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SEPTEMBER 2020 I PCB007 MAGAZINE 111 Table 2 shows Motorola's results. When Mo- torola put in its sorting protocol, it showed yields from 2014–2017. These went up signif- icantly with the protocol, and they learned that there was a breakdown after three-stack or a force that showed in their results anyway. Since the time this was initially presented, Mo- torola has had no failures. Motorola has gone from preferred to unac- ceptable, and the preferred is staggered micro- vias and then two-stacks and with caution on a two-stack or three-stack construction. They re- quire a management sign-off if you need three- stack microvias. In summary, stacked microvias and even some others can fracture at a metallurgical in- terface, particularly during reflow and ther- mal cycling simulations. The failure level is not predictable with what we are doing to- day and how we understand the process. The test methods, like visual cross-section, are not effective in detecting the failures. The tech- nique duplicates reflow assembly and poten- tial problems. T-type testing is the most widely used equip- ment today. You can minimize failures with design, and our team hopes to release a white paper design guide within a year. Is there more than one cause of this? If our industry was not in containment right now, we would have dis- covered the answer already. Please understand that our team has been op- erating for over two years, working to identify the root causes. We are not particularly work- ing on product acceptance. Try to help IPC. We certainly want higher reliability and high-den- sity structures with four stacks or more. Join our working teams and contribute. PCB007 Table 2: Composite test results from years of testing microvias by reflow coupons containing a staggered chain and a stacked chain for each HDI construction type. (Source: Motorola Solutions)

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