To build a reliable test fixture for these boards, and do it in reasonable time at a reasonable cost, fixture manufacturers need the CAD data files for the boards. These files contain the electrical and mechanical details of a PCB assembly. With this data, fixture manufacturers can automatically lay out, drill, wire and test a custom test fixture.

The quality of design data varies from poor to excellent. A poor package would consist of a 1:1 artwork, a manually drawn node map, and a handwritten wire list. While a fixture manufacturer can build a fixture from this information, it will take several days. There is also opportunity for error as the fixture manufacturer has to enter the data.

A better package consists of the PCB assembly's CAD-data files and tester output files. The fixture manufacturer will merge the CAD data with the tester data. Using CAD data eliminates the errors made in entering design data by hand.

Unfortunately, CAD data comes in a variety of database formats, and each CAD system has its own proprietary data structure. Usually, the data is in binary format and is difficult to interpret. From the CAD data, the fixture manufacturer must extract a set of ASCII files containing the needed mechanical and electrical information. Most CAD systems provide a way to generate these ASCII files. If your system doesn't have this feature, the fixture manufacturer may have to develop translators. These translators reformat the data into files the manufacturer can use.

ATE Vendors Supply Best Tools
The best package comes from test development tools supplied by ATE vendors. These tools process the PCB assembly CAD data and produce output files containing the information necessary to build a fixture. As shown in Figure 1, the output files include x-y coordinates of drill locations and pin locations, as well as wiring information. This information helps fixture manufacturers produce a fixture that will work well on your tester. Often, fixture builders can use this information without translating it.

Once the information is in an useable format, fixture builders input it to their own CAD systems. These systems then produce the fixture design documentation: mechanical layout, wire-wrap, gasketing, insertion, and electrical-test files.

Fixture builders will feed some of this information directly into machines that they use to build the fixtures. For example, drilling information may go directly into a drilling machine to drill holes for the test probes. Because the board manufacturer and the fixture manufacturer are working from the same set of CAD data, the holes in the fixture will more closely align with the targets on the board. This means that test probes are more likely to hit small targets.

Supplying the right CAD data helps both you and the fixture manufacturer. The right information helps the manufacturer reduce data-input errors, mechanical errors, and wiring errors. The result is an accurate fixture with a short delivery time at the lowest possible cost.