Jonathan

This condition has appeared fairly frequently with Sb5 in some industries
when the solder joint is heated to about 150oC for a couple of 100 hours.
Two theories have been advanced that both seem to have validity.

One is that the intermetallic growth uses up the Sn at the interface and a
weak antimony rich layer forms which is easily separated.  Analysis showed
that this layer was 18% antimony at the separation.

The other is that the plating; either copper, or tin or tin-lead may contain
excessive organic plating additives that co-deposit and are left behind in a
weak layer when the copper-tin intermetallic is formed.  These act like
Kirkendahl voids, but are much more profuse.

Note that the lead rich solder that does not have much tin is less likely to
have the problem. Other conditions that do not show the problem is when the
leads or pads are nickel plated, and the intermetallic growth is much
thinner.

Phil Hinton


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