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 Posted: Sun Jun 22, 2026 11:16 pm Post subject: |
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Reposted from SFE's old Assembly Forum
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Date: 11 Dec 2025
Time: 01:01:40
Remote Name: 137.71.23.54
Remote User:
Hi AA,
I apologize for the delay in response. I am not an expert on lead finish, so I decided to talk to some people and do a little research on my own before replying to this question.
Whiskers are usually observed in pure-tin systems, so we are a bit surprised that you are seeing it in your tin-lead system, especially since you are using the ratio 85%Sn/15%Pb. Whiskers may occur in Sn-Pb systems as well, of course, but mostly in cases wherein Pb is less than 3%. Experienced engineers say that >5% Pb virtually eliminates the possibility of encountering solder whiskers.
Whisker formation occurs due to the tendency of the deposit to reduce its internal stress by recrystallization and formation of larger grains. This process has been known to be driven by the following factors: 1) internal and external (compressive) stresses; 2) thickness of the coating; 3) structure of the crystal; 4) substrate used; 5) temperature; and 6) humidity. Thus, the presence of contaminants does not directly induce the formation of whiskers, unless it results in a change to any of the factors above.
As soon as Sn is deposited over the copper leadframe, an oxide layer starts to form over the deposited Sn layer. At the same time, Cu atoms from the substrate start to diffuse into the Sn layer, forming Sn-Cu intermetallics. Since the oxide layer over the Sn coating impedes outward movement of the Sn atoms, this process of Sn-Cu intermetallic formation builds up internal compressive stresses within the Sn layer as more Cu atoms diffuse into the same volume of the Sn coating.
Eventually the stress becomes too large that excess Sn material begins to extrude at the weakest points of the oxide layer. This protrusion originates as an Sn nodule over the oxide, which eventually grows into an Sn filament or 'whisker', as it is more commonly referred to.
Now here's my suggestion: please check the appearance of the attribute you are seeing to confirm that you are indeed dealing with solder whiskers, and not just 'excess solder' that accumulate over the contaminants you are seeing. Solder whiskers would exhibit a topography that is different from the original coating.
At any rate, it looks though that you are proceeding in the correct direction by trying to eliminate the source of contaminants that seem to be inducing this problem. Please let us know how this would turn out, and good luck and more power to you!
Best regards,
EE
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