I've heard that the surface tension of molten Sn62 is lower than that of
molten Sn63, resulting in a higher assembly yield, due to a reduction in 
tombstoning on 0402, 0603, and 0805 chip components.  True?  Are there studies 
or opinions (plenty I'm sure) available where the improvement is quantified?


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                                                              Message#      520
                                                        23-OCT-1996 10:01:43.80
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Subj:   RE: assy:NiAu pwb soldering w/Sn62?

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From: "Foster, Donald C." <[log in to unmask]>
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Subject: RE: assy:NiAu pwb soldering w/Sn62?
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Steve,

There would only be one potential benefit and that would depend on the
application of your assembly. Ag additions at the 2wt% level do give
improved creep performance. This could be important if your PCB is mounted
vertically with heavy, and hot devices. All the data I've seen so far
indicates that Sn62 has no better fatigue performance than Sn63.

The original intent of 2% Ag additions to solders was to  minimize the
dissolution of Ag terminations of chip caps and resistors into the solder
joint by reducing the Ag concentration gradient and thus slowing down the Ag
diffusion rate. Now that most chip device manufactures offer Ni barriers in
their termination metallurgy, Sn62 seems redundant.

Don Foster
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