Answer yes.
 
Melting the object you are joining is welding. Soldering is dissolving.
When you make solder joints to SN plated/coated surfaces they do not melt in
the solder. The plating dissolves rapidly and goes into solution. (The usual
analogy is sugar into coffee). The underlying or base metal is now exposed
to solder but dissolves very slowly.  Naturally we don't wait for that
process to complete :). So on freezing there is a transitional zone which is
neither pure solder nor base metal but a mixture. (usually referred to as
intermetallic layer)
 
Sn/2.5Bi can be regarded as a tin plating. The small amount of Bi is added
to control Tin whisker growth. A smaller amount of lead has the same affect.
Ceramic devices have had a tin plate - with the lead - for donkey's years
without a mention for the most part. Now under Pb-free legislation some
people need to know what their BoM is and the added content information is
causing everyone to look into things more closely.

Best Wishes
 
 
 
Mike

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gerald Bogert
Sent: Thursday, June 06, 2013 9:19 PM
To: [log in to unmask]
Subject: [TN] Soldering to SMD device wth Sn98Bi2.5 leads using Sn63Pb37
solder

June 6, 2013

 

We have an OM who soldered surface mount Opto-couplers with Sn98/Bi2.5
leads using Sn63Pb37 solder.  The reflow solder profile used was the
traditional one for Sn63 solder with a measured peak temperature in the
range of 209 to 215C.  The actual amount of Bi by weight that wound up
in the completed solder joint was calculated to be about 0.19%.  Based
on this, there is no issue with a low temperature alloy forming with the
combination of Bi and Pb.  

 

However, since the melting point of the Sn98/Bi2.5 solder is around
223-231C, the reflow profile used will not result in melting of the
solder on the part terminations.  Is this an issue or will the Sn63
properly bond to the solder on the part terminations to form an
acceptable solder joint?

 

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