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32 DESIGN007 MAGAZINE I JANUARY 2022 Circuit board fabricators remind us that multilayer boards will predictably have more components necessitating greater circuit rout- ing complexities than that experienced on ear- lier applications. Also, with each generation of semiconductors it seems that the terminal count increases and the spacing between ter- minals shrinks, requiring designers to employ conductor lines and spaces that are far nar- rower than previously considered the norm. Designing the circuit board for testability is a key requirement in meeting the overall design for manufacturability (DFM) criteria. roughout the circuit board development process, the designer must contemplate the essential features required for electrical test: testing to identify PCB fabrication or assembly defects. Design for Bare Board Testing e circuit board is the primary platform for electronic assembly interconnect. While there are several stages within the circuit board's fabrication process that receive meticulous inspection, the complexity of the circuit struc- tures is increasing. Some may think that they can assess the finished printed circuit board using automated optical inspection (AOI) or magnified visual inspection, but they oen find these non-contact methods limited. Except for low-density single- and two-layer circuits, multilayer circuit boards, and especially the high-density variations, will require a more sophisticated end-product verification. Certi- fying that the finished board is defect free and will operate as designed, electrical bare board testing is essential. Design for Test, Part 1 Designers Notebook by Vern Solberg, CONSULTANT

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