Just so everyone is clear...The copper dissolution is not specific to
OSP but the effect of soldering to the base copper.  The same affect
occurs with HASL, immersion Tin, immersion Silver and DIG for the same
reasons.

Thanks,
Ryan

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Drew meyer
Sent: Friday, January 04, 2008 5:36 AM
To: [log in to unmask]
Subject: Re: [TN] OSP w selective gold

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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