Some information that I read in this forum seems to indicate that the
formation of intermetallics is a bad thing and that you must form a primary
metallurgical bond to base metal, in this case nickel.  As we all know the
bond between the solder and the nickel is based on the intermetallic of
tin-nickel being an intermediary between the nickel and the solder with at
least two of the nickel/tin intemetallics being present.  The same applies to
copper in which the two low temperature are alway present.  If you solder
with tin-lead solders you form many intermetallics which are the basis for
the solder bond.  Intermetallics form with copper/tin, 2 at normal soldering
temperatures; nickel/tin 3 at normal soldering temperatures; palladium/tin, 7
at normal soldering temperatures: palladium/lead, 6 at normal soldering
temperatures; and gold/tin, 4, I if my memery is correct.

The formation of intermetallics is the normal condition and not a defective
solder structure.  Intermetallics is what makes soldering work in the
electronic industry. 

It is also noted that a "good solderable surface" is mainly due to the
protection of the underlying surface such as nickel or copper by a oxidation
resistant metal such as immersion gold over electroless nickel and the
overplate is mainly sacrificial or in the case of OSP, where the coating
inhibits and protects the copper surface until it can form the copper/tin
intermetallics and produce a bond. 

Also, small amounts of palladium (which is somewhat less soluble than copper
in tin/lead solder but much less soluble than gold) or gold in the solder do
not not automatically make the solder joints weak and unusable for surface
mounted components and in fact may add to their shear strength.

Also, The amount of organics in electrolytic palladium or electroless
palladium in the thin coatings (0.125 to 0.25 microns) do not inhibit
solderability or cause cracking of the joint.  As we also know from reports
over the years, codepositon of excessive organic occlusions in copper and
nickel plating will cause the solder joint to be weak and fracture.

Phil  Hinton

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