Size effect on ductile-to-brittle transition in Cu-solder-Cu micro-joints

We report tensile properties of intermetallic compound (IMC)-based solder joints. A systematic study of the mechanical properties of solder joint covering a broad size spectrum from BGA (ball grid array), flip chip joint, to microbump, which are key components used in 3D IC packaging, has been performed. We found that when the size of solder joint is smaller than 70 μm, brittle fracture tends to occur due to IMC embrittlement. When the solder joint size is greater than 70 μm, ductile fracture within solder itself is dominant. The brittleness for microbump in 5-20 μm in diameter used for 3D IC packaging will be a long term mechanical reliability issue as IMC-based solder joint is highly susceptible to brittle fracture. Besides focusing on strengthening for solder, more of research effort should be paid on how to increase the fracture toughness of the intermetallics for sub-20 μm micro-joints. Four different fracture mechanisms are also summarized in this paper for the mechanical failure of solder joints.