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FEBRUARY 2022 I DESIGN007 MAGAZINE 55 Probe Designs and Test Pad Size e minimum surface area provided for probe interface is influenced by the align- ment accuracy of the test fixture to the circuit board's test sites and the probe contactor design. e test probes furnished for in-circuit test are designed to apply uniform contact pressure at all locations. ese spring-loaded contactors have a broad range of tip designs to access even the most confined test pad loca- tions (Figure 4). Test fixture providers note these variables are not usually considered during the initial quoting process and can impact both fixture development time and cost. Basic ICT Requirements • One test node per net • Probe spacing of 2.0 mm (0.080") preferred • Probe spacing of 1.2 mm (0.050") acceptable 1 • Probe land on PCB should be > 0.3 mm (.012") • All nodal access from one side preferred • Double side test probe access may be required 2 • Provide 1.2 mm (0.050") probe body to device body clearance To enable the in-circuit text fixture develop- ers to prepare the platform for accessing each net of the circuit they will need the schematic diagram for the product, a net list, and the bill of materials defining the components refer- ence designator, as well as each component's physical attributes. e CAD files will deter- mine the location and orientation of each com- ponent and the precise X-Y coordinates for test probe access. In part three, the focus will be on boundary- scan testing, an integrated method for testing interconnects on populated printed circuit boards. An extremely rapid test method, it enables both in-circuit test coverage and sys- tem diagnostics. DESIGN007 References 1. The smaller needle probes are fragile and bend easily. 2. Double-sided text fixtures are typically three times in cost. Vern Solberg is an indepen- dent technical consultant, spe- cializing in SMT and microelec- tronics design and manufactur- ing technology. To read past columns or contact Solberg, click here. Figure 4: Test probe design variations.

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