I know this topic is kind of controversial, but I was wondering what people think about the current lead free alloys being used and which one is the closest to being a drop in replacement for Sn-Pb and why?

If we don't have a drop in replacement, what is lacking from the current lead free alloys?

Jim Foley

Unfortunately, the current thrust forward with the SAC alloys is not close to being a drop-in replacement, and not simply from the melting temperature point-of-view. We have solved most of the processing concerns that immediately popped up with the higher melting temperature alloys: new furnace profiles, improved fluxes, etc. However, as has become evident from the research that has been on-going for the last 10-15 years, it is secondary issues that have posed the most concern. Those factors include IMC layer voiding ("fragility"); higher strength that damages the passivation layers of Si die in flip chip applications and reduces drop strength; moisture sensitivity levels of plastic packages; Sn whiskers brought about by the need for an inexpensive, RoHS compliant surface finish; Cu erosion in wave soldering, etc. Clearly, some of these issues are being addressed by re-engineering the SAC compositons, e.g., SAC 105 for hand held devices and the and modified wave soldering alloys. In fact, by the latter examples, it is evident that the industry may be moving away from a single alloy composition and instead selecting a material that fits the application. (Maybe, that is the correct approach rather than a "one alloy, fits all".) Several the other issues listed above would be solved by returning to one of the original goals that drove the alloy development "era" of the mid-1990s, which was to lower the melting point of the Sn-based, Pb-free solders. But, would an alloy that had solidus and liquidus temperatures of 183C come with a whole new set of secondary issues that would have to be addressed?

Paul, I think you are right on track as usual.

Do you think there is much drive for a ~183C lead free alloy?

If such alloy existed, the cost of such alloy is surely going to fairly high. I am guessing there probably is enough drive to have a lead free 183C alloy that is 1X the cost of SAC105, but is there enough for 2X or 3X?

My guess is no, and industry would prefer 0.1X, but is there a reason to invest resources in persuing a lower temperature lead free alloy if the result is something nobody can afford to use?

Thanks for your time Paul