Flip chip joining with low temperature solders and thermal gradient bonding
In this study, we investigated effects of thermal gradient of joint during flip chip bonding on intermetallic compound (IMC) growth for several solder materials. Cu/Sn-2.5Ag/Cu joint was prepared by bonding electroplated Cu/Sn-2.5Ag bump on Cu pad. Ni/Sn/Ni, Ni/In/Ni, Ni/In-48Sn/Ni, and Ni/Sn-58Bi/Ni joints were prepared by solder capping by IMS on electro-less plated Ni/Au post and bonding it on electro-less plated Ni/Au pad. Joint pitch and diameter were 80 micron and 38 micron respectively. Joints prepared by thermal gradient bonding (TGB) were compared with those by homogeneous temperature bonding (HTB). With Cu/Sn-2.5Ag/Cu joints, firstly we confirmed faster growth of IMCs at a cold end than a hot end of the joint and thicker IMC in the TGB joint than in the HTB joint. These important features of TGB were also confirmed experimentally on joints with low temperature solder materials. It indicates that TGB can be used for the joints with low temperature solders to shorten bonding time for full IMC joint. IMC growth at the cold end of Ni/In/Ni joint prepared by TGB was as fast as that of Ni/Sn/Ni joint by HTB.