Solder Joint Reliability of Fully Homogenised SAC-SnBi Low Temperature BGA Interconnections using Solid Liquid Inter-Diffusion (SLID)

We propose a comprehensive investigation of solder joint reliability that compares eutectic SnBi BGA interconnections to novel hypoeutectic Sn-Bi interconnections formed using commercially viable Solid Liquid Inter-Diffusion (SLID) conditions that enable low temperature reflow at both the package and card levels. The homogeneous and stable hypoeutectic SnBi connections, comprising a lower Bi content than their eutectic counterparts, demonstrate favorable resultant properties. A double SLID method and its diffusion kinetics are examined by employing various mask sizes to ensure that the required volume of eutectic SnBi solder paste for complete homogenization can be supplied in a packaging and card attach production environment. Combinations of fully homogeneous and near homogeneous interconnections are reported, with unreacted SAC being less than 10% by volume and Bi% in homogeneous interconnections ranging from 28 to 32%. Secondly, because of reported issues with ENIG metallization, interconnections with Cu metallization pads are investigated. These homogeneous hypoeutectic SnBi interconnections are evaluated for reliability performance with comparison to eutectic Sn58Bi interconnections. Specifically, three stress tests are used - high temperature storage (HTS), high temperature storage with bias (HTSB) and accelerated thermal cycling (ATC). Progressive electrical and mechanical responses are explained through observations of microstructural evolution (grain size, intermetallic formation, etc.). The results show that hypoeutectic SnBi and eutectic SnBi interconnections remain relatively stable through Telcordia stress conditions with minor degradations observed only in the eutectic case. However, the newly formed hypoeutectic SnBi interconnections exhibit notably superior reliability performance over their eutectic SnBi counterparts under extended JEDEC conditions thus far, recommending their use in such applications.