Diffusion kinetics and mechanical behavior of lead-free microbump solder joints in 3D packaging applications

Objective of this work is to understand diffusion kinetics and mechanical properties of interfacial intermetallic compounds (IMC) in the microbump solder joints. Diffusion studies on different sizes of Sn/Cu joints are conducted to compare their interfacial diffusion kinetics with microbump solder joints. Growth rate constants and activation energy of interfacial IMC layer during multiple reflows in microbump joints are lower than 80μm solder joints. However, in the case of solid-state aging, activation energy for interfacial IMC growth increases with the reduction in the size of solder joints. Besides the diffusion kinetics, microstructural changes such as, morphology, kirkendall voids and grain size of IMCs also affect with reduction in the solder joint. These structural changes adversely affect the mechanical integrity of interconnect system. However, determination of mechanical properties of interfacial IMCs in microbump joints is difficult due to its thicknesses (~ 0.5 to 2μm). Tapper mounting technique for microbump joints is demonstrated to characterize the interfacial IMC layers thickness as low as 500nm. Substrate effects on measured elastic modulus and hardness are removed using post-indentation analysis i.e. S-h and S²-P analysis. Elastic modulus and hardness of interfacial IMCs are extracted from the harmonic contact stiffness-load-displacement data.