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JANUARY 2022 I DESIGN007 MAGAZINE 35 finished circuit board and digital file for the fol- lowing items: • Electrical schematic • Mechanical detail • Layer cross-section • Net list • CAD data (Gerber file) e following guidelines have been furnished by Aspen Test Engineering, a prominent test fixture development specialist in Colorado: 1. Provide test probe accessibility to all nets from the bottom surface of the board. When probe locations are distributed on opposite sides of the board, complexity of the fixture "introduces an entirely new tolerance stack-up and will essentially doubles the cost of fixture development." 2. In regard to test probe land size, a 0.030" (~0.75 mm) diameter land area will ensure reliable probe-to-land interface. When the probe lands must be smaller, the fixture may require "guided probes," further increasing the test fixture's complexity. 3. Probe lands should be "no closer than 0.050" (~1.3 mm)" however, smaller probes are available, but they are generally more delicate and flexible, resulting in probe-to-land registration issues. 4. Test land pattern proximity to components is also a concern. Preferred clearance from the component body to the center of the test land should be no less than 0.060" (~1.5 mm). This will enable access to the same probe locations during assembly test. 5. Tooling holes must be proved to enable board to fixture alignment. The require- ment is that two 0.125" (~3.2 mm) diam- eter non-plated tooling holes be provided in opposite corners of the circuit board. Other tests may be required for checking isolation between conductors—shorts test. A soware generated program that checks for nets that are within a tolerance set and imped- ance testing, measures the frequency of the current passing through conductors. Part 2 will focus on assembly test method- ologies. Low-volume assemblies, for example, may rely only on functional testing while high- volume assembly will most likely adapt some form of automated or semi-automated func- tional test, functional partitioning, or auto- mated in-circuit testing (ICT). DESIGN007 Vern Solberg is an independent technical consultant, specializ- ing in SMT and microelectronics design and manufacturing tech- nology. To read past columns or contact Solberg, click here.

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