A Study of the $\mu$ -Solder Joint on Ajinomoto Build-up Film using the Solder on Pad (SOP) process for Multi-Chip Module

Ajinomoto Build-Up Film (ABF), prized for its superior properties compared to other chip substrates, serves as a crucial insulator for CPUs and has become an integral component in personal computers. The Solder On Pad (SOP) technique enhances the mountability of its flip chip bonding process. This research delved into the microstructural and mechanical evolution of SAC305 solder joints on ABF fabricated by the SOP technique. The results reveal that a partial crack emerged at the Ti/Pt/Au/SAC305 interface after 200 thermal cycles, eventually extending to span the entire width of the solder joint following 500 cycles. The morphologies of Ni-Sn IMCs exhibited a transition from scallop-shaped to needle-shaped, and eventually to chunk- shaped. Thermal cycling significantly affected the indentation work of both
$\beta-\text{Sn}$
and IMC phases. Decreases were observed in both the elastic and plastic work of
$\beta-\text{Sn}$
phase after thermal cycles, along with a decline in the elastic work ratio (
$n$
IT). Consequently, the equivalent elastic modulus and hardness of
$\beta-\text{Sn}$
diminished, indicating a deterioration in its mechanical properties. A similar trend was observed in Ni-Sn IMC phase. Based on our previous research, deformation induced by CTE difference led to rotation of heterogeneous lattice, resulting in deviations in grain orientation and recrystallization. Sub-grain formation occurred concurrently with an increase in dislocation density and activation of various slip systems. The energy stored during this deformation ultimately manifested in crack propagation.