Intermetallic Compound Formation and Growth Kinetics in Flip Chip Joints Using Sn–3.0Ag–0.5Cu Solder and Ni–P under Bump Metallurgy

Abstract

The interfacial microstructure of Sn–3.0Ag–0.5Cu solder with ENIG (electroless Ni/immersion Au) UBM was studied using scanning electron microscopy and transmission electron microscopy. (Cu,Ni)6Sn5 intermetallic compound layer was formed at the interface between the solder and Ni–P under bump metallization upon reflow. However, after isothermal aging, some AuSn4 but with a certain amount of Ni dissolved in it, i.e., (Au,Ni)Sn4, appeared above the (Cu,Ni)6Sn5 layer. Two distinctive layers, P-rich and Ni–Sn–P, were additionally found from the TEM observation. The analytical studies using EDS equipped in TEM revealed that the averaged composition of the P-rich layer was close to that of a mixture of Ni3P and Ni, while that of the Ni–Sn–P layer was analogous to the P-rich layer containing a small amount of Sn in it. The thickness of the (Cu,Ni)6Sn5 layer increased with increasing aging time and temperature. The layer growth of the intermetallic compound satisfied a parabolic law in the given temperature range. The increase of the intermetallic compound layer thickness was mainly controlled by a diffusion mechanism in the temperature range studied, because the values of time exponent (n) were approximately equal to 0.5. The apparent activation energy for the growth of (Cu,Ni)6Sn5 intermetallic compound layer was 55 kJ/mol.