SMT007 Magazine

SMT007-May2020

Issue link: https://iconnect007.uberflip.com/i/1243344

Contents of this Issue

Navigation

Page 12 of 109

MAY 2020 I SMT007 MAGAZINE 13 solder paste, and that's a shame. I'll tell you what I mean by new alloys. Today, 75–80% of the solder paste in the world is lead-free, and a pretty high percentage of that is SAC305. I don't want to hurt anybody's feelings, but I've given talks on SAC305 for an hour and had somebody come up afterward and ask me what SAC305 is. With SAC305, the 30 is 3.0% silver, and then the 5 is 0.5% copper, and the balance of 96.5% is tin. The remainder of the lead-free is mostly SAC105, which has lower silver, which is good to save a little money on the silver. The SAC105 is mostly in solder balls for BGAs. My point is companies like Indium Corporation have per- formed a lot of work developing alloys that would have much less silver than the 3% in SAC305. They put in small quantities of other elements, and by doing this, they have come up with some alloys that are better than SAC305 in both drop shock—to protect your cellphone when you drop it—and thermal cycle—to pro- tect your computer from the thermal-mechan- ical stress generated by turning it on and off. Yet the industry hasn't embraced them. If you think about it, there's a logical reason. It costs a lot of money to qualify a new solder alloy, especially if you're a company that wants a reputation for reliability. And some of the cell- phone and PC companies, especially compa- nies that would be producing something that's mission-critical, need a reliable solder. There was so much pain in qualifying SAC305 that even if there is something that promises to be better, SAC305 is good enough. Their feeling— and they may be right—is if we embrace this new alloy, it may be better in many respects, but SAC305 is the devil we know. The newer alloys might have something that doesn't even come out, maybe in the tests. The world has not embraced some of the exciting alloys that companies like Indium Cor- poration have come up with. I'm sure there are niche customers that buy some of these, but SAC305 is dominant. I don't see that changing because it's pretty good. The thing that changes all the time is not the alloy so much. Again, I don't want to make it sound like there isn't any interest in these other niche alloys, but the thing that changes all the time is the develop- ment of the new flux vehicles to improve the performance of the solder paste. That's hap- pening all the time and continues. I'll give you an example of some things that have happened in solder paste in the last 10 years that continue to happen. Ten years ago, the head-in-pillow defect for BGA packages reared its ugly head, and there were all sorts of changes in processes that people developed to try to minimize this failure mode. The com- panies that make solder paste quickly jumped on the bandwagon and tried to develop solder pastes that would minimize or eliminate the head-in-pillow defect. In one case, an assem- bler had 7% head-in-pillow defects. By chang- ing to a solder paste that was designed to min- imize the head-in-pillow defect, it went away. The same was true for the graping defect. The latest thing, in the last five years, has been a lot of interest in minimizing voiding. Indium Corporation has developed solder pastes that dramatically minimized voiding. Of course, one of the challenges is that there isn't usu- ally one solder paste that will be better in all respects. If voiding is your issue, the best paste to control voiding may not be quite as strong as another solder paste is in eliminating the head- in-pillow defect or eliminating graping or that sort of thing. But the work that the solder paste manufacturers have done to address defects, in my opinion, has been breathtaking and can be attributed to the scientists and engineers that study and improve solder pastes. But I don't see a mainstream leaving of the alloy SAC305. I should mention two alloys that are impor- tant exceptions to SAC305. One of the pains of lead-free solder is the cost of silver, and the 3% silver in SAC305 makes it expensive—espe- cially for bar solder for wave soldering. If you look at a wave solder pot—which could have The latest thing, in the last five years, has been a lot of interest in minimizing voiding.

Articles in this issue

Archives of this issue

view archives of SMT007 Magazine - SMT007-May2020