Genny,

I'm pretty sure that you already know what Tan Geok Ang means but thought 
I'd mention it to you as well.  We just encountered an issue with this on an 
OSP finish board used in a lead-free process.

The term used here was copper dissolution.  An intermetallic layer is formed 
between the copper and the solder during the soldering process.  This layer is 
part of the bond that makes the wonderful soldering process work.  The layer 
is combination of the available metals.  

In the case of OSP the available metal on the PCB is copper.  Intermetallic 
thickness is dependent upon time and temperature.  Higher temperatures and 
or long soldering times, the more intermetallic will be formed.  Copper readily 
forms an intermetallic layer and will continue to form it at each soldering step 
including repairs.

With the higher lead-free soldering temperatures more intermetallic will be 
formed than with lead based eutectic soldering.

So, if repeated soldering or prolonged soldering operations occur it is possible 
to simply consume all of the copper into intermetallic.  A board that reaches 
such a state is scrap.

A carefully controlled soldering process should not lead to this problem.  

With lead-free you want solder wetting to be the best possible to avoid long 
dwell times that can induce copper dissolution.

If you want to see some images let me know and I can send them to you.  It's 
not very pretty.

Drew Meyer

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