The soldering behaviour of the eutectic Au-Sn alloy on two kinds of under bump metallurgy was studied in relation to time and temperature. For a Ni substrate, two types of the intermetallic compounds were observed at the joint: (Au,Ni)(3)Sn-2 and (Au,Ni)(3)Sn. As the soldering temperature increased, the shape of the (Au,Ni)(3)Sn-2 grains generally changed from a long, thin rod-type to a short, thick type. The degree of buildup of the interfacial intermetallic compounds was similar up to 32 min, even if the soldering was conducted at three different temperatures between 300 degrees C and 400 degrees C. In addition, the reaction of the eutectic Au-Sn solder with the sputtered under bump metallurgy (Al/ Ni(V)/Au) was studied at 300 degrees C. By 20 s of soldering, the protective Au layer was dissolved away and the Ni(V) layer started to dissolve into the solder. Thus, some of the Au reacted with the At underlayer to form the Au8Al3 phase, which was accompanied by volume expansion at the joint. The (Au,Ni)(3)Sn-2 layer was then lifted up, and several interlocked (Au,Ni)(3)Sn-2 grains were broken and separated at weak points along the joint interface. In this way, the joint interface was separated from the Si chip, and a resultant failure occurred in the device.