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.