I have seen the directional effect when a rack was left to dry and had not
been  rinsed sufficiently so that the "bottom" of the holes where the rinse
water pooled had
enough foreign material concentrated in them to effect adhesion.

If the boards were made by imaging on electroless, the residues could be
anything from photo resist residues to micro-etch residues.

Susan Mansilla
Robisan Lab

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