Microstructures and shear strengths of W-ball solder joints aged under isothermal and cyclic thermal conditions

The thermal reliability, which results from CTE (coefficient of thermal expansion) mismatch among packaging materials, is essential to BGA (ball grid array) packaging electronic components. For reducing the CTE mismatch, tungsten ball BGA packaging was proposed in the present study. Core-shell-structured W@Cu balls were fabricated by copper electroplating on surfaces of tungsten balls. Single-ball joint specimens were made by reflow soldering a sandwich-structured Cu/W@Cu/Cu using Sn-Ag-Cu (SAC) solder paste. Two kinds of thermal aging treatments, namely isothermal aging and cyclic thermal aging, were conducted. The microstructure and shear strength of the joints were investigated. The results show that the average shear strength of the as-soldered single-ball joint samples is 57.9 MPa. It drops to 36.8 MPa and 33.4 MPa after isothermal aged at 170 degrees C for 250 h and cyclic thermal aged for 500 cycles, respectively. Cu6Sn5 IMC layer forms at boundaries of solder/Cu and grows with either isothermal aging time or cyclic thermal aging time. The roughness of the IMC increases with cyclic thermal aging time, but it changes little during isothermal aging process. The mechanism of microstructure and property evolution of single-ball solder joints under thermal aging have been briefly discussed.

Automated summary

In this study, tungsten ball BGA packaging was proposed to reduce the CTE mismatch between packaging materials. Core-shell-structured W@Cu balls were fabricated using copper electroplating. Single-ball joint specimens were made and thermal aging treatments were conducted. The results show that the shear strength decreases after thermal aging and the formation and roughness of Cu6Sn5 IMC layer were observed.