Failure analysis of thermally and mechanically stressed plastic core solder balls

Plastic Core Solder Balls (PCSB) offer improved mechanical compliance compared to traditional solder balls in assemblies where materials with significantly different coefficient of thermal expansion are present. However, failure mechanisms for systems with lead free PCSB have not been studied sufficiently to allow direct replacement in applications for harsh environments. An assembly of a ceramic carrier onto an organic circuit board intended for application in a harsh environment subsea has been studied with regard to replacement of traditional lead free solder balls with lead free PCSB. Assembled test vehicles were exposed to mechanical and thermal environmental stress testing and failure analysis performed. Degradation of the PCSBs was observed in electrical measurements and in studies of cross sectioned test vehicles. Cracks were observed in relation to formation of intermetallic compounds and a suggestion is made with regard to improvement of the metal system of the PCSBs.