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JULY 2021 I DESIGN007 MAGAZINE 75 Carl Schattke: CTE mismatch is the root cause of these defects. Metal expands at a different rate than fiberglass and resin. A shorter via stack will have less contraction and expansion than a longer one. Here are a few more reasons to use staggered vias: ere is a far wider pool of vendors that can make those boards. Also, it's less risky, as the staggered via processing does not rely on the same sintering process as the stacked via, so it is generally considered a bit more reliable. Results may vary. Lee Ritchey: e so-called "weak stacked via" is caused by the expansion of the resin in a PCB during soldering. It breaks the bond between the bottom of a via and the copper layer below it. It is a very real problem and is only solved by not stacking vias. DESIGN007 The IPC reliability committee has been studying the "weak microvia" problem in defense PCBs. They recommend using staggered microvias vs. stacked as a temporary workaround. What do you think is causing this defect? Heidi Barnes: It might be time to get out the EM simulation soware and start looking at reducing capacitors on your power delivery networks. is would also help with finding alternative less popular values with shorter lead times. Lee Ritchey: It depends on the capacitor type and size. When I select capacitors, I start with what is the most abundant in the supply chain and make my design work with them. Carl Schattke: Some distributors charge higher prices, so they maintain some stock. Part of the engineering process of choosing parts is to look at lead times and availability for those parts. e cost of ignoring this, is potentially going to be some redesign effort later to address it. You are certainly not alone in having sourcing issues for critical parts. DESIGN007 Where can we buy capacitors with less than 60 weeks lead time? Q Q

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