SMT007 Magazine

SMT007-Dec2019

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26 SMT007 MAGAZINE I DECEMBER 2019 After failures related to QFNs started to pile up, a light was shone on that component and all the difficulties related to processing it. Com- ponent manufacturers continue to learn from their users, so they started offering options for crosshatch and windowpane ground pads, which help increase the standoff height from the surface of the PCB. That simple bit of engi- neering helped increase reliability and reduce field returns, which isn't to say that everyone has learned that lesson and aren't still battling the problem. There are hundreds of research papers and presentations available for free on the internet, I have been the author of quite a few, but I still regularly see customers come to us with the same old QFN/BTC problems. In the spirit of being a continuous learner, feel free to use the Google machine between fits of cursing at your once rock-solid process. Lots of good, free information is available for those who search for it. I have only been in the industry for just shy of 20 years, so I wasn't around when surface mount started replacing through-hole tech- nology on a large scale, but I assume that it was quite the shift in electronics manufactur- ing. The shift I have been exposed to is the one we are still in the midst of, which is min- iaturization. We all want a 48" 4K television that fits in our pockets or purses, and we are getting closer every day. But throughout this endeavor, the hurdles have become substantial roadblocks to reliability. One of the main sources of failure I see around here is electrical leakage, or electro- chemical migration. Doing things the way you have always done them simply does not work when you start reducing spacing and stand- off height. In the old days, spacing was almost never an issue on primarily PTH assemblies because it would take a quart of contamina- tion to bridge the gap that would set up the electrical leakage path. Remember, it takes some sort of conductive residue, bias differ- ential, and moisture to complete the dendrite fire triangle. If you have a good 5 mm or more of open space between leads, the risk of electrical leakage was much lower than what it is today when parts routinely have 0.5-mm spac- ing between conductors a single small drop of active flux or process residue can create a dead short. This means education on how to optimize the assembly process to ensure that a wash process is fully removing all of the res- idues, or in the case of no-clean flux, all of the bad actors are properly outgassing and not lurking in the dark corners of your parts wait- ing to attack. When PCB real estate resem- bled a small town in Nebraska, there were standard assembly profiles that would work across many different part numbers, but now that PCBAs look like a topographical map of New York City, the days of one size fits all are gone (Figures 1 and 2). Being aware of things like large thermal mass components or parts tied to heavier ground planes that wick thermal energy away from the component and the effect on solder joint quality and reliability is of paramount importance. If the parts are to be washed after assembly, being aware of large body com- Figure 1: "Small town," Nebraska. Figure 2: New York City, New York.

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