Dave,

Echoing Denny's input, I would repair Lead Free (SAC) with Lead Free
(SAC) and Tin Lead (SnPb) with Tin Lead (SnPb). I would keep the two
technologies separated.

Just a warning, there are several papers out there that indicate that
you could do first piece production mixing technologies (SAC BGA / SnPb
paste or SnPb BGA / SAC paste), and some might imply that you can do
rework in the same manner. I am very skeptical on this since I work in
the aerospace arena and the test data documented does not come close to
what I have to deal with.

One more thing - your soldering temperatures with SAC are about 20C
higher than Tin Lead (SnPb). This means that you have to control your
processes better so you do not disturb the neighboring solder joints.

Hope this helps. Good Luck.

Lee Whiteman, PMP
Senior Member Engineering Staff
L-3 Communications East
Telephone: (856) 338-3508
FAX: (856) 338-2906
E-Mail: [log in to unmask]
 
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Schaefer
Sent: Tuesday, October 27, 2009 11:53 AM
To: [log in to unmask]
Subject: [TN] Reworking LF(SAC) Assemblies with SnPb Solder

Working on a product that is exempt from RoHS requirements but does
require
LF(SAC) processing because of BGA ball solder composition. Most other
products are standard SnPb process, and rework is performed with SnPb
solder.

Am I correct to question the use of SnPb solder to rework the LF(SAC)
assemblies?

It would seem that mixing SnPB and SAC solder would result in an unknown
solder composition.

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