Pratap, immersion tin coatings will loss their solderability in less than 24
hours.  You may be able to use very aggressive fluxes and solder to immersion
tin coatings after 2 weeks, but I would be worried about ionic contamination.

If you are using an active flux anyway, consider using OSP coatings like Entech
106A or just "Bare Copper" instead of immersion tin.  I use immersion tin to
cover bare copper for cosmetic purposes only.

The problem with immersion tin is that there is no solder to protect the
copper/tin intermetallic.  The IMC layer oxidizes very quickly and is difficult
to reactivate.

In my experience the immersion tin/lead isn't much better than immersion tin.
However, if you can plate it thick enough (>0.0003") and reflow it immediately,
it works fine.  After reflow the tin/lead surface should be bright and shinny
not matt gray.

Hope this helps.
                                Regards,

                                Jim Reed, Texas Instruments, Austin, TX

Voice = 512 250-7172,  
Fax =   512 250-7967,  
Email = [log in to unmask]
Pager = 800-759-8888 ID 289-7077

------------------
Original text

From: Pratap Singh <[log in to unmask]>, on 1/16/97 11:35 AM:
sam mccorkel wrote:
> 
> I'm looking for any cautions or advice you may have on the following.
> We are considering the use of immersion tin as a finish on bare printed
> circuit boards.
> The boards are thru-hole only and they will be wave soldered within the next
> 3-4 weeks.
> Do you consider this a "no no" or is this an acceptable finish until wave
> soldering?
> 
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Do not use TIN coatings on PCB. Have you not read about Tin-Whisker 
growth on tin coated boards that cause random short failures.

This mechanism has ben documented in several papers. Recently, a few 
emails have beeen posted about the same mechanism. 


-- 
Pratap Singh
Tel./Fax: 512-255-6820
e-mail: [log in to unmask]

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