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Design007-June2019

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JUNE 2019 I DESIGN007 MAGAZINE 19 Shaughnessy: In the designers' defense, they would probably say, "We don't know where it's going to be built or whether it's going to be built in Asia." Is it up to them to try and find out where it's going to be built, if it's going to volume, and then get in touch with those man- ufacturers? Ellis: Absolutely. In the last year, I've had to convert several designs that could have been simple through-hole designs, but they worked with North American suppliers who must have said, "Yes, we can drill a 5-mil mechanical drill with a 10-mil pad on a 0.62" board." That is greater than 10:1 aspect ratio; it's using a 5-mil bit that's not even common for prototype in America, much less in mass production over in China. The poor customers think that they can use very small pads with small drills on through-holes and transfer it to China, but no- body in China can do it. I may seem overly regimented, but I tell my customers that I'm an equal opportunity—and conservative—designer. With the rules I give them for standard design guidelines, if they're using a halfway decent supplier, they will be able to fabricate their design at many loca- tions, including our competitors. Sometimes, you give away information that's going to help somebody else. But if you don't train your de- signers correctly in the first place, you're going to be battling designs that you cornered into one factory, and then it's going to be hard for everybody to maneuver, move forward, and grow into higher volumes. I recommend fair design rules for standard production, design mil because of the small distance between the pads. Most fabricators don't have the etching capability to do that, and we have to be careful to follow the design guidelines of the fabrica- tors that can support the small lines. Shaughnessy: What sort of mistakes do you see when the design moves to volume production overseas? Ellis: Here's a recent list of common design er- rors for jobs going into mass production: • Multiple semiconductor reference designs using a 6-mil drill and 10-mil or 11-mil pad as a through-hole to route a 0.5-mm pitch BGA. A multi-row 0.5mm pitch BGA requires layers of microvias, because we only use mechanical drills, not lasers, for through-holes, and mechanical drills have larger tolerances than laser drills. So with a minimum standard pad of 16mil (= 8-mil drill + 8-mil to maintain Class 2 annular ring), mechanical drill pads are too large for fine-pitch BGA routing. And laser pads, at laser drill diameter plus min 6-mil = 10-mil min, are way too small for mechanical drills. • Designs that are prototyped in North America with 6-mil drills transferring to China, where the vast majority of fabri- cators can handle a minimum 8-mil drill size. Check your fabricator's min drill size before routing. • A circuit board was designed, fabbed, and assembled before anyone realized there were no access pads for functional or in-circuit test anywhere. • A 6-layer stack-up with 6-mil drills on 10-mil pads going from L1 to L4 and L6 – L3 with dielectrics that are too thick to use stacked microvias and maintain a max aspect ratio of 0.75:1. • Another semiconductor reference design had 3mil lines and spaces on an outer through-hole layer with VIPPO—via- in-pad (filled) and plated over—which requires and extra plating process, which increases the copper thickness before etch. The poor customers think that they can use very small pads with small drills on through-holes and transfer it to China, but nobody in China can do it.

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