Werner,

I'll attempt to pull this circular argument at least into a bit of an
oval.  It doesn't make a lot of sense that if P is the problem, more P is
the solution.  At least not on the surface - but that's where the problem
lies.

>if more P can prevent BIF due to 'Black Pad' we are talking
>about a different mechanism.
>If BIFs are caused by "the board fabricator letting the phosphorus
>concentration in the electroless nickel plating bath get too low,' than
what results is
>not 'Black Pad' by definition.
>

Black Pad on a bare board is generally a result of a hyperactive Gold bath
dissolving too much of the nickel and leaving a layer of Phosphorus between
the remaining Nickel and the Gold.  If solder can't get to the Nickel, no
intermettalic, no joint. Bath suppliers have found that increasing the
amount of P to 9-13% causes the Gold dissolution to be self limiting.  So
while there is more Phosphorus in the Nickel, less of it is concentrated at
the top of the pad.  That makes it much easier to control black pad on bare
boards.

That extra P is still available to migrate to the top of the pad during
soldering.  If the solder process is not well controlled, there will be an
increased risk of BIFs caused by an enriched layer of P at the
intermetallic.  A kind of "soldering process induced black pad".  An
intermettalic forms, that's what increases the amount of P at the joint.
Too much P, weak joint, stress creates a BIF.

Low P Nickel, increased risk of no joints.  Black Pad.
High P nickel, increased risk of weak joints.  Black Line.  But these are
easier to avoid.

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