Electro-migration and Thermo-migration Studies in Nano-scaled Solder Joints and Interfaces Under High Electric Current Stressing and Cumulative Service Loading for Nanoelectronics Applications

Description

Advances in chip-scale packaging technologies have prompted a rapid increase in the density of solder joints in microelectronic products. The ICs are beginning to move nano-scale with less than 100nm lithography, 100 million transistors, requiring more than 10,000 I/Os on a 10-40micrometres area array pitch. Downscaling traditional solder bump interconnects will not satisfy the thermo-mechanical reliability requirements at very fine pitches (< 20micrometres). Electromigration (EM) and thermomigration (TM) are the two basic concerns in electronic packaging with the high current density requirement and the high thermal gradient effect at an ultra fine-pitch solder joint. Stress-migration may also occur under cyclic stresses due to the mismatch of coefficients of thermal expansion. There is no way to solve future problems except with an increasing knowledge-base for Pb-free interconnects through in-depth understanding of all interacting failure mechanisms. As new materials emerge, and the industry incorporates many different materials into the assembly process, it is also critical to be able to describe the interactions between materials and process parameters, and the accurate characterisation and modelling is an important pre- requisite to achieve high yield and reliability. This project will explore the failure mechanism of ultra-fine Pb-free solder joints for high density chip-to- substrate interconnections under combined electro-thermo-mechanical-chemical loading. Due to the intrinsic low current capability of solder alloy (compared with Al or Cu), these tiny solder joints will not survive up to the warranty period of an electronic product while high current density capability is becoming an obvious requirement of these interconnections. Therefore, systematic experimental investigations will be carried out for each type of service loading individually and as well as cumulatively in this project. Under-bumpmetallization (UBM) dissolution, intermetallic compound (IMC) formation, current crowding, atomic migration, plastic flow, void formation in the solder joint will be investigated by using a Physics-of-Failure (PoF) approach in conjunction with revolutionized concepts of innovative EM and TM models. Electrical resistance measurement and advanced metallographic techniques will be employed for damage studies. Failure distribution during accelerated tests will be extrapolated to reveal the service life of electronic products based on the PoF mechanisms. It is expected that this novel PoF approach coupled with carefully planned Design-of-Experiments (DoE) will revolutionize the EM and TM study in nano-scaled solder joints and interfaces, thus laying a strong foundation for a better scientific understanding of the capabilities and limitations of such physical phenomena in nanoelectronics applications on a generally wider scope.