Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-03-31
2001-01-16
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S754090, C324S755090
Reexamination Certificate
active
06175243
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to test fixtures used in the process of testing and verifying the operation printed circuit boards, and more specifically to an improved test fixture and an improved apparatus and method for assembling test fixtures.
BACKGROUND AND SUMMARY OF THE INVENTION
Grid test fixtures, also commonly known as “grid translator fixtures,” are used in conjunction with automated, computer-based testing equipment to test the functioning of printed circuit boards. In the process of testing the circuit boards, the test fixture serves as a framework structure that facilitates the establishment of an electrical contact between test points on the circuit board being tested on the one hand, and the testing equipment on the other hand. The number of circuits to be tested on any given circuit board can be quite large, numbering in the tens of thousands, and there typically is a switch for each individual test point. During the testing process the test equipment transfers test signals to selected circuits on the circuit board that is being tested, and a pass, no pass result is obtained. In this way the proper functioning of a circuit board can be quickly checked and verified.
Test fixtures of this type typically include a series of parallel, spaced apart plates, each having a plurality of test probe holes drilled therethrough in a predetermined specific pattern that corresponds to the pattern of test points on the circuit board to be tested. The plates are typically manufactured of a plastic material such as Lexan, G
10
or FR
4
. Test probes, also called “test pins” or “translator pins” extend through the test probe holes in the test fixture plates. The test probes are generally spring-loaded and are used to establish electrical contact between test points on the circuit board on one side of the test fixture, and switches interconnecting the probes to the test analyzer on the opposite side of the test fixture. Because the array pattern of the test points on the circuit board is different from the array pattern of the test analyzer, many of the test probe holes drilled through any one plate in the test fixture will be in a slightly different position from the corresponding test probe holes drilled through the next adjacent plate. This results in the test probes being arranged in an image pattern on the circuit board side of the test fixture, and a gridded pattern on the opposite side of the test fixture. Many of the test pins are thus tilted in the test fixture in an angular orientation relative to the plane of the plates such that the pins are inclined at oblique angles relative to the plate plane. This has led to the test pins sometimes being called “tilt pins.” Given the many tens of thousands of test probes that may be included in a test fixture, the positioning of the test probe holes drilled through the plates must be precisely controlled. This is typically done with sophisticated computer controlled drilling equipment. But it is also critical that the test fixture is assembled in a controlled and precise manner to minimize any errors in the alignment of the plates. Thus, the plates must be properly oriented with respect to one another so that the test probe holes from one plate to the next are precisely aligned so that the test probes correctly fit through the holes.
In the testing process the circuit board that is to be tested (i.e., the board under test) is brought into physical contact with the grid test fixture such that the ends of the test pins, which extend beyond the upper plate in the test fixture, are brought into physical contact with associated test points on the board under test. The tip of each test pin contacts, or “probes” a specific associated test point on the board. The spring probes of the test equipment, which as noted are in a regular grid array, are electrically interfaced with the test pins. As well known in the art, and as noted above, the test probes are resilient, typically with spring loaded tube-in-tube arrangements. Typically the board under test is sandwiched between a pair of grid test fixtures, one probing each side of the board, so that both sides of the board are tested. As the test equipment is brought into contact with the test probes a compressive force is applied to the test probes, compressing each probe so that each makes positive contact with the test point (through the test pins) on the board under test that is associated with the probe. The compressive force applied to the test fixtures insures good electrical interconnectivity between points on the test machines, through the test probes and the test pins to the associated test points on the board under test.
The testing equipment may apply a substantial compressive force to the test fixtures, and thus on the board under test. With some test equipment up to 8 tons of hydraulic force is applied to the test probes. While a substantial compressive force helps ensure good electrical interconnectivity between the test machine and the board under test, it also can cause significant problems. One such problem occurs when the compressive force causes the plates in the test fixtures to be forced out of their ideally aligned positions relative to one another. As noted above, many of the test pins in the test fixtures are oriented in tilted positions relative to the plane of the plates in the test fixture. In other words, many of the pins are at oblique angles with respect to the plane defined by the parallel plates in the test fixture. Occasionally the orientation of test pins (which is dictated by the positioning of the test points on the board under test) calls for many of the test pins to tilt in the same general direction. When a substantially greater number of test pins tilt in one direction over another, the ideal alignment of the test plates in the fixture can be “skewed” when the test fixture is compressed in the test equipment. In other words, under compressive force the plates in the test fixture move relative to one another, shifting them out of their original, aligned position. This skewing is caused by the uneven forces applied to the fixture as a result of the large number of pins tilting in one general direction.
Test fixture skewing causes several problems, including test pin binding, and possible faulty electrical probing due to misaligned test pins. Significant skewing can also cause substantial problems for the hydraulic rams that compress the test fixtures, resulting in expensive repair problems.
In one traditional method of assembling grid test fixtures, the test fixture is assembled with a series of posts spaced around the periphery of the fixture that secure and separate the plates. The posts are constructed of a series of plastic or metal spacers that are inserted between the plates. The spacers separate the plates and hold them in a parallel array. Hollow rods are inserted through bores through the spacers to hold the spacers in the proper orientation. As described above, test fixtures are assembled so that they can be inverted. This allows both sides of the circuit board to be tested. As such, with such traditional assembly arrangements, long connectors such as threaded bolts are typically inserted through the rods in order to hold the entire test fixture together in a fixed position. This manner of assembling test fixtures has several limitations. First, several different sizes of spacers are required because although the plates are parallel, they generally are not all evenly spaced from one another. This results in an increased number of parts that must be kept in inventory. Further, variances in the thickness of the spacers and the plates as a result of manufacturing tolerances for those parts can lead to misalignment of the plates when the test fixture is assembled. Since in any test fixture there are multiple plates, the cumulative effect of size variances in the spacers and the plates can lead to the plates being assembled in a non-parallel orientation. This in turn can lead to misalignment of the test probe holes between plates. Fin
Gordon Jon
Kocher Douglas
AQL Manufacturing Services, Inc.
Ipsolon LLP
Metjahic Safet
Sundaram T. R.
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