Effect of aging conditions on interfacial reaction and mechanical joint strength between Sn-3.0Ag-0.5Cu solder and Ni-P UBM

The interfacial microstructure of Sn-3.0Ag-0.5Cu solder with electroless Ni/immersion Au (ENIG) was studied using SEM, EPMA and TEM. (Cu,Ni)6Sn5 intermetallic compound layer was formed at the interface between the solder and Ni-P UBM upon reflow. However, after isothermal aging the AuSn4, 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 in TEM revealed that the averaged composition of the P-rich layer is close to that of a mixture of Ni3P and Ni, while that of the Ni-Sn-P layer is analogous to the P-rich layer but containing a small amount of Sn in it. Shear force of the flip chip solder joints decreased during aging at various temperatures. The decrease in shear force was mainly caused by the coarsening of microstructure within the solder and increase of the interfacial intermetallic compound thickness. Nearly all of the test specimens showed ductile failure mode and this could be well explained with the results of FEM analyses.