Test fixture for matched impedance testing

Details Test fixture for matched impedance testing Test fixture for matched impedance testing

1999-02-22

2001-02-20

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S754090

Reexamination Certificate

active

06191601

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the automatic testing of printed circuit boards, and more particularly, to a test fixture for transmission of test signals from a board under test to a tester wherein the test fixture includes a printed circuit board with matched impedance traces as a top plate for testing of high frequency sites on the board under test.
BACKGROUND OF THE INVENTION
Automatic test equipment for checking printed circuit boards has long involved used of a “bed of nails” test fixture in which the circuit board is mounted during testing. This test fixture includes a large number or nail-like spring-loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test, also referred to as the unit under test or “UUT.” Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed of nails arrangement for contacting test points in the board must be customized for that particular circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture superimposed on the array of test probes. During testing, the spring-loaded probes are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board.
Various approaches have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a “wired test fixture” in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures are often referred to as “vacuum test fixtures” since a vacuum is applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. Customized wired test fixtures of similar construction also can be made by using mechanical means other than vacuum to apply the spring force necessary for compressing the board into contact with the probes during testing.
The wire-wrapping or other connection of test probes, interface pins and transfer pins for use in a wired test fixture can be time intensive. However, customized wired test fixtures are particularly useful in testing circuit boards with complex arrangements of test points and low-volume production boards where larger and more complex and expensive electronic test analyzers are not practical.
As mentioned previously, the customized wired test fixtures are one class of fixtures for transmitting signals from the fixture to the external circuit tester. A further class of test fixtures is the so-called “dedicated” test fixtures, also known as a “grid-type fixture,” in which the random pattern of test points on the board are contacted by translator pins which transfer test signals to interface pins arranged in a grid pattern in a receiver. In these grid-type testers, fixturing is generally less complex and simpler than in the customized wired test fixtures.
A typical dedicated or grid fixture contains test electronics with a huge number of switches connecting test probes in a grid base to corresponding test circuits in the electronic test analyzer. In one embodiment of a grid tester as many as 40,000 switches are used. When testing a bare board on such a tester, a translator fixture supports translator pins that communicate between a grid pattern of test probes in a grid base and off-grid pattern of test points on the board under test. In one prior art grid fixture so-called “tilt pins” are used as the translator pins. The tilt pins are straight solid pins mounted in corresponding pre-drilled holes in translator plates which are part of the translator fixture. The tilt pins can tilt in various orientations to translate separate test signals from the off-grid random pattern of test points on the board to the grid pattern of test probes in the grid base.
In the past, translator fixtures have been constructed and assembled with a plurality of translator plates made from a plastic material such as Lexan. The translator plates are stacked in the fixture between corresponding sets of spacers aligned with one another vertically to form “stand-offs” spaced apart around the periphery of the fixture. The spacers hold the translator plates in a fixed position spaced apart vertically from one another and reasonably parallel to each other. The translator plates at each level of the fixture have pre-drilled patterns of alignment holes that control the position of each tilt pin in the translator fixture.
Testing match impedance test points on a circuit board requires contacting the test points by test probes and analyzing the test signals with a time domain reflectometer. Previous TDR technology consisted of separate hand held probes consisting of a signal probe and a ground probe which were manually operated to contact the desired test locations. When multiple test locations exist for testing, manually testing each site is an inefficient and time consuming task. Consequently, more automated TDR configurations were developed consisting of an x-y prober wherein only one of the prober heads was used and was modified such that the signal wire and the ground shield of the TDR cable extended to the tip of one of the heads of the x-y prober. This single head was designed to be adjustable and rotatable for different test pad spacings. Incorporating a rotatable and adjustable tip on a single prober head is a very expensive and complicated device for testing matched impedance test points. Consequently, a need exists for a TDR test fixture capable of testing matched impedance test points that addresses the problems of the prior art and is easy and inexpensive to manufacture.
SUMMARY OF THE INVENTION
Briefly, one embodiment of this invention comprises a translator fixture for a printed circuit board tester of the type having a pattern of test probes on a base upon which the translator fixture is mounted. The translator fixture comprises a plurality of essentially parallel and vertically spaced apart rigid translator plates having selected patterns of pre-drilled holes for supporting translator pins for contacting test points on a printed circuit board supported on one side of the translator fixture. The translator pins translate electrical test signals between test points on the printed circuit board and the test probes at the base of the tester. The translator fixture includes a plurality of translator plate stacking towers of identical construction for supporting the translator plates in their fixed position in the translator fixture. Each stacking tower comprises a rigid support member formed as an integral piece having vertically spaced apart translator plate support surfaces with corresponding upright alignment posts at spaced apart levels of the stacking tower. The translator plate support surfaces and their corresponding alignment posts having respective diameters at each level which are progressively shorter along the length of the tower to define a stair-step arrangement. The different translator plates are assembled onto the stacking towers at predetermined levels in a progressive stacking sequence, so that each translator plate with its specific pattern of pre-drilled holes is supported by a corresponding translator plate support surface and retained thereon in a fi

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