Test fixture for testing backplanes or populated circuit boards

Details Test fixture for testing backplanes or populated circuit boards Test fixture for testing backplanes or populated circuit boards

1998-12-08

2001-02-27

Brown, Glenn W. (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

06194908

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the testing of printed circuit boards. The invention has particular utility in connection with determined test fixtures of the type having test probes on a grid pattern, in which a translator pin fixture is used for translating electrical current from an off-grid pattern on a board under test to the channels of a tester in which the channel contacts are arranged in a grid pattern. An especially particular utility for the present invention is found in the area of facilitating electrical testing of electrical connectors mounted on a circuit board (commonly referred to as “backplane” or “mid-plane”). These connectors typically are surrounded by a sidewall extending beyond the ends of the electrical contact posts. The invention includes a large area probe plate having a plurality of spring probes fastened to the top plate of the translator fixture to provide compliant parallel translation of test signals between the connectors on the backplane and the translator pins of the translator fixture.
BRIEF DESCRIPTION OF RELATED PRIOR ART
Automatic test equipment for checking printed circuit boards has long involved the use of “bed of nails” test fixtures on which the circuit board in mounted during testing. A typical test fixture includes a large number of nail-like test probes arranged to make electrical contact between spring loaded contacts in the test equipment and 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, arrangement of test probes for contacting test points on the board must be customized in a test fixture for that particular circuit board. Board design and fabrication data is used to determine what specific board features are to be tested by the fixture. A “determined” grid test fixture is typically fabricated by drilling patterns of holes in several rigid and non-conducting plates, e.g. of Lexan®, assembling those plates with suitable fasteners and spacers to maintain said plates in a parallel, aligned position and then mounting test pins or probes in the drilled holes. Each plate has a hole pattern which is unique such that the test pin can only be inserted to provide an xy and z translation between a unique feature on the UUT and a unique tester grid channel. The circuit board is then positioned in the fixture precisely aligned with the array of test probes. During testing, the pins in the fixture are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred between the board and the tester through the fixture so that a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board can perform the actual test.
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 or a “dedicated” 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.
One method traditionally used to connect the electronic channels of a tester to a backplane of a unit under test, is the dedicated fixture which consists of a collection of spring loaded contacts arranged in a drilled probe plate to match the pattern of test points on the unit under test. The spring probes are wired to transfer points or connectors which plug in, or otherwise make contact to, the channels of the tester. An advantage of a dedicated test fixture is it permits testing of non-uniform patterns of contacts, including different height contacts by using varying length spring probes. Dedicated testers can be configured to have an appropriate number of channels for the type of product to be tested so that the channels are generally used quite efficiently. There are a number of disadvantages, however, to dedicated testers, the most notable of which is cost. The complexity of wiring dedicated fixtures, together with the relatively high material cost of spring loaded contacts makes the dedicated arrangement extremely expensive for large, high point count backplanes. Testing a backplane for opens and shorts requires that all nodes of every net be contacted. This is in contrast to dedicated fixtures for (loaded board test), and also known as in-circuit or functional testing, which requires only one test point per net. A backplane might have ten thousand test points for opens and shorts testing, and only one thousand nets. Further, revision changes in the unit under test cannot be easily accommodated and the serviceability and reliability of dedicated fixtures is somewhat lower than determined translator grid fixtures.
A further class of test fixtures is the so-called “grid-type fixture” test fixture, also known as a “determined” fixture, in which the random pattern of test points on the board are contacted by translator pins which transfer test signals to spring loaded interface pins arranged in a grid pattern in the tester. In these grid-type testers, fixturing is generally less complex and can be produced at a lower cost than in the customized wired test fixtures; but with a grid system, the grid interfaces and test electronics are substantially more complex and costly.
A typical grid tester may have thousands of switches and channels. Each channel may have several switches, and is addressable and serves as one coordinate in the “grid”. The tester usually has spring-loaded contacts which comprise the grid. The fixture usually contains rigid translator pins which conduct current from the grid channels to the UUT. In this way, the tester's computer can be made to test continuity and isolation in the UUT through the fixture. When testing a bare board on such a tester, a translator fixture supports and guides rigid pins that conduct between a grid pattern of spring-loaded probes in a grid base and an 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.
Determined fixtures, also known as translator fixtures for a universal grid tester, offer a lower cost alternative which is suitable for many backplane testing requirements. Translator fixtures, however, are less commonly used for testing backplanes than dedicated fixtures mainly because the software requirements are unique and beyond the capabilities of general purpose test fixture software. Translator fixtures usually employ rigid pins, often with a cup-shaped tip, for testing backplanes. Translator fixtures usually rely on the spring loaded contact of the grid to provide the contact force needed for a reliable electrical connection. As a result, any difference in the contact height on the unit under test, must be matched by a corresponding lengthening or shortening of the rigid pin contact between the grid and the unit under test.
Translator fixtures offer a number of advantages when they can be employed for testing backplanes. Given the much lower lab

Affiliated with

Also associated with

Rating

Test fixture for testing backplanes or populated circuit boards does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Test fixture for testing backplanes or populated circuit boards, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Test fixture for testing backplanes or populated circuit boards will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2559346