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PCBD-June2016

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26 The PCB Design Magazine • June 2016 Thomas: That's a somewhat "loaded" question. To answer straight out, yes. Here's the caveat: Some PCB design tools do handle very tight tolerances well and some do not. So the "right tool for the job" rule applies, i.e., expecting a free/web-based PCB tool to design an HDI board would be rather troublesome. But remember that these are "design tools." Once the "right tool for the job" is selected, then, as with any other tool, it is the applied knowledge of the user that provides the level of results and qual- ity for the design activity. Now for the fine print. The PCB tool of choice may have issues with handling data of a tight tolerance board with generating outputs for fabrication/assembly. Meaning the as-de- signed PCB tool board data is not represented correctly in the fabrication/assembly outputs. Due to rounding issues, the Gerber format/ ODB++/data output translation from the PCB tool may not have been correlated as expected. Or there may be other not-so-nice nuances of the PCB tool. Another point would be the preferred/se- lected fabricator has not been informed/con- tacted (or received their feedback) as to the specific tight tolerance implementation. This means that the tight design constraint "as de- signed/specified" in the PCB tool could cause a resultant of fabrication issues. Shaughnessy: With finer features, how do you de- termine whether it is cost-effective to use HDI? Thomas: First, start by asking, "Is HDI the right technology application/approach? Or can oth- er PCB technology be used to accomplish the same as HDI with lower cost, while still meet- ing requirements?" Second, what form would the implementation of HDI take in this particu- lar application (i.e., using stacked vias, buildup layers, via-in-pad, laser/plasma/photo drilling vias, plating vias closed)? If these listed factors are negligible, then HDI would likely be a great design technology to utilize. It should also be noted that HDI may not be used for some cases and types of high-reliability designs. Customer specifications may also call out "No HDI." Shaughnessy: Do you work with ultra-fine-pitch BGAs? If so, do you have any special techniques for doing this type of fanouts? Thomas: Yes. Design techniques ensure the PCB footprint and fanout are correct. You have to take into account signal integrity, and getting the signal on and off the device with minimal impact. Refer to the device manufacturers' rec- ommendations (electrical and mechanical), fab- rication tolerances and assembly methodolo- gies. For a bit more detail, always confirm that the fanout has been routed with PCB fabrica- tion tolerances in mind. It might be a good idea to run the design by your fabricator to get their recommendations. Shaughnessy: What do you think is the most im- portant thing to remember when facing tight toler- ances? Thomas: I believe there are a multitude of items to be aware of when doing tight tolerance de- signs. First would be to meet signal integrity. Second, ensure the design is workable at the fabricator and assembly houses. Third, confirm customer requirements are met. Shaughnessy: Thanks for speaking with us, David. Thomas: Thank you, Andy. PCBDESIGN DESIGNING PCBS WITH TIGHTER TOLERANCES David Ledger-Thomas

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