There are known compatibility issues with some alloys and joint
configurations and in your case you would have determine the unknown ones.
My question to you would be: Why go there?
1) ROHS allows the use of leaded solders for repair of pre ROHS products.
2) Infrastructure [power stations ] is presently excluded from ROHS along
with anything else where high reliability is required  (Aviation, medical,
automotive etc).
3) Suggest that even assuming Pb free solders were environmentally superior
(and some doubt this), in your application the environmental impact of using
Pb-free solders would be negligible and far outweighed by the possible
consequences of unreliable joints. 
Regards
 
 
Mike
ü Please consider the environment before printing this e-mail
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Goodyear, Patrick
Sent: Sunday, May 01, 2011 7:12 AM
To: [log in to unmask]
Subject: [TN] Repair / Rework on non-in-house RoHS compliant boards

As a technician that does frequent board level repair rework some
questions have come to mind.   While the majority of my work is legacy
i.e. vintage (1968) we will face these concerns in the near future.  The
boards I primarly work on are Nuclear graded Class IE, (high
reliability, seismically qualified, etc.) high dollar boards, typically
in the 3-10g range and are not throw-aways, most have components that
are no longer manufactured. 
I am looking to the industry to find best practices, this is one of the
places that was suggested for posting.   Any help you can give me would
be appreciated. 

With pre RoHS solders most if not all of major manufacturers used 63/37
solders and repair rework was easy, one just used 63/37. 

With the adoption of RoHS controls and the subsequent use of many
different alloys of lead free solders, (at least 6 I know of), the
following comes to mind:
1. Are there compatibility issues mixing between the various alloys of
lead-free employed currently in electronics manufactures?   In strength
and reliability primarly. 
2. How does the technician determine the correct alloy of lead-free
solder to use? 

3. How does the technician determine what alloy was originally used by
the manufacturer? 

4. Will IPC 7711A / 7721A standards be updated to delineate alloy
determination and / or an alternate methods for the complete removal of
lead-free solders and subsequent repair using 63/37 or lead-free alloys?
Pat Goodyear 
Control Technician 
Pacific Gas and Electric Company 
Diablo Canyon Nuclear Power Plant 
PO Box 56 
Avila Beach, CA. 93424 
805-545-4312 


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