I hear two phrases way too often on a production floor: “We have always done it this way,” and its first cousin, “We have been building this board for 20 years and never had a problem.” Inevitably, these phrases are always uttered by a “seasoned” engineer in the industry 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 failures associated with it to this day.
What we learned from processing many bottom-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 ability 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 during a thermal excursion, leaving behind nearly benign levels of ionics. With such a low standoff height, the gasses accumulate between the outer edge of the component body and ground pad, which leaves a thick residue that will readily absorb any atmospheric moisture and easily set up electrical leakage paths, or even electrochemical migration creating a dead short.
To read this entire column, which appeared in the December 2019 issue of SMT007 Magazine, click here.