High reliability? With oxidisable edge fingers? Your pealing rings a 
bell, or rather several bells, for a change. Peeling indicates no IMCs 
between the underlying copper and the metal. It would seem that this may 
not be a hot process but either immersion (if the peeled metal is very 
thin) or, more likely, an electrolytic plating of either tin or tin/lead 
in a fluoborate process, over a badly prepared surface. What would sound 
a warning bell is that the pads and PTHs could well have a similar 
defect. In the 1970s, it was quite common to have electrolytic 
co-deposited tin/lead plating followed by an IR or hot liquid reflow to 
alloy the metals and ensure IMC formation with the copper but this 
process was replaced by hot air levelling. Is the board you describe 
that old?

Incidentally, are the contacts in the socket also tin plated? I hope so, 
because the usual gold-plated ones would suggest corrosion may occur 
under humid conditions with condensation. Edge connectors should mate 
like-with-like.

Repair? I'm not sure you could make a reliable repair as the copper 
surface must be contaminated. It would have to be carefully 
decontaminated before re-applying a finish of choice. This may entail 
using chemicals that I would not like near electronics, the last one 
being a persulfate etch or equivalent before re-tinning. Probably, if my 
surmise is correct, replacement would be better than repair.

On second thoughts, there is another explanation for the peeling. If the 
fingers were gold plated before tinning, you may have a brittle IMC 
layer at the copper-gold-tin interface. unplugging/replugging the board 
several times could possibly break that down causing the peeling. In 
that case, careful retinning with an iron may be possible, removing the 
excess molten solder by blowing or with a fluxed copper braid. 
Cleaning/defluxing would then be necessary.

Brian

On 17.02.2014 07:24, Patrick Goodyear wrote:
> Ok industry experts, I’m back to my old stomping grounds for a refuel /
> repair outage.
>
> I have a board that has what appears to be solder reflow or tin plating
> over the top of copper plated edge fingers on the power connections. The
> plated surface was pealed off during cleaning using dust-off
> aerosol.     The pealed pad readily melts under a soldering iron,
> material would be 63-37 solder or pure tin.    It does not appear to be
> silver plating.    The back side of the pealed pieces have a copper
> patina to them, indicating the bonds have failed.
>
> My question is, what is the best way to repair/reflow the fingers?    Do
> they need to be re-plated with copper prior to reflow?   I looked in the
> copy of 7721 we have but can find nothing specific only for replating
> with gold or silver.    This board is used in a HIGH Reliability system.
>
> Pat Goodyear
> Temp PG&E.
>

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