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34 SMT007 MAGAZINE I MAY 2020 Solder is inarguably one of the required building blocks for electronic assemblies, and apart from a few exotics, every assembly in the world has it. Solder, in general, has been around for over 5,000 years, by some accounts, and used for weapons, jewelry, and stained glass, among other items. Fast forward about 4,900 years (give or take), and now it's in pretty much everything with a battery or a plug. For the first 100 years, tin-lead solder dom- inated the electronics assembly process with proven results. In 2006, everything changed for the vast majority of contract manufactur- ers with the RoHS directive that effectively removed lead from the soldering process for all products to be built or imported into the EU. There are plenty of opinions on both sides of this issue to this day on whether this was a good idea, or even necessary. When it comes to meeting the requirement, those opinions and historical reliability data are not taken into consideration. Now, tin-lead is used for very few assem- blies, and those are almost always high- reliability applications like medical and aero- space. Directive 2002/95/EC ushered in the era of lead-free solder, as well as questions about any impact removing lead may have on solder joint quality. Removing lead increased the amount of thermal energy required to melt the solder and create a good IMC. Removing lead also increased the risk of tin whisker for- mation. I talk a lot about dendrite growth, but although whiskers have the same failure mechanism as a dendrite, they don't require moisture or conductive residues. According to the NASA Goddard website, the first pub- New Solder, Same Old Testing Quest for Reliability Feature Column by Eric Camden, FORESITE INC.

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