The growth kinetics of intermetallic compound (IMC) layers formed between Sn-3.5Ag-0.75Cu BGA (ball grid array) solder and electroless Ni-P/Cu substrate by solid state isothermal aging were examined at temperatures between 70 and 170°C for 0 to 100 days. In the solder joints between the solder ball and electroless Ni-P/Cu pads, the IMC layer was (Cu,Ni)6Sn5. Also, a P-rich Ni layer formed at the interface between (Cu,Ni)6Sn5 and original Ni-P deposit layer because of the phosphorous accumulation. These IMC layer thicknesses increased linearly with the square root of aging time and the growth was faster for higher aging temperatures. On the contrary, the shear strength decreased with the increasing temperature and time. The growth of IMC layer was mainly controlled by diffusion-controlled mechanism over the temperature range studied. The apparent activation energy calculated for the growth of the (Cu,Ni)6Sn5 IMC was 69.75 kJ/mol.