Package-on-Package Micro-BGA Microstructure Interaction with Bond and Assembly Parameter

Package on Package (PoP) technology can play a vital role in advanced packaging for various reasons. The use of a thin interposer allows for tighter wiring ground rules and can reduce die-to-die latency when placing either multiple die on a single interposer or placing multiple interposers on a larger carrier laminate. This assembly technology can also be of benefit for heterogeneous integration applications. The main challenge for bond and assembly process development with organic to silicon bonding, particularly with thin organic interposers, is to overcome solder join defects (non-wets, bridges) created by highly warped components. To address this issue, two different assembly process flows were developed, namely process A (chip first) and process B (laminate first) with PoP type test vehicle hardware. The assembly was composed of a 23×29mm die with 185μm pitch SAC solder connected to an organic interposer of 37.5×37.5mm dimension and 0.7mm thickness. The bottom side of the interposer, with either CuOSP or NiPdAu pads, was soldered to a 68.5×68.5mm carrier laminate of 1.8mm thickness and CuOSP pads with SAC 305 μBGA solder balls of 0.4mm pitch. To ensure bond and assembly integrity, over 30 cross-sections were performed on the modules from these two process sequences. μBGA's were of first interest as warpage issues forced a thorough optimisation of joining parameters. Microstructure appears with undeniable variations between process sequences and interposer pad surface finishes. Cross-polarized microscopy observation and etching of cross-sections, with over 2500 μBGA characterized, showed that process flow A microstructure was mostly constituted by very large and few grains of beta-Sn phase compared to mainly small interlaced grains for process flow B. IMC's with "chip first" (process flow A) assembly were composed of rare AgSn crystals and several CuSn filaments, which were also observed on "laminate first" (process flow B), but only with NiPdAu interposer pads. Also, for process sequence B with NiPdAu pads, thick plates of AgSn were observed, as opposed to very thin sheets with CuOSP pads. Interim cross-section in the bond and assembly process flows allowed the formulation and verification of a hypothesis on the mechanism that leads to a "laminate first" atypical microstructure. Cooling rates during reflow and interposer pad surface finish will change all the kinetics of intermetallic nucleation and produce eccentric growth. A thorough mechanism understanding will allow a proper process parameter intervention to modulate a desired microstructure from a reliability stand point.