‘The Purple of Cassius’ : A Recurrent Gold Finger, Tab‐plating Problem

Over the past 20 years there has been a recurrent problem of a purple‐bluish stain appearing on the laminate materials of finished printed circuit boards between the gold‐plated fingers. The origin of this staining has been traced back to the PWB fabrication plant and it is generally accepted to be related to either the gold plating bath and/or the solder stripper chemistry. In this paper the authors report their investigation of this phenomenon and show that, far from being a benign, cosmetic defect, this purple stain poses a potentially serious metallic contamination to the laminate surface of the PWB. The purple colour arises from generation of a colloidal gold chromophore known as the ‘Purple of Cassius,’ which has been known since ancient times and has been in commercial use in the glass and ceramics industry for at least 300 years.