corresponding in location to the at least one protrusion location. The at least one cavity of the second side receives the at least one protrusion when the second side of the stencil is adjacent to the substrate.

As implemented in the first preferred embodiment, and to illustrate the principle, with reference to Fig. 8, substrate 2' includes a protrusion 38, such as a semiconductor chip, and stencil 10 includes a cavity 40 in its second side 14 corresponding in location to the location of protrusion 38. Cavity 40 receives protrusion 38 when second side 14 of stencil 10 is adjacent to substrate 2'.

In accordance with another aspect of the invention, an apparatus is provided for applying a liquid solder flux in a pattern to a substrate. The apparatus comprises a first side for distribution of the liquid solder flux to the apparatus; a second side opposite the first side; and a plurality of apertures passing through the apparatus from the first side to the second side so that the liquid solder flux passes in fluid communication from the first side through the apertures to the substrate. The apertures correspond in location to the pattern. Each of the apertures has a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end. The first section and the second section meet at a planar intersection substantially parallel to the first and second sides. The planar intersection has a third cross section. The second section includes a second aperture in fluid communication with the first section and the second end. Each second aperture of the second section has a third end adjacent to the third cross section and a fourth end adjacent to the second cross section.

A second preferred embodiment of the invention, one which illustrates this aspect of the invention, will now be described with reference to Figs. 9-12. In accordance with this aspect of the invention, an apparatus in the form of a stencil 110 is provided which is similar in most respects to stencil 10 described above. The reference numerals for the various components of stencil 10 apply to stencil 110, except that each two-digit reference numeral is preceded in this embodiment by a 1, i.e., 10 becomes 110, and so on. Stencil 110 differs from stencil 10 of the first preferred embodiment primarily in that the second section 128 of stencil 110 comprises a single second aperture 132 for each first aperture 116, whereas in stencil 10 each first aperture 16 includes a plurality of second apertures 32.

The options for the geometry of the first and sections as described above also apply here. Preferably, for example, the third end 134 is located within the third cross section 130 and the third end 134 is smaller than the first cross section 122a. More preferably, the first section is substantially conical in shape. It may, however, take other shapes, such as cylindrical, an example of which is illustrated in Fig. 12. The second section preferably is substantially cylindrical.

In an alternative embodiment similar to the one described above with respect to Fig. 8, where the substrate includes at least one protrusion having at least one corresponding protrusion location, the second side of substrate 110 may include at least one cavity 140 corresponding in location to the at least one protrusion location. The at least one cavity of the second side again would receive the at least one protrusion when the second side is adjacent to the substrate. In accordance with another aspect of the invention, a method is provided for applying a liquid solder flux in a pattern to a substrate. The method comprises a first step of providing an apparatus having a first side and a second side opposite the first side, and a plurality of first apertures passing through the apparatus from the first side to the second side in fluid communication from the first side through the first apertures to the second side. The first apertures correspond in location to the pattern. Each of the first apertures has a first end adjacent to the first side with a first cross section_^ substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side. Each of the first apertures also has a first section adjacent to the first end and a second section adjacent to the second end. The first section and the second section meet at a planar intersection substantially parallel to the first and second sides. The planar intersection has a third cross section. The second section includes a plurality of second apertures in fluid communication with the first section and the second end. Each of the second apertures has a third end adjacent to the third cross section and a fourth end adjacent to the second cross section.

Although not limiting, an apparatus suitable for use in carrying out this step of the method, in fact the presently preferred apparatus, comprises stencil 10, which is described above as the first preferred embodiment.

The method according to this aspect of the invention includes a second step of positioning the second side of the apparatus adjacent to the substrate. This step according to the presently preferred version of the method involves placing stencil 10 adjacent to, and preferably in contact with, the surface of substrate 2 upon which the solder lands 8 are located. This causes the first apertures 16 to align with the lands 8 according to pattern 6. The preferred embodiments of the apparatus typically would be mounted in a frame, such as frame 42, which retains the periphery of the apparatus to provide support as the apparatus is moved to and away from the substrate, for example, as depicted in Figs. 13 and 14. The most common frame designs capture the apparatus by clamping it between a first and a second gripping surface, 44 and 46, respectively, at the first and second sides 12 and 14 of the stencil. Many such frames are designed to place the apparatus in tension so as to stretch the apparatus to assure that is remains flat across its surface during use. This helps assure that the distance between the apparatus and the substrate is constant over the entire surface of the substrate. The substrate is placed adjacent to the second side of the apparatus to apply liquid solder flux to the substrate. The substrate is placed so that the pattern on the substrate is aligned with the corresponding aperture pattern on the apparatus. In this manner, the substrate pattern is in fluid communication with the first side of the apparatus. The substrate is located at a distance 34 from the second side of the apparatus. This distance varies from substantially zero (direct contact between the substrate and the second side of the apparatus) to several hundredths of an inch depending upon the application.

Preferably, the first step includes locating each third end within each respective third cross section. According to another aspect, the first step may include providing each second aperture with a substantially cylindrical shape. This could correspond to the cylindrical second apertures, for example, as shown in Fig. 4. According to another aspect, the first step includes providing the first cross section and the third cross section with a substantially circular shape. According to still another aspect, the first step includes providing the first cross section and the third cross section with substantially the same size and each first section is substantially cylindrical. According to yet anther aspect, the first step includes providing the first cross section to be larger than the third cross section and each first section is substantially conical.

The preferred method according to this aspect of the invention further includes a third step of applying the liquid solder flux to the first side of the apparatus so the liquid solder flux flows through the first and second apertures passing from the first side of the apparatus to the substrate.

The liquid solder flux 48 is typically applied to the first side 12 of stencil 10 by the use of a flooding blade 50, for example, as illustrated in Fig. 15. The liquid solder flux 48 is first placed onto the apparatus (stencil) adjacent to the pattern 18 on the apparatus. The flooding blade 50 is then used to spread the liquid solder flux 48 over the pattern by dragging the flooding blade 50 over the first side 12 of the apparatus 10 in a first direction 52 with the liquid solder flux in front of the flooding blade 50. During this flooding operation, the flooding blade 50 typically does not contact the first side 12 of the apparatus. The flooding blade 50 is typically positioned at an blade angle 54 relative to the first side 12. The angle 54 is generally about 40 to 50 degrees, although other angles may be used. The angle 54 is selected so that liquid solder flux 48 is captured in front of the flooding blade 50 while the flooding blade is being drawn across the first side 12. Once the blade 50 is drawn across the pattern 18 and the liquid solder flux has been distributed to the pattern 18 on the first side 12 of the apparatus, the flooding blade is removed from the apparatus.

The flooding blade may be made of metal, plastic, rubber, ceramic, glass, or a variety of other materials provided they are compatible with the liquid solder flux. Preferred materials are metal and rubber.

When liquid solder flux 48 is applied to the first side 12 of the apparatus by the flooding blade 50, as described previously, the first apertures 16 are filled with liquid solder flux 48. After the flooding blade is removed from the apparatus, a squeegee 56 is placed on the apparatus 10 at the periphery of the pattern 18 of first apertures 16 on the apparatus opposite the original starting position of the flooding blade 50. The squeegee 56 is typically located at a squeegee angle 58 so that the squeegee blade 56 contacts the first side 12 of the apparatus 10 with a light force, for example, several pounds. This results in a liquid seal between the squeegee 56 and the first side 12 so that liquid solder flux 48 is wiped from the first side 12 when the squeegee 56 is moved along the first side 12. The squeegee angle is typically about 40 to 50 degrees, but other angles can be used. The squeegee 56 is then moved along the first side of the apparatus in a second direction 60- opposite the first direction 52 the flooding blade 50 was moved along the apparatus. This wipes the liquid solder flux 48 from the first side of the apparatus and forces the liquid solder flux through the first apertures 16 and second apertures 32 to the substrate, as shown in Fig. 16. The squeegee 56 is moved along the first side 12 until the squeegee has traversed the entire aperture pattern 18. At this point the squeegee is removed from the apparatus, the substrate 2 is removed from the apparatus, or vice versa, and the cycle is ready to repeat with a new substrate.

The squeegee 56 is most often made of some form of rubber, but metal blades such as stainless steel and plastic blades are common as well. Other materials may be used as long as they are compatible with the liquid solder flux, but metals and rubbers are the most common. When rubber materials are used, the durometer is selected to give the optimum sealing characteristics to the apparatus for that particular application.

Another alternative method of applying the liquid solder flux to the first side of the apparatus is to use the squeegee for both the flooding operation as well as the squeegee operation. In this case, the squeegee 56 is first be located above the apparatus and then drawn across the apparatus in a first direction 52 similar to the flooding operation previously described. After flooding is complete, the squeegee is moved until it contacts the apparatus and then it is drawn across the apparatus in the second direction 60 in a manner similar to the squeegee operation describe previously. Other methods of applying the liquid solder flux to the first side of the apparatus may be used as well. For example, the liquid solder flux may be applied to the apparatus in a single squeegee operation without the flooding operation. Alternatively, liquid solder flux could be applied to the first side of the apparatus be a manifold. In this arrangement, the pressure of the liquid solder flux in the manifold could be varied to selectively apply the liquid solder flux to the substrate when the substrate is located adjacent to the second side of the apparatus.

Once liquid solder flux 48 is applied to the first side of the apparatus, the first apertures fill with liquid solder flux, generally from the first end to the second end. The first cross section defines the flow area of the first end for receiving the liquid solder flux when it is distributed on the first side. The area of the first cross section must be adequate to allow liquid solder flux to flow freely into the first apertures, for example, during the flooding operation. The preferred shape of the first cross section is circular, but other shapes are acceptable as well, for example, oval, square, rectangular, or triangular to list a few other possibilities. The principal requirement is that the first cross section should have sufficient area to allow the liquid solder flux to flow freely into the first apertures when the liquid solder flux is applied to the first side. As explained previously, the substrate is located adjacent to the second side of the apparatus when the apparatus is being used to apply flux to the substrate. The pattern 6 on the substrate 2 is comprised of solder pads 8. The substrate is located so that the second end of the first apertures is located adjacent to the solder pads so flux can be accurately applied to the solder pads. The second cross section is the flow area for the transfer of the liquid solder flux to the solder pads on the substrate. As a general rule, the area of the second cross section may be less than or equal to the area of the solder pad.

The preferred method according to this aspect of the invention further includes a fourth step of removing the substrate from the apparatus. This would typically involved using the frame and the mechanical armature and control mechanisms associated with it to physically move the stencil away from the substrate.

In accordance with a modified version of this method, the first step includes providing the substrate to include at least one protrusion having at least one corresponding protrusion location, and the second step includes providing the second side with at least one cavity corresponding in location to the at least one protrusion location. The at. least one cavity of the second side receives the at least one protrusion when the second side is adjacent to the substrate.

An example of a substrate which includes a protrusion is provided in Fig. 8, and was discussed above. This aspect of the method may be accomplished by providing a stencil 10' as shown in Fig. 8.

In accordance with yet another aspect of the invention, a method is provided for applying a liquid solder flux in a pattern to a substrate. The method comprises a first step of providing an apparatus having a first side and a second side opposite the first side, and a plurality of first apertures passing through the apparatus from the first side to the second side in fluid communication from the first side through the first apertures to the second side, the first apertures corresponding in location to the pattern, each of the first apertures having a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end, the first section and the second section meeting at a planar intersection substantially parallel to the first and second sides, the planar intersection having a third cross section, and the second section including a second aperture in fluid communication with the first section and the second end, the second section having a third end adjacent to the third cross section and a fourth end adjacent to the second cross section. This apparatus differs from that described above with respect to the previous preferred method, once again, in that the second section of each first aperture includes a single second aperture, instead of a plurality of second apertures as described above.

The preferred method according to this aspect of the invention (the second preferred method) also includes a second step of positioning the second side of the apparatus adjacent to the substrate. This step is performed in the same manner as the second step of the previously-described preferred method.

The method according to this aspect further includes a third step of applying the liquid solder flux to the first side of the apparatus so the liquid solder flux flows through the_first and second apertures passing from the first side of the apparatus to the substrate. This step of the second preferred method also is carried out as is described above for the second step of the first preferred method. The second preferred method also includes a fourth step of removing the substrate from the apparatus. This also is performed as described above for the fourth step of the first preferred method.

In a modification of this second preferred method, the first step includes providing the third end located within the third cross section, the third end being smaller than the first cross section.

This second preferred method differs from the first preferred method primarily in that an apparatus wherein each first aperture includes a single second aperture, rather than an apparatus wherein each first aperture includes a plurality of second apertures. The various shapes, dimentions, etc. for the first and second sections as discussed above apply in this method as well. The apparatus according to this preferred method also may include a cavity, for example, as shown in Fig. 8.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for applying a liquid solder flux in a pattern to a substrate, the apparatus comprising: a first side for distribution of the liquid solder flux to the apparatus; a second side opposite the first side; and a plurality of first apertures passing through the apparatus from the first side to the second side so that the liquid solder flux passes in fluid communication from the first side through the first apertures to the substrate, the first apertures corresponding in location to the pattern, each of the first apertures having a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end, the first section and the second section meeting at a planar intersection substantially parallel to the first and second sides, the planar intersection having a third cross section, and the second section including a plurality of second apertures in fluid communication with the first section and the second end, each of the second apertures having a third end adjacent to the third cross section and a fourth end adjacent to the second cross section.

2. An apparatus as recited in claim 1, wherein each third end is located within the third cross section.

3. An apparatus as recited in claim 1, wherein each second aperture is substantially cylindrical.

4. An apparatus as recited in claim 1, wherein the first cross section and the third cross section are substantially circular.

5. An apparatus as recited in claim 4, wherein the first cross section and the third cross section are substantially the same size, and each first section is substantially cylindrical.

6. An apparatus as recited in claim 4, wherein the first cross section is larger than the third cross section and each first section is substantially conical.

7. An apparatus as recited in claim 1, wherein: the substrate includes at least one protrusion having at least one corresponding protrusion location; and the second side includes at least one cavity corresponding in location to the at least one protrusion location, the at least one cavity of the second side receiving the at least one protrusion when the second side is adjacent to the substrate.

8. An apparatus for applying a liquid solder flux in a pattern to a substrate, the apparatus comprising: a first side for distribution of the liquid solder flux to the apparatus; a second side opposite the first side; and a plurality of first apertures passing through the apparatus from the first side to the second side so that the liquid solder flux passes in fluid communication from the first side through the first apertures to the substrate, the first apertures corresponding in location to the pattern, each of the first apertures having a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end, the first section and the second section meeting at a planar intersection substantially parallel to the first and second sides, the planar intersection having a third cross section, and the second section including a second aperture in fluid communication with the first section and the second end, each second aperture having a third end adjacent to the third cross section and a fourth end adjacent to the second cross_section.

9. An apparatus as recited in claim 8, wherein the third end is located within the third cross section and the third end is smaller than the first cross section.

10. An apparatus as recited in claim 9, wherein the second section is substantially cylindrical.

11. An apparatus as recited in claim 9, wherein the first cross section and the third cross section are substantially circular.

12. An apparatus as recited in claim 11, wherein the first cross section is larger than the third cross section and each first section is substantially conical.

13. An apparatus as recited in claim 12, wherein the first cross section and the third cross section are substantially the same size, and each first section is substantially cylindrical.

14. An apparatus as recited in claim 9, wherein: the substrate includes at least one protrusion having at least one corresponding protrusion location; and the second side includes at least one cavity corresponding in location to the at least one protrusion location, the at least one cavity of the second side receiving the at least one protrusion when the second side is adjacent to the substrate.

15. A method for applying a liquid solder flux in a pattern to a substrate, the method comprising:

(a) a first step of providing an apparatus having a first side and a second side opposite the first side, and a plurality of first apertures passing through the apparatus from the first side to the second side in fluid communication from the first side through the first apertures to the second side, the first apertures corresponding in location to the pattern, each of the apertures having . a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end, the first section and the second section meeting at a planar intersection substantially parallel to the first and second sides, the planar intersection having a third cross section, and the second section including a plurality of second apertures in fluid communication with the first section and the second end, each of the second apertures having a third end adjacent to the third cross section and a fourth end adjacent to the second cross section; (b) a second step of positioning the second side of the apparatus adjacent to the substrate;

(c) a third step of applying the liquid solder flux to the first side of the apparatus so the liquid solder flux flows through the first and second apertures passing from the first side of the apparatus to the substrate; and

(d) a fourth step of removing the substrate from the apparatus.

16. A method as recited in claim- 15, wherein the first step includes locating each third end within each respective third cross section.

17. A method as recited in claim 15, wherein the first step includes providing each second aperture with a substantially cylindrical shape.

18. A method as recited in claim 15, wherein the first step includes providing the first cross section and the third cross section with a substantially circular shape.

19. A method as recited in claim 18, wherein the first step includes providing the first cross section and the third cross section with substantially the same size and each first section is substantially cylindrical.

20. A method as recited in claim 18, wherein the first step includes providing the first cross section to be larger than the third cross section and each first section is substantially conical .

21. A method as recited ^'in claim 15, wherein: the first step includes providing the substrate to include at least one protrusion having at least one corresponding protrusion location; and the second step includes providing the second side with at least one cavity corresponding in location to the at least one protrusion location, the at least one cavity of the second side receiving the at least one protrusion when the second side is adjacent to the substrate.

22. A method for applying a liquid solder flux in a pattern to a substrate, the method comprising:

(a) a first step of providing an apparatus having a first side and a second side opposite the first side, and a plurality of first apertures passing through the apparatus from the first side to the second side in fluid communication from the first side through the first apertures to the second side, the first apertures corresponding in location to the pattern, each of the first apertures having a first end adjacent to the first side with a first cross section substantially parallel to the first side and a second end adjacent to the second side with a second cross section substantially parallel to the second side, and a first section adjacent to the first end and a second section adjacent to the second end, the first section and the second section meeting at a planar intersection substantially parallel to the first and second sides, the planar intersection having a third cross section, and the second section including a second aperture in fluid communication with the first section and the second end, the second section having a third end adjacent to the third cross section and a fourth end adjacent to the second cross section; (b) a second step of positioning the second side of the apparatus adjacent to the substrate;

(d) a fourth step of removing the substrate from the apparatus .

23. A method as recited in claim 22, wherein the first step includes providing the third end located within the third cross section, the third end being smaller than the first cross section .

24. A method as recited in claim 23, wherein the first step includes providing the second section with a substantially cylindrical shape.

25. A method as recited in claim 23, wherein the first step includes providing the first cross section and the third cross section with a substantially circular shape.

26. A method as recited in claim 25, wherein the first step includes providing the first cross section to be larger than the third cross section and each first section is substantially conical .

27. A method as recited in claim 26, wherein the first step includes providing the first cross section and the third cross section to be substantially the same size, and first section is substantially cylindrical.

28. A method as recited in claim 23, wherein: the second step includes providing the substrate to include at least one protrusion having at least one corresponding protrusion location; and the first step includes providing the second surface to include at least one cavity corresponding in location to the at least one protrusion location, the at least one cavity of the second surface receiving the at least one protrusion when the second surface is placed adjacent to the substrate.