Darrel 

Looking at the phase diagram for Pb-Sn-Ag at the low silver end except for
the 3 eutectics with L/S temps. of  304, 221 and 179 degrees C, I do not see
any phase that would cause the solidus temperature to be different than the
Sn-Pb binary composition.  ie. when you reach 10 % tin the solidus temp is
268 Deg. C, at 15% it drops to 225 Deg. C and at 19% it drops to the the 183
Deg C. Obviously the liqidus temperature is higher, but when any melting
occurs the joint strength is essentially zero.  

If you are soldering components with leads with a tin-lead coating or tin
electroplate the probability of the solder reaching a low solidus temperature
by mixing with the Pb is very good.  The diffusion of the solder during SM is
poor and you can have localized areas with high tin and high lead in the same
joint when using the HMP solders and tin or tin-lead coated leads.  Even with
good difusion and mixing the tin content of the solder joint could reach 20 %
easily, especially if you have components  with 500 micron [0.020inch] of tin
electroplate.  Therefore, you could be using HMP solder to solder joints that
really have a solidus temperature of 179-183 Deg. C.

Any other question or thoughts give me a ring at 520-745-1013

Phil Hinton 
Hinton "PWB" Engineering.  

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