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 degrees C to 300 degrees C, the electroless-plated Ni-5 wt%B alloy film showed a marked increase in tensile stress at approximately 270 degrees 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 degrees C to 30 degrees C, after heating to 300 degrees 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 degrees 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. (C) The Author(s) 2018. Published by ECS.