|Ask the Experts|
February 21, 2020
Rise of Bismuth Levels in our Solder Pots
We have an issue of concentrating bismuth in our selective solder machines. We have two machine and both have very similar rise in bismuth levels to 0.724% in our solder pots after six months.
We are using bar solder that contains .45% to .55% bismuth. IPC-J-STD-001 has the max level for bismuth at 0.25%. We are starting with twice that amount and concentrating to 0.724%.
Where could the bismuth be coming from? Most of the boards that use these machine are Immersion Gold. Could it be coming from the components?
|Expert Panel Responses|
J.G.does not state which solder he uses. From the appendix of the J-STD-609A standard, I noticed that 2 solder alloys have some bismuth in them. I think J.G. should check with his solder supplier as to what levels and variation of such levels of Bi are in his new solders.
It may be that Bi is not consumed in the soldering process, but builds in the solder pot as he is experiencing. Also, I suggest that he verify with his solder supplier what analytical method is used by the supplier to determine Bi content and ensure that his solder pot analyses use the same analytical method.
First of all, you are correct in that J-STD-001 sets the limit for Bismuth at .25% for standard Sn60 or Sn63 alloy. Is that the alloy you are using or intend to use? If so, please realize that at room temperature, bismuth has solubility in lead of up to 18% and solubility in tin of about 1%.
A concentration of less than 0.25% bismuth will cause a very small reduction in the working temperature, but that is OK. Excess Bismuth concentrations above the limit of .25% may cause a gray appearance in the solder joint and that is NOT OK. So you never want to exceed the maximum .25% level of bismuth in the solder.
If you are using lead-free solder, the allowed impurity levels are completely different. Please refer to J-STD-006. During soldering, a certain amount of drag-out takes place as the solder flows over the bottom of the CCA, the through-hole component leads, and back down into the sump. Over time, the tin percentage of Sn63 solder typically drops, and unless it is periodically replenished with new solder containing Sn63 (or even a small amount of pure tin), pretty soon you will be soldering with Sn62, then Sn61,etc.
Conversely, the lead percentage can increase from 37%, to 38%, to 39%,etc, but only because there is now less tin, not because more lead was added.As the percentage of lead content increases, so does the amount of bismuth,because it has a much higher solubility in lead than it does in tin (the 18%versus 1% that we just discussed).
So to answer the first part of your question; yes, you could be increasing your bismuth concentration from soldering components that have been pre-tinned with a lead content higher than 37%, but also because as the tin content drops, the lead content (and thus the bismuth) naturally increases.
Having said that, the other statement you made about starting with bar solder containing .45% to .55% bismuth really bothers me, because you are starting outwith a bismuth level in the new solder supply that is already twice the maximum than what is allowed per the standard!
So if you are starting out with bismuth at .45% - .55%, it won't take much tin loss to cause the bismuth percentage to climb to the .74% concentration that you describe. My suspicion is that you are starting out using wire or bar solder in your selective solder machine that isalready out of specification, and then over time it goes even further out.
I would suggest you simply take a small sample of your new bars of solder and send it out for sampling. You can get a full report back showing exactly what the alloy makeup is. For a good quality bar or wire solder I would expect the initial readings of bismuth to be very low, perhaps less than .02%(nowhere near the starting concentration of .45% to .55% that you report).
If the bismuth levels are actually .45% to .55%, and the solder is supposed to meet the Certificate of Compliance to J-STD-006 for Sn60/Sn63, then I would definitely find a different solder supplier. What you are probably getting is"recycled" solder, and it may never have been intended to meet J-STD-006.
Could bismuth be coming from the components? Short answer, yes, it could. Is it likely? Not so much.
I know of some components that use SnBi alloy on the lead finish to comply with aerospace tin whisker mitigation requirements. These components have up to 5% Bi in the lead finish. These are not common components though, so unless for some reason you have a large number of them in your assemblies, I doubt this is the source.
Another possibility is that the bismuth is being concentrated because some tin in the pot is being consumed by dross production, leaving bismuth behind. Given the relatively high starting concentration, and the typically very small volume of the solder pot on selective soldering machines, this may be more likely.
The real question is, why does your incoming solder not comply with the J-STD-006 requirement of 0.10% maximum for bismuth? I would compare your independent lab analysis with the solder manufacturer's analysis for that lot(which you may not have, but should be available on request). If your analysis is consistently different from theirs, you may need to determine why.
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