Thermal Expansion and Thermal Stress Behavior of Electroless-Plated Fe–Ni–B Alloy Thin Film for High-Density Packaging

Abstract

Fe–Ni–B alloy thin films were prepared by using an electroless plating method, and their thermal-expansion behavior, thermal-stress generation, and crystal structure were evaluated. By heating from 30°C to 300°C, the electroless-plated Ni–5 wt%B alloy film showed a marked increase in tensile stress at approximately 270°C, which is caused by a phase separation from the amorphous Ni–B alloy into a highly crystalline Ni and Ni3B compound. The coefficient of thermal expansion (CTE) of electroless-plated Ni–B alloy film from 300°C to 30°C, after heating to 300°C, was 14 ppm/K. In contrast, the electroless-plated Fe–Ni–B alloy film in the Invar composition range had a small CTE (8 ppm/K to 9 ppm/K) and no substantial change in crystal structure during heating to 300°C. As a result, the Fe–Ni–B alloy film on the silicon substrate (CTE = 3 ppm/K) exhibited a low thermal stress compared with the electroless-plated Ni–B alloy film. Thus, it is suggested that the electroless-plated Invar Fe–Ni–B alloy films are suitable as an under-bump metal layer from the viewpoint of the thermal-expansion behavior.