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.