A low thermal conductivity material layer covers a peripheral portion of the bottom surface of the conductive plate of a chip bonder head. The center portion of the conductive plate is exposed or covered with another conductive plate laterally surrounded by the low thermal conductivity material layer. During bonding, the chip bonder head holds a first substrate upside down and heats the first substrate through the conductive plate. Heating of a fillet, i.e., the laterally extruding portion, of a pre-applied underfill material is reduced because the temperature at the exposed surfaces of the low thermal conductivity material layer is lower than the temperature at the bottom surface of the conductive plate. The longer curing time and the more uniform shape of the fillet in the bonded structure enhance the structural reliability of the bonded substrates.