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September 2004

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From:
R Sedlak <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, R Sedlak <[log in to unmask]>
Date:
Fri, 10 Sep 2004 11:25:06 -0700
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In the "good old days", say last year, when Tin Lead plating was acceptable, the typical electroplating bath electrolyte was Tin/Lead Fluoborate, and Fluoboric Acid.  This electrolyte is VERY forgiving, and if you allowed some of the Tin dissolved in the electrolyte to oxidize from the normal Stannous Tin to Stannic, it remained in solution, and simply became inactive, and nobody noticed or cared, and there was no effect on the plating.

Today, pure Tin is typically being plated from Sulfate and Methanesulfonate baths, which are much less forgiving.  When the Tin oxidizes from the normal Stannous, to the Stannic state, it forms the insoluble Stannic Oxide, and the bath changes from clear, to milky white.  This is caused by atmospheric Oxygen, or more quickly, by having too low an anode area.

When there is Stannic Tin oxide is in the plating bath, it is co-deposited with the Tin metal.  This could be the cause of the problems you are seeing, as the Tin oxide can absorb, and retain water.  It may even be the cause of the problem, without any water present.
(FYI, I have never seen a clear Sulfate of Methanesulfonate Tin plating bath in my life, they all have had some level of Stannic Oxide present.)

How to determine if you have Stannic Tin oxide present?  Unsure.  Suspect, at some level it could be seen under microscope, or by SEM.

There are a number of ways to prevent this issue, not the least of which is to electroplate pure Tin from the old Fluoborate system...duh... but, you probably do not need to know about that...

Hope this helps.

Rudy Sedlak
RD Chemical Company

Gabriela Bogdan <[log in to unmask]> wrote:
Dear friends,
After assembly, on all circuits there was one fine pitch PLCC from a certain vendor which had lots of small solder balls between the leads, even on the solder mask dam between them.
Other PLCC's with the same pitch were clean.
We tried to see the amount of solder balls by X-ray, and found many more behind the leads.
At first we thought that our solder paste was the culprit, but seeing that other components were "normal" , we looked closer to the leads by X-ray, and saw a lot of small voids only along the leads of the bad component, these voids reaching to the upper end of the leads, where the solder paste did not reach. Visually, the upper part of the leads was very rough.
I suppose that the tin plating of the leads contained additives which epand during soldering, even burst out, and "spit" the molten solder all around.
If this is the case,how could I reject such components during incoming inspection?
Should I do a solderability test for each batch on a printed ceramic plate?
Gaby

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