SMT007 Magazine

SMT007-Dec2019

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24 SMT007 MAGAZINE I DECEMBER 2019 What we learned from processing many bot- tom-terminated components (BTCs) is that standoff height is a key parameter when it comes to reliability. When building products with a water-soluble flux, the impedance is the abil- ity of the wash and rinse solution to fully flush underneath the component body and remove the flux residues. If a no-clean flux is used, the issue is the ability for the active parts of the flux to outgas as they are designed to do dur- ing a thermal excursion, leaving behind near- benign levels of ionics. With such a low stand- off height, the gasses accumulate between the outer edge of the component body and ground pad, which leaves a thick residue that will read- ily absorb any atmospheric moisture and easily set up electrical leakage paths, or even electrochemical migration creat- ing a dead short. I hear two phrases way too often on a pro- duction floor: "We have always done it this way," and its first cousin, "We have been build- ing this board for 20 years and never had a problem." Inevitably, these phrases are always uttered by a "seasoned" engineer in the indus- try that probably should know better. Don't get me wrong, these phrases are going a long way in my effort to send two kids to college, but they aren't very helpful regarding reliability. Times change, and technology changes even faster, and if you don't keep up, you will be left behind. This means focusing on emerging technologies and the associated risk that may be unique to that package. As an industry, we experienced this with QFNs several years ago, and to be very honest, we are still dealing with that exact package and the fail- ures associated with it to this day. Old Dogs, New Tricks Quest for Reliability by Eric Camden, FORESITE INC.

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