Reliability Assessment and Microstructure Characterization of Cu Pillars Assembled on Si and Glass Substrates

With the challenges of moving to 2.5/3D packaging structures, it has become imperative to improve our understanding of the materials science of fine pitch Pb-free solder joints. The use of Cu pillars capped with thin layers of SnAg solder provides for tighter bump pitches reducing the chance of solder bridging at chip joining. However, changes in geometry, materials and processes associated with 2.5 D packaging create new materials challenges. The thinner solder regions mean that a larger volume fraction of joints is consumed by the formation of intermetallic compounds at the pillar/SnAg solder interface. The final concentration of Ag in the joint can vary, and the Ag
₃
Sn precipitate morphology in the solder joint may change, directly affecting the reliability of the joint. This can occur through the formation of Ag
₃
Sn plates, or simply because of different distributions of much smaller Ag3Sn precipitates. Or, the entire solder joint may be transformed into intermetallic compounds during assembly or operation of fine pitch joints. The presence of interposer materials with a different CTE compared to FR-4 laminates may also affect the lifetime of the package during drop/shock or thermal fatigue. In the current study, relations between processing, microstructure and reliability of assemblies enabled through Cu pillar/interposer technology were examined. The effects of solder cap composition, thickness and volume on microstructure of assemblies on Si and glass substrates were examined. Effects of multiple reflows on the microstructure of solder joints were also studied. Significant variation in Ag
₃
Sn precipitate morphology was observed under nominally identical fabrication conditions. These were correlated with relatively large variations in mechanical behavior, for instance in measured values of shear strength. Large variations in Ag
₃
Sn precipitate size and number were also observed with changes in composition and upon aging, as would be expected. Cu pillar assemblies revealed small, but continuous solder layers. After failure during ATC, cracks were found to have propagated through these continuous solder layers.