Issue link: https://iconnect007.uberflip.com/i/1243344
14 SMT007 MAGAZINE I MAY 2020 1,000 kilograms of solder in it—and you are using SAC305, that much solder would cost as much as a car! Some metallurgists developed a very high tin content around 99.3% tin with 0.7% copper solder for wave soldering. Ini- tially, that didn't work very well, but metal- lurgists found that if they put a small amount of nickel in it—say 0.06%—it made the sol- der perform much better. For wave soldering, that alloy—99.3% tin and 0.7% solder with a trace of nickel—has been a real winner for the wave solder market. One other thing that that's emerging is there are a few disadvantages to lead-free solder, such as the higher cost and higher melting temperature. Tin-lead solder melts at 183°C, and lead-free solder melts at a little less than 220°C. That's quite a temperature increase. iNEMI and others have made a large effort to develop a low-temperature solder alloy, and the main candidate is tin-bismuth. This melts at a lower temperature than tin-lead solder, about 138°C. The main reason to choose tin-bismuth is the lower temperature, which is less stressful on the components when reflow soldering to a board. However, bismuth-tin has a disadvantage in that it's brittle. If you use it for a mainframe computer, it's probably okay, as it performs well in thermal cycling. IBM used an alloy like that for many years. But, if you use it in a cellphone, you have to improve the drop shock performance. A lot of this iNEMI project is to try to modify the tin-bismuth alloy to make it so that it performs better in drop shock and maybe even with some help like using a polymer coating to strengthen the solder joints. Feinberg: Would nickel take away a little bit of that brittleness? Lasky: No. We're talking about a tin-bismuth solder. An addition of nickel will not bene- fit this solder. It's 99.3% tin and 0.7% cop- per solder that a small amount of nickel helps. The main disadvantage for that solder is that it requires an even higher temperature, up to 227°C to melt. And when it cools, there's like a frostiness in the solder joint. Not only does it look bad, but it could have reliability implica- tions. When you put this little bit of nickel in, the frostiness goes away. Feinberg: When it comes to lead-free solder, most people don't realize that there are indeed some negatives to it. Lasky: The big issue when going to lead-free was a lot of people thought the negatives were almost too much; perhaps it wasn't going to work from a reliability perspective. Researchers spent hundreds of millions of dollars, trying to find an alloy to replace tin-lead that would be acceptable that would melt at approximately the same temperature, but they couldn't do it. Indium Corporation and others had devel- oped alloys that melt at exactly 183°C, but for several reasons, they ended up not being widely accepted. Thus, after hundreds of millions of dollars of research worldwide, the SAC alloys pretty much ended up being the only choice that was reasonable in all respects. When all the dust has settled, we would not have a modern cellphone if it wasn't for lead-free solder because it does not wet as well as tin-lead solder. This initially supposed drawback of lead-free solder has enabled closer spacings of the pads without the short- circuiting that tin-lead solder would cause. In other words, tin-lead solder wets so well that it would short the pads out. What seems like a disadvantage in that lead-free solder doesn't After hundreds of millions of dollars of research worldwide, the SAC alloys pretty much ended up being the only choice that was reasonable in all respects.