Dual stage in-circuit test fixture for circuit board testing

Details Dual stage in-circuit test fixture for circuit board testing Dual stage in-circuit test fixture for circuit board testing

2001-08-08

2003-03-18

Le, N. (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

Reexamination Certificate

active

06535008

ABSTRACT:

The present invention relates to a fixture which positions a circuit board with respect to a plurality of probes of a circuit board tester and in particular to a fixture for dual stage testing using probes of two or more different heights or lengths.
BACKGROUND INFORMATION
Circuit board testers are used for testing a variety of circuit boards or similar devices to assure that the circuit boards operate as intended. In at least one type of circuit board tester, such as Hewlett Packard Model No. 3070, Series 3, a separate device, referred to as a fixture, is used to position the circuit board such that a plurality of electrically conductive probes (which are part of, or coupled to, the tester) contact predetermined components or positions of the circuit board. The particular components or positions which are contacted by the test or probes depend on the tests which are desired. When the probes are in contact with the desired locations on the circuit board, electrical signals with predetermined parameters (e.g., predetermined magnitudes or patterns of current, voltage frequency, phase and the like are applied, by the tester, typically under control of a computer, to certain of the probes. Some or all of the probes are used to measure the performance or response of the circuit board (i.e., to measure electrical parameters of at some or all of the probes contacting the circuit board). In this way, it is possible to rapidly perform a number of tests or measurements characterizing the performance of the circuit board while simulating the conditions the circuit board would have, or could have, during actual use. Although it is possible to use these types of tests (and testing devices) for a variety of possible purposes (such as “spot checking” selected circuit boards at a production facility, testing circuit boards which may be malfunctioning, testing prototype circuit boards as part of a design program and the like), in at least some applications, circuit board testing is used to provide quality assurance on all or substantially all products of a given type or class which are produced by a company. Even with the relatively rapid test procedures which can be achieved by in circuit testing, it is not unusual for desired testing of each circuit board to require on the order of 30 seconds to 90 seconds or more.
Because, in at least some applications, circuit board testing is performed on substantially all devices on a production line or production facility, speed and reliability of testing can be especially important since delay or failure at a testing station can delay or interrupt the overall production in a production line or facility. Accordingly, it would be useful to provide a fixture, useable in connection with in-circuit testers, which provides desired speed of positioning the circuit board or other unit under test (UUT) and which achieves a relatively high degree of reliability, e.g., so as to avoid interrupting or delaying production rates at a production line or facility.
The effect of such testing on overall production rates is at least partially related to the rate at which each UUT can be placed in the fixture and the rate at which the fixture can accurately and reliably move the UUT to the desired position or positions. In at least some situations, it is desired to provide a tester with probes at two or more levels (with respect to a direction normal to the plane of the UUT) e.g., by providing some probes having a first height and other probes having a second height. This arrangement affords the opportunity to perform two or more different sets of tests such that the points at which probes contact the UUT during one set of tests are different from (or a subset of) the points at which probes contact the UUT during another set of tests. Typically, in such a “dual stage” testing situation, the UUT is first positioned so as to contact all probes (and perform a first set of tests), and then positioned to contact only the taller set of probes (at points of the UUT which are determined by the location of the tall probes) and a second set of tests are performed using only the taller probes. Although many different testing procedures can be used, as will be understood by those of skill in the art, in at least some situations, the taller probes may be used for functional tests and/or boundary scan tests (such as boundary scan tests as described in IEEE Standard No. 1149.1).
In at least one previous approach, the circuit board is moved in the direction of the probes, typically causing the taller probes, which may be provided with a spring-urged telescoping structure, to partially collapse or telescope, down to the level of the smaller probes, such that substantially both sets of probes (the taller probes and the shorter probes) contact the UUT at desired positions. With the board held in this position, a first set of tests (such as functional tests and/or boundary scan tests) can be performed. After tests are performed using the full set of probes the vacuum is released such that the UUT is positioned to contact only the taller probes (which telescope upwardly) and a second set of tests, (such as tests directed to measuring performance or characteristics of individual components on the UUT) can be performed.
In order for fixtures used in dual stage testing to operate well, especially in the context of a production line or facility, it is also desirable to avoid delays or malfunctions in moving the UUT between the stages. Accordingly, it would be desirable to provide a fixture, useable connection with dual stage in-circuit testing, with a relatively high degree of reliability and operating speed.
In at least one type of fixture, the force of atmospheric pressure is used to move the UUT towards the probes, e.g., by drawing a (partial) vacuum in a sealed area above or near the probes. In these devices, in order to accommodate dual stage testing, such fixtures have, in the past, been provided with a shuttle plate positioned in the area somewhere above the probes and defining one or more standoff structures which engage or contact a surface of the fixture (or of the UUT) to limit the amount of downward movement that the vacuum can effect on the UUT. In this way, the shuttle plate, in a first position, can cause the UUT to be positioned so as to contact only the taller probes. After a first set of test is performed, the vacuum is at least partially released and the shuttle plate is then moved, typically laterally, such that the standoffs slide against a surface of the fixture sufficiently to become aligned with notches or other openings, allowing the (reapplied) vacuum to pull the UUT down farther, so as to contact the full set of probes (so that a second set of tests can be performed). It is also possible to perform tests with the full set of probes before performing the tests using only the taller probes. In at least one previous device, a shuttle mechanism is located in the lid structure to hold the board down onto the long probes. This device requires pneumatic cylinders, and requires an additional operator connection of compressed air lines to the fixture.
While this arrangement can achieve dual stage positioning, it has been found that such a shuttle plate approach can lead to delays or failures in testing. For example, the shuttle plate approach can provide a relatively high amount of friction when the shuttle plate is moved laterally, particularly when components of the fixture are made of a G10 or similar relatively high-friction material. This can lead to binding (inability of the shuttle plate to move smoothly to the second position or return to the first position). Such binding can not only cause delays and slow down a production line or production facility but can cause failures which may require repairs or replacement of parts, thus creating a substantial interruption. of production. Accordingly, it would be useful to provide a fixture which can achieve dual stage testing while avoiding the type of binding, delay, failure, or interruption associated with the use of a shuttle

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