WO2000073807A1

WO2000073807A1 - Multi-stage test fixture

Info

Publication number: WO2000073807A1
Application number: PCT/AU2000/000426
Authority: WO
Inventors: Karl Berger
Original assignee: Berger Testfacilities Pty Ltd
Priority date: 1999-05-31
Filing date: 2000-05-11
Publication date: 2000-12-07
Also published as: AUPQ072999A0

Abstract

A multistage test fixture including means for mounting a unit under test, an array of spring loaded contact pins of varying height for selectively contacting the unit under test, and means for generating multistage relative movement between the unit under test and the array of spring loaded contact pins, the means for generating multistage relative movement including a pair of actuators in series.

Description

"MULTI-STAGE TEST FIXTURE"

TECHNICAL FIELD

This invention relates to a multi-stage test fixture for use by the electronics manufacturing industry to test printed circuit boards (PCBs) and/or componentry mounted thereon.

BACKGROUND ART

Testing PCBs is a necessary quality control step in the manufacture of electronic devices and it is well known to utilise test fixtures for this purpose.

A test fixture includes a "bed of nails" comprised of an array of spring- loaded electrical contact pins which are brought into contact with the printed circuit board at selected locations so that functional or diagnostic tests can be carried out.

A number of different tests can be carried out either on the board as a whole, or on selected components mounted on the board. Most commonly, the tests which are performed include a full functional test and a full in-circuit test (ICT) or manufacturing defect analysis (MDA).

Rather than change the "bed of nails" between tests, it is known to utilise a dual-stage test fixture in which the "bed of nails" includes "high" spring-loaded contact pins and "low" spring loaded contact pins. Thus, it is possible to move the "bed of nails" towards the PCB (or vice versa) in two stages such that, at the first stage, the "high" pins contact the PCB and a full functional test can be performed, and thereafter the "bed of nails" and the PCB can converge further so that, at the second stage, the "low" contact pins also contact the PCB and an ICT or MDA can be performed. A 3-stage test, for example, could be performed if the contact pins were

"high", "intermediate" and "low".

It will be appreciated that a dual-stage test fixture requires accurate and robust actuators to position the "bed of nails" relative to the PCB (or vice versa) to enable performance of both the first and second tests. Whilst relative positioning of the PCB and "bed of nails" could be achieved by electromechanical or other actuators, for example, it is conventional for the actuator to be a pneumatic single-acting ram.

Of course, a serious drawback with pneumatic actuators is that they can only be "on" or "off", with no ability to produce an intermediate position which would allow a dual-stage function.

To overcome this limitation of pneumatic actuators, it is known to utilise a transversely directed blocking pneumatic ram which can selectively be operated to block the travel of the pneumatic actuator at the intermediate position or first-stage. After the first test has been performed, the pneumatic actuator and blocking ram are both turned "off" such that both return to their

"off" position under spring bias. Subsequently, the pneumatic actuator is turned "on" again so that it proceeds to the second-stage test unhindered in the absence of the transverse blocking ram. DISCLOSURE OF INVENTION

According to one aspect the invention resides in a multi-stage test fixture including:- means for mounting a unit under test; and an array of spring-loaded contact pins of varying height for selectively contacting the unit under test; and means for generating multi-stage relative movement between the unit under test and the array of spring-loaded contact pins, said means for generating multi-stage relative movement including a pair of actuators in series.

In the preferred embodiments the actuators are piggybacked in back-to- back relationship.

BRIEF DESCRIPTION OF DRAWINGS

Reference will now be made to the accompanying Figures which illustrate preferred embodiment(s) of the invention and in which:-

FIG 1 is a series of three sequential views of a first embodiment of the invention;

FIG 2 is a series of three sequential views of a second embodiment of the invention.

BEST MODE With reference firstly to Figure 1 there is illustrated a first embodiment of a dual stage test fixture according to the invention.

A pair of double acting pneumatic rams 10, 12 are arranged in piggyback (back-to-back) relation between support point 30 and probe plate 40 which carries an array of upwardly directed spring-loaded contact pins (not illustrated). In alternative embodiments, the rams may be arranged in front-to- back or front-to-front relation.

The piggy-backed pneumatic rams 10, 12 are connected to probe plate 40 via a linkage mechanism which will now be described. The piston associated with pneumatic ram 12 is pivotally connected to a first link member 14. The other end of first link member 14 is pivotally attached relative to the support at pivot point 16. Also pivotally supported at 16 is a second link member 18 which rotates about 16 in unison with first link member 14. Second link member 18 is also pivotally connected to a third link member 20 at connection point 22.

Third link member 20 is pivotally connected relative to the probe plate at 24. As discussed previously, probe plate 40 carries the upwardly directed array of contact pins. The operation of the dual stage test fixture will now be described. As shown in Figure 1A the probe plate is shown at a height Zi above the datum defined by the fixed support.

In Figure 1 B pneumatic ram 12 has been actuated with the effect being that the piston associated with pneumatic actuator 12 has extended by a distance Xi with the resultant rotation of the first and second link members about 16 and lifting of the probe plate 40 by a distance Z₂ as shown.

In Figure 1 C the second pneumatic actuator 10 has been actuated resulting in an extension of the piston associated with actuator 10 by a distance X₂ which has resulted in a further lifting of the probe plate by a distance Z3 as shown.

It will be understood that in the position shown in Figure 1 A the upwardly directed array of contact pins do not contact the printed circuit board which is supported above the probe plate. However, in the position shown in Figure 1 B the "high" or longer contact pins mounted in the probe plate do contact the printed circuit board and a full functional test can be conducted.

Thereafter, in the configuration shown in Figure 1 C both the "high" and

"low" spring-loaded contact pins contact the printed circuit board thereby enabling a MDA or ICT.

Subsequent to completion of the MDA/ICT test, both actuators 10 and 12 are turned "off" with the test fixture returning to the orientation shown in Figure 1A. The return may occur under gravity or may occur due to spring bias or may occur positively via the use of double-acting rams.

The above described embodiment provides significant advantages over the prior art. Firstly, the present invention avoids the need to retreat to the initial position before advancing to the second stage position. Rather, the present invention can advance directly from the first stage to the second stage without any retreat to the initial position. Thus, the present invention provides efficiencies over the prior art.

It should also be noted that the contact pins or probes have finite life which is defined in compression/expansion cycles. With the prior art, the probes completed two compression/expansion cycles during a dual-stage test due to the need to retreat to the initial position between tests. However, with the present invention the probes only go through a single compression/expansion cycle thereby effectively doubling the usable life of the probes or contact pins.

Referring now to Figure 2 a second embodiment of the invention is shown in which the printed circuit board is moved rather than moving the array of contact pins as was the case in the first embodiment.

This second embodiment has advantages over the first embodiment in that, in practice, it is easier and simpler to move the unit under test (ie. the PCB) into contact with the test pins rather than moving the test pins into contact with the unit under test. In this regard, it will be appreciated that electrical connections are associated with the test pins and therefore it is desirable to keep the array of test pins immobile whilst the unit under test is moved. In the embodiment shown in Figure 2, the unit under test is mounted on the down-holding plate 50 and then down-holding plate 50 is brought down into contact with the upwardly-directed contact pins (not illustrated).

The actuators and linkage mechanisms are substantially identical to the first embodiment and will not be described again. The test fixture is initially as shown in Figure 1 A with both actuators "off".

Thereafter, the test fixture is moved to a first stage testing position in which actuator 12 is turned "on" as shown in Fig 2B.

Thereafter, the test fixture is moved to a second testing position at which the actuator 10 is also turned "on" as shown in Fig 2C. Thus, the unit under test (PCB) is moved from a first position at which it is located at height Zi above the datum defined by the support. It is then moved to a first stage testing position at which it is at height Zi - Z₂ and, thereafter, it is moved to a second stage testing position at which the unit under test is located at a height Zi - Z₂ - Z3 above the datum. It will, of course, be realised that the above has been given by way of illustrative example(s) of the invention. Any variations, modifications, or omissions, as would be apparent to persons skilled in the art, are deemed to fall within the broad scope of this invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

1. A multi-stage test fixture including:- means for mounting a unit under test; and an array of spring-loaded contact pins of varying height for selectively contacting the unit under test; and means for generating multi-stage relative movement between the unit under test and the array of spring-loaded contact pins, said means for generating multi-stage relative movement including a pair of actuators in series.

2. A test fixture as claimed in claim 1 , wherein the actuators are piggybacked in back-to-back relationship, back-to-front relationship or front-to-front relationship.

3. A test fixture as claimed in claim 1 or 2, wherein the actuators are mounted intermediate a support and the means for mounting a unit under test or the array of spring-loaded contact pins.

4. A test fixture as claimed in claim 1 , 2 or 3, wherein the actuators are pneumatic rams.

5. A test fixture as claimed in claim 4, wherein the pneumatic rams are double- acting.