Millsap, Pete (AZ77) wrote:
> 
> The Navy did a surface mount repair study in June 1987.    The conclusion of
> this study is that the "long term reliability of a solder joint will be
> jeopardized with multiple rework cycles. With copper dissolving away during
> thermal processing, the pads will inherently lose their structural
> compliancy.  The copper goes into solution forming brittle compounds such as
> Cu6Sn6.  These brittle intermetallic layers may cause long-term solder joint
> fatiguing problems."  They state that on 2 oz copper  100% of the pad would
> dissolve in 12 cycles.
> 
>  My question does anybody know of any recent studies that would support or
> counter this study.  Do other companies limit the number of heat cycle on a
> pad?
> 
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There is literature out there, that shows repeated rework decreases 
reliability. The factors responsible for reduced reliability are:

1. Increase in Cu-Tin intermetallic.
2. Copper desolution with each rework cycle - upto 5 um (0.2mil) per
cycle. Rework temperature of,solder reflow and rework duration is    
critical. For PTH solder joints, the number of rework cycles has been, 
therefore, limited to 4 for i mil nominal copper plating. There is risk 
of dissolving copper entirely at barrel/pad corner.

Several unpublished IBM reports have internally documented this data.
Also C. Lea and F.H. Howie have mentioned similar observations in their 
paper "Blowholing in PTH Solder Fillets, Assessment of the Problem, Part 
1";  Circuit World Vol 12, No. 4, 1986.

You can run your own experiment to verify on your product to convince 
your management. Good Luck.
-- 
Pratap Singh
Tel./Fax: 512-255-6820
e-mail: [log in to unmask]


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