Microstructural stability enhancement and mechanical reinforcement of TLP-bonded Cu/Sn-3.5Ag/Cu microbumps under multiple reflow cycles through Zn Alloying and Ni substrate integration

Abstract

The transient liquid phase (TLP) bonding process is effective for constructing stacked structures in advanced packaging, as it allows for multiple reflow cycles without remelting. However, the various reflows can cause phase transformations, leading to internal stress-induced voids. Thus, the stability of IMC phases is particularly challenged in 3D stacking structures. Common configurations include Cu/Sn/Cu and Cu/Ni/Sn/Cu. Although Ni improves the stability of the Cu₆Sn₅ phase, phase transformation to Cu₃Sn can still occur, compromising reliability. This study investigates microstructure stability by doping Zn into the Cu/Sn-3.5Ag/Ni system across five reflow cycles. Results demonstrate that Cu-15Zn/Sn-3.5Ag/Ni microbumps reduce void formation and ensuring the phase stability of the (Cu,Ni)₆(Sn,Zn)₅ to maintain the microstructure stability. The Zn addition inhibits the Cu₃Sn layer, while optimizing grain size and orientation of (Cu,Ni)₆(Sn,Zn)₅. (Cu,Ni)₆(Sn,Zn)₅ also exhibits increased hardness and reduced modulus (E_r). These findings provide critical insights for designing sub-10-μm scale TLP-bonded microbumps in advanced packaging.