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December 2017 • SMT Magazine 23 When it comes to considering lead-free, the question becomes what's worse between the thermal damage, and the questionable reliabil- ity of lead free. "Lead free has got a decade of experience behind it, and the planes are still flying. So, at some point in time, you'd like to think that they'll feel more comfortable," says O'Neill. "Additionally, new alloys are being developed, but the elimination of lead is reopening the op- portunity to reintroduce other elements that were sidelined because lead was still present in the system, specifically bismuth. This may come as a surprise, but SAC305 was not the industry recommendation by iNEMI, which was like the Jedi council. Where do we go to once lead is gone? They actually came out with SAC387, and that was 3.8% silver, 0.7% copper. What was realized was that you could eliminate 0.8% silver, which saved a few bucks a pound, which was huge when people were dumping solder pots. It solved the issue with higher silver al- loys, because they developed this silver inter- metallic that becomes a fracture boundary and reduces drop shock performance. SAC305 kind of grew into the standard and displaced what the original recommendation was. SAC305 isn't written anywhere as being the standard; it's just what the industry kind of settled on. The other important thing was it was compatible with the lead that was still present. Not only was it ade- quate, the reflow temperatures were within rea- son, but it could be combined in case lead got mixed in. The reliability wouldn't suffer great- ly. If you combine a lead-free solder ball with a leaded process, there's a measurable degra- dation in reliability, but it's not the end of the world." Bismuth, if you were to incorporate that into an alloy, and it was exposed to tin and lead, would create an alloy with a melting tem- perature of 97°C, and it would fail very rapid- ly, according to O'Neill. Thereby the reason bis- muth got sidelined. "What AIM has pursued, and I'm seeing it with my competitors as well, is revisiting bis- muth because it does some cool things. We know for a fact that it dramatically improves thermal cycling performances. SAC305, when exposed to high temperatures for an extend- ed period of time, the grain structure of the al- loy itself changes and morphs. It coarsens over time, and as the grain structure coarsens, the mechanical characteristics of the alloy degrade significantly. A SAC305 solder joint, if you get it hot enough for long enough, will literally just disin- tegrate, which is significant in the DoD market spe- cifically because you're getting very powered dents with these things. You're jamming 30 W into something the size of a post- age stamp, so it's getting hot. Hot, cold, hot, cold, hot cold, and if you're joining that with SAC305, that's a failure mechanism waiting to happen. The in- corporation of the right amount of bismuth has a significant, positive influence on that," says O'Neill. "We think what will end up happen- ing is that designer alloys will evolve. Niche al- loys, customized alloys for the application re- quirements will evolve. We've already seen it happen on a widespread basis with the incorpo- ration of an alloy like SN100C, which was not SAC305, performed adequately, reduced costs by anywhere from 30–50%, and so was quick- ly adopted as a result. I think you'll continue to see that type of evolution of alloy preference. I think it's just going to continue to fracture, pun intended. SAC305 will no longer be the de fac- to standard. I think it will continue to be imple- mented, but that the other alloys will start to get introduced and solve these high-reliability concerns, specifically." Jet Printing: The Next Evolution Jet printers now have the capability to print more than one million dots per hour. Since they are mainly driven by software, jet printers no longer need stencils—thus eliminating the need to clean them—and offer faster change- EQUIPMENT MATTERS IN SOLDER PASTE PRINTING Matt Kan