Transcript
Phil
And welcome to Board Talk with Phil Zarrow and Jim Hall, The Assembly Brothers. Here to help you solve your dilemmas in electronic assembly. Here is an interesting inquiry today Jim.
This is from JL. If we switch to low temp solder paste, will we have issues with BGAs and SAC balls? Well, I think the answer is potentially yes. This is a forward compatibility issue.
Jim
For those of us who were around with the conversion from tin lead to SAC, lead-free, we will be familiar with a similar issue but it was backward compatibility where the new SAC BGA would be taken backward into an old process using tin lead paste such as a military exempt product which was certified as a tin lead product. Now all of a sudden, has a BGA that is only available with SAC ball.
So we have the newer SAC BGA with SAC balls going backwards into an older process of tin lead processing, tin lead paste. Here we have the opposite. We have the SAC ball is now the existing or the old guy and the low temperature solder is the new guy. We are trying to build a product with low temperature solder, but we have a BGA that has SAC balls. The same issue is there, how can I assure reliability.
I want to point out that this is new but because of the industry’s experience with going to tin lead to SAC, we know what to do.
We know what the issues are. They are already being studied. Data is being published that has some promising reports.
It is a complex issue from just reading some of the initial reports. Bigger difference between thermal cycle reliability and drop test reliability. Also, with low temperature solder alloys Bismuth based, there is a much wider variation particularly in the Bismuth content.
If we think about SAC, all of the SAC alloys that we deal with have at least 94-95% tin and maybe they go up to 98%. So there is a small variation. With lead-free low temperature alloys, with Bismuth and tin, the range of those compositions varies much more significantly by 10s of percents, 20%. Having much greater variation in the potential properties of these alloys when we think about reliability.
As I said, thermal cycle drop test, all the other migration impact. Again, comparing it to SAC and tin lead, one of the techniques that was applied to deal with this issue was to raise the temperature of the tin lead reflow process so that it melts, or at least partially melts, the SAC balls on the BGA. So, they have a melting point of 217. If my tin lead profile was 210, maybe crank it up by 8 or 10 degrees depending on what level of mixing I believe is necessary to ensure the reliability of my product.
As we said, some people did not accept that technique. Now we are never going to take the reflow temperature above 190. That is the reason we are using this, to prevent warpage and to allow the use of other materials. We are going to have a joint where you have the low temperature solder alloy melting in. In the pictures that I have seen of the tests at least partially surrounding this SAC ball, which does not melt. Then you have some diffusional melting between the alloys of the SAC, even below temperatures due to the solubility of tin and Bismuth.
These issues are being dealt with. Yes, JL you are going to have to deal with this. Fortunately, some of the data being published says that soldering this SAC ball with low temperature paste, at a 180, 190 without melting the SAC ball and getting a full mixing is still going to produce a solder joint that has acceptable reliability under certain conditions.
So yes, you will have to deal with it. It may be relatively easy. It may require some special processing on your count. The jury is still out on that.
Phil
There is a whole plethora of lower temp alloys. A lot of work has been done on this, particularly in the last ten years on this. Some of the problems and the concerns with the Bismuth content have been resolved. As Jim said, a lot of possibilities and a lot of different directions.
Jim
Well Phil, there was some very fundamental research that was done way back 20 or 30 years ago when some people were using the Bismuth tin for step soldering. We started off a little further ahead of the game than we did going from tin lead to SAC, what to test, how to test for it and so forth. What the final answer will be, I don’t know. It will probably be, as we always say, application specific.
Phil
You have been listening to Board Talk with Jim Hall and Phil Zarrow, The Assembly Brothers. I would like to just add, whatever you do you, whatever temperature range solder paste you are using or not using, don’t solder like my brother.
Jim
And don’t solder like my brother.
|
