WO2015121532A1

WO2015121532A1 - A circuit board and associated apparatus and methods

Info

Publication number: WO2015121532A1
Application number: PCT/FI2015/050058
Authority: WO
Inventors: Samiul Haque; Richard White; Jani Kivioja
Original assignee: Nokia Corporation
Priority date: 2014-02-14
Filing date: 2015-01-30
Publication date: 2015-08-20
Also published as: CN106465540A; EP3106007A4; GB2523145A; EP3106007A1; GB201402589D0

Abstract

A circuit board comprising a primary substrate and a secondary substrate, the primary substrate comprising a recess configured to receive the secondary substrate, and a conducting trace which extends towards an edge of the recess, the secondary substrate comprising an electronic component and a conducting trace which extends from the electronic component towards an edge of the secondary substrate, wherein the secondary substrate is positioned within the recess and attached to the primary substrate, and the conducting trace of the secondary substrate is joined to the conducting trace of the primary substrate to form an electrical connection between the electronic component and the primary substrate.

Description

A circuit board and associated apparatus and methods Technical Field The present disclosure relates to the field of device fabrication, associated methods and apparatus, and in particular concerns a circuit board formed using a primary substrate and a secondary substrate. Certain disclosed example aspects/embodiments relate to portable electronic devices, in particular, so-called hand-portable electronic devices which may be hand-held in use (although they may be placed in a cradle in use). Such hand- portable electronic devices include so-called Personal Digital Assistants (PDAs) and tablet PCs.

The portable electronic devices/apparatus according to one or more disclosed example aspects/embodiments may provide one or more audio/text/video communication functions (e.g. tele-communication, video-communication, and/or text transmission, Short Message Service (SMS)/ Multimedia Message Service (MMS)/emailing functions, interactive/non- interactive viewing functions (e.g. web-browsing, navigation, TV/program viewing functions), music recording/playing functions (e.g. MP3 or other format and/or (FM/AM) radio broadcast recording/playing), downloading/sending of data functions, image capture function (e.g. using a (e.g. in-built) digital camera), and gaming functions.

Background

Research is currently being done to develop flexible circuit boards for modern electronic devices. These circuit boards must be able to comply with varying form factors and device configurations and be suitable for use in ever-shrinking device casings.

One or more aspects/embodiments of the present disclosure may or may not address these issues.

The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge. Summary A circuit board comprising a primary substrate and a secondary substrate, the primary substrate comprising a recess configured to receive the secondary substrate, and a conducting trace which extends towards an edge of the recess,

the secondary substrate comprising an electronic component and a conducting trace which extends from the electronic component towards an edge of the secondary substrate,

wherein the secondary substrate is positioned within the recess and attached to the primary substrate, and the conducting trace of the secondary substrate is joined to the conducting trace of the primary substrate to form an electrical connection between the electronic component and the primary substrate.

The secondary substrate may or may not be formed from part of the primary substrate.

The conducting traces of the primary and secondary substrates may extend to at least the edges of the recess and secondary substrate respectively. The conducting traces of the primary and secondary substrates may extend onto the lateral surfaces of the recess and secondary substrate, respectively. The conducting trace of the secondary substrate may be directly joined to the conducting trace of the primary substrate. The conducting trace of the secondary substrate may be indirectly joined to the conducting trace of the primary substrate by a conducting bridge.

The primary and secondary substrates may be configured such that the conducting trace of the primary substrate is at substantially the same vertical height as the conducting trace of the secondary substrate when the secondary substrate is positioned within the recess of the primary substrate. The primary and secondary substrates may be configured such that the conducting trace of the primary substrate is in contact with (or at least in proximity to) the conducting trace of the secondary substrate when the secondary substrate is positioned within the recess of the primary substrate. The recess of the primary substrate may be a blind hole in the primary substrate. The recess of the primary substrate may be a through hole in the primary substrate. The secondary substrate may have substantially the same dimensions as the recess of the primary substrate. The conducting traces of the primary and secondary substrates may be formed on the upper or lower surfaces of the respective substrates. The conducting traces of the primary and secondary substrates may be formed within the respective substrates. The secondary substrate may comprise a plurality of electronic components. The primary and secondary substrates may comprise a plurality of corresponding conducting traces for connecting the electronic components to the primary substrate. The primary substrate may comprise a plurality of recesses. The circuit board may comprise a plurality of respective secondary substrates positioned within the recesses of the primary substrate.

The primary substrate may comprise a recess and conducting trace on each of its upper and lower surfaces. The circuit board may comprise a respective secondary substrate positioned within the recess of each surface. The primary substrate may comprise first and second primary substrates attached back-to-back. Each primary substrate may have a recess and a conducting trace on its exposed surface. The circuit board may comprise a respective secondary substrate positioned within the recess of each primary substrate.

The primary and secondary substrates may each be formed from reversibly deformable materials. The primary and secondary substrates may each be formed from flexible plastic materials. The primary and secondary substrates may each be formed from one or more of polyethylene terephthalate and polyethylene naphthalate. The primary and secondary substrates may comprise the same materials or different materials. The primary and secondary substrates may comprise the same materials which have been functionalized differently (e.g. using one or more of oxygen plasma, UV curing and spray coating of materials). The primary and secondary substrates may be spray coated with one or more of graphene, graphene oxide, boron nitride and molybdenum disulfide. The conducting traces of the primary and secondary substrates may each be formed from one or more of silver, gold, copper, carbon and conductive ink. The conductive ink may comprise a composite or mixture of two or more different inks.

The primary substrate may be a motherboard for an electronic device. The secondary substrate may be a module for the electronic device. The electronic component may be one or more of a sensor, an organic light-emitting diode, an actuator and a microelectromechanical system. According to a further aspect, there is provided an electronic device or apparatus comprising any circuit board described herein. The electronic device or apparatus may be one or more of a portable electronic device, a portable telecommunications device, a mobile phone, a PDA, a desktop computer, a laptop computer and a tablet computer. The apparatus may be a module for the electronic device.

According to a further aspect, there is provided a primary substrate for a circuit board which comprises a primary substrate and a secondary substrate,

the primary substrate comprising a recess configured to receive the secondary substrate, and a conducting trace which extends towards an edge of the recess,

wherein the primary substrate is configured such that the conducting trace of the primary substrate can be joined to the conducting trace of the secondary substrate to form an electrical connection between the electronic component and the primary substrate when the secondary substrate is positioned within the recess and attached to the primary substrate. According to a further aspect, there is provided a secondary substrate for a circuit board which comprises a primary substrate and a secondary substrate,

wherein the secondary substrate is configured such that the conducting trace of the secondary substrate can be joined to the conducting trace of the primary substrate to form an electrical connection between the electronic component and the primary substrate when the secondary substrate is positioned within the recess and attached to the primary substrate.

According to a further aspect, there is provided an apparatus comprising a primary circuit board substrate and a secondary circuit board substrate, the primary circuit board substrate comprising a recess configured to receive the secondary circuit board substrate, and a conducting trace which extends towards an edge of the recess,

the secondary circuit board substrate comprising an electronic component and a conducting trace which extends from the electronic component towards an edge of the secondary circuit board substrate,

wherein the secondary circuit board substrate is positioned within the recess and attached to the primary circuit board substrate, and the conducting trace of the secondary circuit board substrate is joined to the conducting trace of the primary circuit board substrate to form an electrical connection between the electronic component and the primary circuit board substrate.

According to a further aspect, there is provided a method of making a circuit board which comprises a primary substrate and a secondary substrate, the method comprising:

positioning the secondary substrate within a recess of the primary substrate configured to receive the secondary substrate, the primary substrate comprising a conducting trace which extends towards an edge of the recess, the secondary substrate comprising an electronic component and a conducting trace which extends from the electronic component towards an edge of the secondary substrate;

attaching the secondary substrate to the primary substrate; and

joining the conducting trace of the secondary substrate to the conducting trace of the primary substrate to form an electrical connection between the electronic component and the primary substrate. The method may comprise positioning the secondary substrate within the recess of the primary substrate by aligning the secondary substrate with the recess of the primary substrate, and subsequently stamping/punching the secondary substrate from a larger substrate into the recess. The method may comprise attaching the secondary substrate to the primary substrate using one or more of adhering, sintering, welding, laser sintering and laser welding. The method may comprise joining the conducting trace of the secondary substrate to the conducting trace of the primary substrate directly using one or more of sintering, welding, laser sintering or laser welding. The method may comprise joining the conducting trace of the secondary substrate to the conducting trace of the primary substrate indirectly by printing a conducting bridge between the ends of the conducting traces. The conducting traces of the primary and secondary substrates may extend to at least the edges of the recess and secondary substrate, respectively. The method may comprise attaching the secondary substrate to the primary substrate, and joining the conducting trace of the secondary substrate directly to the conducting trace of the primary substrate, in a single step using sintering, welding, laser sintering or laser welding.

One or more of the positioning, attaching and joining steps may be performed using roll- to-roll processing of the primary and secondary substrates.

The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated or understood by the skilled person.

Corresponding computer programs (which may or may not be recorded on a carrier) for implementing one or more of the methods disclosed herein are also within the present disclosure and encompassed by one or more of the described example embodiments.

The present disclosure includes one or more corresponding aspects, example embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. Corresponding means for performing one or more of the discussed functions are also within the present disclosure.

The above summary is intended to be merely exemplary and non-limiting. Brief Description of the Figures

A description is now given, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 a illustrates schematically formation of multiple secondary substrates from a larger single substrate (plan view);

Figure 1 b illustrates schematically a primary substrate comprising a recess configured to receive a secondary substrate (perspective view);

Figure 1 c illustrates schematically positioning of a secondary substrate within the recess of the primary substrate (perspective view); Figure 1 d illustrates schematically attachment of the secondary substrate to the primary substrate, and joining of the conducting trace of the secondary substrate to the conducting trace of the primary substrate (perspective view);

Figure 1 e illustrates schematically the completed circuit board after attachment of the primary and second substrates and the joining of their respective conducting traces (perspective view);

Figure 2a illustrates schematically a circuit board in which the conducting trace of the primary substrate is directly joined to the conducting trace of the secondary substrate (plan view and cross-section);

Figure 2b illustrates schematically a circuit board in which the conducting trace of the primary substrate is indirectly joined to the conducting trace of the secondary substrate (plan view and cross-section);

Figure 3a illustrates schematically a circuit board in which the conducting traces of the primary and second substrates extend onto the lateral surfaces of the recess and secondary substrate, respectively (cross-section);

Figure 3b illustrates schematically a circuit board in which the recess, secondary substrate and conducting traces are located on the lower surface of the primary substrate (cross- section);

Figure 3c illustrates schematically a circuit board in which the conducting traces are located within the primary and secondary substrates (cross-section);

Figure 3d illustrates schematically a circuit board in which the conducting trace of the secondary substrate extends onto the lateral surface of the secondary substrate and the conducting trace of the primary substrate is formed within the primary substrate (cross- section);

Figure 3e illustrates schematically a circuit board in which the recess is a through hole of the primary substrate (cross-section);

Figure 3f illustrates schematically a circuit board in which the primary substrate comprises a recess, secondary substrate and conducting trace on each of its upper and lower surfaces (cross-section);

Figure 3g illustrates schematically a circuit board comprising two primary substrates attached back-to-back each having a recess, secondary substrate and conducting trace on its exposed surface (cross-section);

Figure 4 illustrates schematically an electronic device comprising a circuit board described herein;

Figure 5 shows the main steps of a method of making a circuit board described herein; Figure 6 shows a computer-readable medium comprising a computer program configured to perform, control or enable one or more of the method steps of Figure 5; and

Figure 7 shows a roll-to-roll processing setup for making a circuit board described herein. Description of Specific Aspects/Embodiments

Flexible electronics are often fabricated by direct printing of several different materials to form electronic components on a substrate. Each printed material must undergo a post- process to cure or cross-link the material before further printing can take place. In some circumstances, and especially as the complexity of the circuitry increases, different post- processes may be incompatible with one another and/or repeated post-processing may degrade the performance of previously deposited materials.

Another issue in the manufacture of flexible electronic devices is the stacking of substrates to form circuit boards. Substrate stacking requires vertical interconnect access (VIA) connections which increase the complexity of the fabrication process and can result in contact failures when the materials are bent or stretched.

There will now be described an apparatus and associated methods that may or may not provide a solution to one or more of these issues.

Figure 1 shows a method of making a circuit board using a primary substrate 101 and a secondary substrate 102. The secondary substrate 102 comprises an electronic component (not shown) and a conducting trace 103 which extends from the electronic component towards an edge of the secondary substrate 102. As illustrated in plan view in Figure 1A, a plurality of secondary substrates 102 may be formed from a larger substrate 104 which is subsequently divided into the individual secondary substrates 102. The conducting trace 103 may be formed using an additive process such as printing or patterned deposition (e.g. evaporation or sputtering) and/or a subtractive process such as etching.

The primary substrate 101 (Figure 1 b) comprises a recess 105 configured to receive the secondary substrate 102, and a conducting trace 106 which extends towards an edge of the recess 105. The recess 105 may be formed using any conventional subtractive process, such as wet or dry chemical etching, cutting, ablation or dissolution. In the example shown, the primary substrate 101 comprises a plurality of recesses 105 and associated traces 106, but any number may be incorporated. In practice, the required number of recesses 105 (and associated traces 106) will depend on the number of electronic components required to form the circuitry. The secondary substrate 102 could be formed from part of the primary substrate 101 which has been removed to form the recess 105. This could be achieved, for example, by cutting the primary substrate 101 to define the recess 105 and using suction or force to remove the part defined by the cut. This approach is advantageous in the sense that a secondary substrate 102 could be made to exactly fit the recess 105 of the primary substrate 101.

The secondary substrate 102 is positioned within the recess 105 (Figure 1 c) and attached to the primary substrate 101 (Figure 1 d). Attachment of the secondary substrate 102 to the primary substrate 101 may be achieved using an adhesive on the bottom or lateral surfaces of the secondary substrate 102 and/or recess 105. Additionally or alternatively, the substrates 101 , 102 may be sintered or welded together at the edges of the recess

105 (e.g. using a laser 107 as shown). The conducting trace 103 of the secondary substrate 102 is then joined to the conducting trace 106 of the primary substrate 101 to form an electrical connection between the electronic component (on the secondary substrate 102) and the primary substrate 101 .

Depending on the materials used to form the substrates 101 , 102 and conducting traces 103, 106, attachment of the substrates 101 , 102 and joining of the conducting traces 103,

106 may be performed in the same processing step, or in different processing steps. For example, if the primary 101 and secondary 102 substrates are each formed from a flexible plastic material such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), and the conducting traces 103, 106 of the primary 101 and secondary 102 substrates are each formed from silver, gold, copper or carbon, it is possible to sinter or weld the substrates 101 , 102 and conducting traces 103, 106 in the same step. With other materials, however, it may be necessary to use separate sintering/welding steps using different parameters.

Whether or not the attachment and joining processes can be performed in a single step also depends on the relative positioned of the conducting traces 103, 106. For example, if the primary 101 and secondary 102 substrates are configured such that the conducting trace 106 of the primary substrate 101 is in contact with (or at least in proximity to) the conducting trace 103 of the secondary substrate 102 when the secondary substrate 102 is positioned within the recess 105 of the primary substrate 101 , then a single sintering/welding step may be possible. Figure 1 e shows the circuit board 108 comprising both the primary 101 and secondary 102 substrates after the attachment and joining steps have been completed.

The present fabrication process provides a number of advantages over existing techniques. For example, since the electronic components are formed on secondary substrates 102 and then subsequently transferred to the primary substrate 101 to produce the circuit board 108, the chemical or physical processes used to form each component does not affect the other components or circuit board 108. That said, if the individual secondary substrates are formed from a larger substrate 104 (as shown in Figure 1 a), the electronic components may need to be formed on their respective secondary substrates 102 after the larger substrate 104 has been divided up. Formation of the electronic components on the secondary substrates 102 also facilitates attachment of the components to the circuit board 108. This is particularly beneficial for delicate components such as chemical vapour deposited (CVD) graphene components where direct attachment of the component to the circuit board 108 is difficult and can damage the component. Furthermore, by positioning the secondary substrates 102 within the recesses 105 of the primary substrate 101 rather than stacking multiple layers of material on top of one another, the electronic components can be electrically connected to the circuit board 108 without the need for VIAs. Figure 2a shows (in plan view and cross-section) a circuit board 208 in which the conducting trace 203 of the secondary substrate 202 is directly joined to the conducting trace 206 of the primary substrate 201 to form the electrical connection between the electronic component 218 and the primary substrate 201 . This type of join may be formed by sintering or welding the conducting traces together, and requires the conducting traces 203, 206 of the primary 201 and secondary 202 substrates to extend to at least the edges of the recess 205 and secondary substrate 202, respectively, such that the conducting traces 203, 206 are in contact with one another when the secondary substrate 202 is positioned within the recess 205 of the primary substrate 201 . Figure 2b shows (in plan view and cross-section) another circuit board 208 in which the conducting trace 203 of the secondary substrate 202 is indirectly joined to the conducting trace 206 of the primary substrate 201 by a conducting bridge 209 to form the electrical connection between the electronic component 218 and the primary substrate 201 . A conducting bridge 209 may be used when the traces 203, 206 of the primary 201 and secondary 202 substrates do not extend to the edges of the recess 205 and secondary substrate 202, respectively, and can be formed by depositing an electrically conductive material between the ends of the traces 203, 206 (e.g. by printing graphene ink). The electrically conductive material could also be deposited over a direct join (Figure 2a) of the conducting traces 203, 206 to improve the electrical connection. To facilitate formation of a direct join, the conducting traces 303, 306 of the primary 301 and secondary 302 substrates may extend onto the lateral surfaces of the recess 305 and secondary substrate 203, respectively. This is illustrated in cross-section in Figure 3a in which a gap is shown between the lateral sections 310, 31 1 of the traces 303, 306 for visualisation. In practice, however, the secondary substrate 302 would have substantially the same dimensions as the recess 305 of the primary substrate 301 to aid positioning of the secondary substrate 302 within the recess 305 and help ensure contact between the respective traces 303, 306. In this way, the resulting circuit board 308 has a substantially planar configuration. Extending the traces 303, 306 beyond the edges of the recess 305 and secondary substrate 302 serves to increase the interface therebetween. The larger interface increases the amount of material which is available to form the join thereby providing a more structurally robust connection. Although both traces 303, 306 can be seen to terminate at the bottom of the lateral surfaces, one or both of the traces 303, 306 could be further extended onto the lower surface 312 of the recess 305/secondary substrate 302.

In the examples shown above, the primary 301 and secondary 302 substrates are configured such that the conducting trace 306 of the primary substrate 301 is at substantially the same vertical height as the conducting trace 303 of the secondary substrate 302 when the secondary substrate 302 is positioned within the recess 305 of the primary substrate 301. This is not an absolute requirement, however. For example, the conducting traces 303, 306 of the primary 301 and secondary 302 substrates may be formed on the upper 313 or lower 314 surfaces of the respective substrates 301 , 302, or even within the respective substrates 301 , 302 (i.e. embedded). Figure 3b shows one example where both traces 303, 306 are formed on the lower surfaces 314 of the respective substrates 301 , 302, Figure 3c shows another example where both traces 303, 306 are formed within the respective substrates 301 , 302, and Figure 3d shows a further example where one trace 306 is formed within the primary substrate 301 and the other trace 303 is formed on the upper surface 313 of the secondary substrate 302. In the example of Figure 3b, the recess 305 and secondary substrate 302 are also positioned on the lower surface 314 of the primary substrate 301 , and in the example of Figure 3d, a lateral extension 310, 31 1 of at least one of the traces 303, 306 is required to enable a connection to be formed.

As shown in Figure 3e, the recess 305 of the primary substrate 301 could be a through hole (i.e. the recess 305 extends from one surface 313 of the primary substrate 301 to the other 314) in the primary substrate 301 rather than a blind hole (i.e. the recess 305 terminates within the bulk of the primary substrate 301 ). In this scenario, the primary 301 and secondary 302 substrates could be placed on top of a supporting substrate or other surface to hold them in position during attachment of the secondary substrate 302 to the primary substrate 301. Additionally or alternatively, the recesses 305 could have substantially the same dimensions as the respective secondary substrates 302 to provide a friction fit. Attachment of the secondary substrates 302 to the primary substrate 301 may be provided by the friction fit, or by an additional attachment process (e.g. adhering, welding or sintering) in combination with the friction fit for a more robust attachment. In either case, conducting bridges may be printed between the ends of the conducting traces of the primary and secondary substrates to help form the electrical connections.

If a particular device circuitry requires multiple electronic components, as would normally be the case, the primary substrate 301 may comprise a plurality of recesses 305, and the circuit board 308 may comprise a plurality of respective secondary substrates 302 positioned within the recesses 305 of the primary substrate 301 . This configuration may be achieved in different ways, as shown in Figures 3f and 3g. In Figure 3f, the primary substrate 301 comprises a recess 305 and conducting trace 306 on each of its upper 313 and lower 314 surfaces, and the circuit board 308 comprises a respective secondary substrate 302 positioned within the recess 305 of each surface 313, 314 (although there could be more than one recess 305/secondary substrate 302 on each surface 313, 314). In Figure 3g, on the other hand, the primary substrate 301 comprises first 315 and second 316 primary substrates attached back-to-back each having a recess 305 and a conducting trace 306 on its exposed surface 317, and the circuit board 308 comprises a respective secondary substrate 302 positioned within the recess 305 of each primary substrate 301 (although there could be more than one recess 305/secondary substrate 302 on each exposed surface 317).

A further option is to form a plurality of electronic components 318 on the same secondary substrate 302 (not shown) with a plurality of corresponding conducting traces 303, 306 on or within the primary 301 and secondary 302 substrates for connecting the electronic components 318 to the primary substrate 301 . The latter option may, however, require the processing steps used to form each component 318 on the secondary substrate 302 to be compatible with the other components 318 in order to prevent damage thereto.

A variety of different materials could be used to form the substrates 301 , 302 and conducting traces 303, 306 of the circuit board 308 depending on the particular application and/or the fabrication process. For example, if the circuit board 308 is to be housed within a particularly small casing (flex-to-install) or is to be used in a dynamic flex application (e.g. folding laptop or mobile phone), the substrates 301 , 302 may be formed from reversibly deformable materials such as flexible/stretchable plastics. Such materials also enable roll-to-roll processing to be used during fabrication of the circuit board 308. Roll- to-roll processing could be used in the present case to perform one or more of the positioning, attaching and joining steps described previously, which may help to reduce the associated fabrication costs if such circuit boards 308 were to be mass produced.

As illustrated in Figure 7, the roll-to-roll process may involve the use of at least two separate rolls of material: roll A (to form the primary substrate 701 ) and roll B (to form the secondary substrates 702). The chemistry, roll speed and/or bobbin diameter associated with each roll may or may not be the same. The roll-to-roll process may comprise: cutting/etching the recesses 705 and corresponding secondary substrates 702 from the respective roll A and roll B materials using a laser; aligning the recesses 705 of the roll A material with the secondary substrates 702 of the roll B material; and punching/stamping the secondary substrates 702 from the roll B material into the recesses 705 of the roll A material, as indicated by arrow 727.

The roll-to-roll process may also comprise attaching the secondary substrates 702 to the primary substrate 701 (i.e. within the recesses 705 of the roll A material) by adhering, laser welding or laser sintering; and printing conducting bridges to form electrical connections between the conducting traces of the primary 701 and secondary 702 substrates. When an adhesive 728 (e.g. an optically clear adhesive (OCA) or an anisotropic conductive adhesive (ACA)) is used to attach the secondary substrates 702 to the primary substrate 701 , an additional lamination step may be required in the roll-to-roll process to deposit the adhesive 728 within the recesses 705 of the primary substrate 701 and/or on a surface of the secondary substrates 702. In this scenario, the adhesive 728 may be provided on a further roll of material.

Once the secondary substrates 702 are attached and electrically connected to the primary substrate 701 , one or more post-processes (e.g. curing or chemical treatment) may be performed before the resulting circuit board is cut from the roll A material. Furthermore, the roll-to-roll process may be continuous (i.e. constant movement of the rollers) from start to finish, or it may be stopped intermittently to perform one or more of the above- mentioned steps. Although continuous processing may increase the overall production rate, greater precision during some of the steps (such as alignment) may help to reduce the number of fabrication defects in the finished circuit board.

All thermoplastics and almost any thermoplastic elastomer can be welded/sintered together provided that they are chemically compatible and their respective melting temperature ranges overlap sufficiently. Some specific examples of thermoplastic elastomers which can be welded/sintered include polyethylene, polypropylene, polystyrene, acrylonitrile butadiene styrene, styrene acrylonitrile resin, polyamide 6, polyamide 6.6, polycarbonate, poly(methyl methacrylate), polysulfone, polyether ether ketone, polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate. Figure 4 shows one example of an electronic device/apparatus 419 comprising a circuit board 408 as described herein, a processor 420 and a storage medium 421 , which are electrically connected to one another by a data bus 422. The processor 420 and storage medium 421 may or may not form part of the circuit board 408. The electronic device/apparatus 419 may be one or more of a portable electronic device, a portable telecommunications device, a mobile phone, a PDA, a desktop computer, a laptop computer, a tablet computer and a module for any of the aforementioned devices.

The circuit board 408 comprises a primary substrate and at least one secondary substrate attached thereto, each secondary substrate comprising one or more electronic components. The primary substrate may be a motherboard of the electronic device/apparatus 419 and the secondary substrates may be modules of the electronic device/apparatus 419. Furthermore, the one or more electronic components may comprise sensors, organic light-emitting diodes (OLEDs), actuators or microelectromechanical systems (MEMS). The processor 420 is configured for general operation of the electronic device/apparatus 419 by providing signalling to, and receiving signalling from, the other components to manage their operation. The storage medium 421 is configured to store computer code configured to perform, control or enable operation of the electronic device/apparatus 419. The storage medium 421 may also be configured to store settings for the other components. The processor 420 may access the storage medium 421 to retrieve the component settings in order to manage the operation of the other components. The processor 420 may be a microprocessor, including an Application Specific Integrated Circuit (ASIC). The storage medium 421 may be a temporary storage medium such as a volatile random access memory. On the other hand, the storage medium 421 may be a permanent storage medium such as a hard disk drive, a flash memory, or a non-volatile random access memory.

The main steps 523-525 of a method of making a circuit board using a primary substrate and a secondary substrate are shown schematically in Figure 5. The method generally comprises: positioning 523 a secondary substrate within a recess of a primary substrate; attaching 524 the secondary substrate to the primary substrate; and joining 525 a conducting trace of the secondary substrate to a conducting trace of the primary substrate. As described previously, a variety of different processes may be used to perform the attachment 524 and joining 525 steps.

Figure 6 illustrates schematically a computer/processor readable medium 626 providing a computer program according to one embodiment. The computer program may comprise computer code configured to perform, control or enable one or more of the method steps 523-525 of Figure 5. In this example, the computer/processor readable medium 626 is a disc such as a digital versatile disc (DVD) or a compact disc (CD). In other embodiments, the computer/processor readable medium 626 may be any medium that has been programmed in such a way as to carry out an inventive function. The computer/processor readable medium 626 may be a removable memory device such as a memory stick or memory card (SD, mini SD, micro SD or nano SD). Other embodiments depicted in the figures have been provided with reference numerals that correspond to similar features of earlier described embodiments. For example, feature number 1 can also correspond to numbers 101 , 201 , 301 etc. These numbered features may appear in the figures but may not have been directly referred to within the description of these particular embodiments. These have still been provided in the figures to aid understanding of the further embodiments, particularly in relation to the features of similar earlier described embodiments.

It will be appreciated to the skilled reader that any mentioned apparatus/device and/or other features of particular mentioned apparatus/device may be provided by apparatus arranged such that they become configured to carry out the desired operations only when enabled, e.g. switched on, or the like. In such cases, they may not necessarily have the appropriate software loaded into the active memory in the non-enabled (e.g. switched off state) and only load the appropriate software in the enabled (e.g. on state). The apparatus may comprise hardware circuitry and/or firmware. The apparatus may comprise software loaded onto memory. Such software/computer programs may be recorded on the same memory/processor/functional units and/or on one or more memories/processors/functional units. In some embodiments, a particular mentioned apparatus/device may be pre-programmed with the appropriate software to carry out desired operations, and wherein the appropriate software can be enabled for use by a user downloading a "key", for example, to unlock/enable the software and its associated functionality. Advantages associated with such embodiments can include a reduced requirement to download data when further functionality is required for a device, and this can be useful in examples where a device is perceived to have sufficient capacity to store such pre-programmed software for functionality that may not be enabled by a user.

It will be appreciated that any mentioned apparatus/circuitry/elements/processor may have other functions in addition to the mentioned functions, and that these functions may be performed by the same apparatus/circuitry/elements/processor. One or more disclosed aspects may encompass the electronic distribution of associated computer programs and computer programs (which may be source/transport encoded) recorded on an appropriate carrier (e.g. memory, signal). It will be appreciated that any "computer" described herein can comprise a collection of one or more individual processors/processing elements that may or may not be located on the same circuit board, or the same region/position of a circuit board or even the same device. In some embodiments one or more of any mentioned processors may be distributed over a plurality of devices. The same or different processor/processing elements may perform one or more functions described herein.

It will be appreciated that the term "signalling" may refer to one or more signals transmitted as a series of transmitted and/or received signals. The series of signals may comprise one, two, three, four or even more individual signal components or distinct signals to make up said signalling. Some or all of these individual signals may be transmitted/received simultaneously, in sequence, and/or such that they temporally overlap one another.

With reference to any discussion of any mentioned computer and/or processor and memory (e.g. including ROM, CD-ROM etc), these may comprise a computer processor, Application Specific Integrated Circuit (ASIC), field-programmable gate array (FPGA), and/or other hardware components that have been programmed in such a way to carry out the inventive function.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole, in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that the disclosed aspects/embodiments may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the disclosure.

While there have been shown and described and pointed out fundamental novel features as applied to different embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. Furthermore, in the claims means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.

Claims

1 . A circuit board comprising a primary substrate and a secondary substrate,

2. The circuit board of claim 1 , wherein the conducting traces of the primary and secondary substrates extend to at least the edges of the recess and secondary substrate respectively, and wherein the conducting trace of the secondary substrate is directly joined to the conducting trace of the primary substrate.

3. The circuit board of claim 1 or 2, wherein the conducting traces of the primary and secondary substrates extend onto the lateral surfaces of the recess and secondary substrate, respectively, and wherein the conducting trace of the secondary substrate is directly joined to the conducting trace of the primary substrate.

4. The circuit board of any preceding claim, wherein the conducting trace of the secondary substrate is indirectly joined to the conducting trace of the primary substrate by a conducting bridge.

5. The circuit board of any preceding claim, wherein the recess of the primary substrate is a blind hole or a through hole in the primary substrate.

6. The circuit board of any preceding claim, wherein the secondary substrate has substantially the same dimensions as the recess of the primary substrate.

7. The circuit board of any preceding claim, wherein the conducting traces of the primary and secondary substrates are formed on the upper or lower surfaces of the respective substrates, or within the respective substrates.

8. The circuit board of any preceding claim, wherein the secondary substrate comprises a plurality of electronic components, and wherein the primary and secondary substrates comprise a plurality of corresponding conducting traces for connecting the electronic components to the primary substrate.

9. The circuit board of any preceding claim, wherein the primary substrate comprises a plurality of recesses, and wherein the circuit board comprises a plurality of respective secondary substrates positioned within the recesses of the primary substrate.

10. The circuit board of any preceding claim, wherein the primary substrate comprises a recess and conducting trace on each of its upper and lower surfaces, and wherein the circuit board comprises a respective secondary substrate positioned within the recess of each surface.

1 1 . The circuit board of any of claims 1 to 9, wherein the primary substrate comprises first and second primary substrates attached back-to-back, each primary substrate having a recess and a conducting trace on its exposed surface, and wherein the circuit board comprises a respective secondary substrate positioned within the recess of each primary substrate.

12. The circuit board of any preceding claim, wherein the primary and secondary substrates are each formed from flexible plastic materials.

13. An apparatus comprising the circuit board of any preceding claim.

14. A primary substrate for a circuit board which comprises a primary substrate and a secondary substrate,

wherein the primary substrate is configured such that the conducting trace of the primary substrate can be joined to the conducting trace of the secondary substrate to form an electrical connection between the electronic component and the primary substrate when the secondary substrate is positioned within the recess and attached to the primary substrate.

15. A secondary substrate for a circuit board which comprises a primary substrate and a secondary substrate,

16. An apparatus comprising a primary circuit board substrate and a secondary circuit board substrate,

the primary circuit board substrate comprising a recess configured to receive the secondary circuit board substrate, and a conducting trace which extends towards an edge of the recess,

17. A method of making a circuit board which comprises a primary substrate and a secondary substrate, the method comprising:

attaching the secondary substrate to the primary substrate; and

joining the conducting trace of the secondary substrate to the conducting trace of the primary substrate to form an electrical connection between the electronic component and the primary substrate.

18. The method of claim 17, wherein the method comprises attaching the secondary substrate to the primary substrate using one or more of adhering, sintering, welding, laser sintering and laser welding.

19. The method of claim 17 or 18, wherein the method comprises joining the conducting trace of the secondary substrate to the conducting trace of the primary substrate directly using one or more of sintering, welding, laser sintering or laser welding, and/or indirectly by printing a conducting bridge between the ends of the conducting traces.

20. The method of any of claims 17 to 19, wherein the conducting traces of the primary and secondary substrates extend to at least the edges of the recess and secondary substrate respectively, and wherein the method comprises attaching the secondary substrate to the primary substrate, and joining the conducting trace of the secondary substrate directly to the conducting trace of the primary substrate, in a single step using sintering, welding, laser sintering or laser welding.

21 . The method of any of claims 17 to 20, wherein one or more of the positioning, attaching and joining steps are performed using roll-to-roll processing of the primary and secondary substrates.

22. The method of any of claims 19, wherein the conducting bridge comprises graphene ink.