Universal grid interface

Details Universal grid interface Universal grid interface

1999-04-23

2001-09-18

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

Reexamination Certificate

active

06292004

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to test fixtures used in the process of testing and verifying the operation of bare and loaded printed circuit boards, and more specifically to a test fixture utilizing a removable universal interface adaptable to test boards and components having high densities and fine pitches.
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 and verify the functioning of both bare printed circuit boards and loaded boards. The testing routines ensure that the boards are electrically and physically correct, and that they meet specifications. Comprehensive board testing is necessary since loading complex and expensive components onto a partially tested or defective bare board can be very costly. For bare circuit boards the goal of the board testing and verification process is 100% fault coverage in the electrical network or net list. Since one side of a board usually has a different array of electrical contacts from the other side of the board, both sides of a bare board must be tested. Loaded circuit boards, on the other hand, do not require 100% fault testing of the net since the bare board should have already been tested, and the components that are loaded onto the board also will have been tested. However, loaded boards do require that a significant testing routing be followed to ensure compliance with standards for the board and that, for instance, all soldering has been done to specification.
The test fixtures of interest in the present invention are adapted to test both bare boards and boards that have been loaded with electrical and/or electronic components. But regardless of whether a bare or loaded board is being tested, the test fixture serves as a structural framework that facilitates the establishment of an electrical connection between test points on the circuit board being tested on the one hand, and the testing equipment on the other hand. This is done by electrically interfacing the test equipment with test points on the board through a plurality of test pins that probe specific contacts on the board, one test probe for each test point. The “test points” thus are the electrical contacts, via holes or plated through holes on a bare circuit board that require testing, or the contacts found on a board that has been loaded with components and which require testing.
As is standard in the industry, the test points on a circuit board are irregularly arranged and spaced, depending upon such factors as the physical size of the board, and the type, number and spacing of components that are to be used with the board. In contrast, the test equipment used to test the boards has test contacts arranged in a regular grid array. The spacing of the test contacts in the grid array varies according to the manufacturer and model of the test equipment. Regardless of the spacing of the regularly spaced test contacts in the grid array, the test fixture therefore serves to “translate” the irregular array of test points on the circuit board into the regular grid array of the test equipment.
The number of points to be tested on any given circuit board can be quite large, numbering in the thousands. During the testing process an electrical connection is made between the each test point on the board under test and the test equipment. The test equipment transfers signals through switched circuits to selected and predetermined test points or circuits on the circuit board that is being tested, and a pass, no pass result is obtained. In this way the proper functioning of the entire net of circuits on a board—the so-called net list—can be checked and verified. Faults in the board are also readily identified.
Grid test fixtures are uniquely designed for each specific board design. The test fixture typically includes a top plate and a bottom plate and series of parallel, spaced apart guide plates arranged in a stacked fashion between the top and bottom plates. Each plate in the test fixture has a plurality of test probe holes drilled through it in a predetermined array pattern that corresponds to the array pattern of test points on the circuit board to be tested. Again, the pattern or array of test points varies widely from board to board. For instance, one board might have tens of thousands of test points that are entirely or partially grid-oriented. Another board might be adapted for an entirely different set of components with only several hundred test points spaced irregularly around the board.
The center-to-center distance between test points in a given component is referred to as “pitch.” With some integrated circuit (IC) components the pitch is very fine. As an example, in some component packages such as ball grid array (BGA) packages and chip scale packages (CSP)—package referring to the I/O packaging for the die—the pitch may be less than 1 millimeter, and down to 0.2 mm. A bare board adapted for utilizing such a package has a component mounting location with connecting holes (i.e., vias) having the same pitch. The “density” of a package or a board refers to the number of test points per unit area in the package, or the component mounting location on the board. Current densities may be up to 900 test points in an area of 35 mm
2
for some IC packages, and the density of packages is constantly increasing. The pattern of test points in either a bare board as a whole, a component mounting location on a bare board, or in a single component is referred to herein as the “image pattern.” The image pattern is specific to the board under test or to a component. Since there are many, many different components, and of those, many different pitches and densities, the image pattern varies according to the specific component or component mounting location on a board.
In contrast to the variable image pattern of test points on the board under test, the test equipment, as noted, has a regularly arranged grid pattern of test contacts. This regular grid pattern is a result of various factors, including the fact that spring probe connections are used to interface the test machine contacts to test probes in the test fixture, and these introduce certain physical limitations on contact spacing. In addition, since the electrical connections on switch cards have certain standard dimensions, these limitations also effect the spacing of contacts on the test machine. Due to these and other factors, on most commercially available test equipment the test contacts or connecting points are provided in, for instance, a regular grid having ten contacts per inch, one hundred test contacts per square inch. Some test equipment has been built with up to double this number of test points per inch, but no equipment has a grid pattern density that approaches the density and pitch of, for example, BGA and CSP packages. Thus, the distance between adjacent test contacts found on the test equipment is nearly always greater than the pitch of components to be tested.
The test fixture facilitates an electrical interface between the test points in the image pattern of the board under test, and the regular “grid pattern” of the test contacts of the test equipment. This interface is accomplished with test probes, also called “test pins” or “translator pins,” that extend through the test probe holes drilled through the plates in the test fixture. The test probes are used to establish electrical contact between the test points on the circuit board on one side of the test fixture, and associated switches interconnecting the probes to the test equipment on the opposite side of the test fixture. Standard test probes are around 3¾ inches in length. The diameter of the probes varies according to the type and size of the test point that the probe will contact. The test probes extend completely through the test fixture. One end of the probes is connected to the test equipment and the other end probes a test point on the board under test. In this way an electrical conn

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