Gold Plating Defect Trivia – “Purple of Cassius”
by Karl Dietz
August 16, 2007
A curious gold plating defect called “Purple of Cassius” caught my attention when
I read an article by Karl Minten some time ago (“The Purple of Cassius”: A
Recurrent Gold Finger, Tab-plating Problem, K. Minten and J. Messer, Circuit
World, Vol. 19 No. 1, 1992, pg.44). A purple-colored stain is observed in gold
contact plating on the laminate between gold plated contacts. This stain was
identified as “Purple of Cassius,” a colloidal gold colorant used for glass
staining and named after the 17th century Dutch alchemist who
first observed it when he added stannic acid to a very dilute solution of gold.
The
most likely scenario that leads to this laminate stain is the swelling of the
epoxy; e.g., by the glycol ethers used in the desmear swell and etch, the
retention of tin from the HASL process in the porous laminate surface, and the
oxidation of the tin to stannic acid in the oxidative environment of tin/lead
stripping. The formation of the insoluble beta-form of the stannic acid that
survives post-solder strip rinses finds its way into the gold bath where
it forms the gold colloid "Purple of Cassius" on the laminate. This colloidal gold
is not just a cosmetic blemish but may lead to high resistance shorts.
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