The presence of lead in the joint allows for the theoretical possibility of the Sn/Pb/Ag eutectic forming. This has a solidus of 179C  [The liquidus of the solder will still be >220C]
If this Pb is washing off your boards/components, there is a real possibility of a locally high concentration of Sn/PB and therefore of a depressed Sn content in the adjacent area leaving a surplus of ...it begins to be hard to forecast everything. Whatever there is the distinct possibility of a different strength/composition/flexibility layer in the solder joint. This discontinuity would be visible in micrographic section and is undesirable from a reliability point of view.
 
You may also be seeing effects of this in terms of surface appearance  - slight frostiness - although probably joint shape and wetting etc are not [yet] effected.
 
In a perfect world you solder lead free components with your Sn/Ag solder, in a real world you have an analysis program so you can monitor these increases and take corrective action. The only thing which puzzles me is the amount - 1% represents a lot of devices and I would have thought regular drag out and replenishment would not normally allow this build up. Perhaps you have a nearby Sn63 machine?
 
 
Regards

Mike Fenner

Applications Engineer, European Operations
Indium Corporation
 T: + 44 1908 580 400
M: + 44 7810 526 317
 F: + 44 1908 580 411
 E: [log in to unmask]
W: www.indium.com 
Leadfree: www.Pb-Free.com

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Stolar, Paul W
Sent: Tuesday, January 14, 2003 9:49 PM
To: [log in to unmask]
Subject: [TN] Pb

Does anyone no what 1% Pb in SnAg solder will do? I just got a solder analysis done for my wave.

 

Paul Stolar

Materials Engineer

Baker Atlas

713-625-5376

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