The effect of IMC microstructure of solder joint on the mechanical drop performance in SnxAgCu and SnAgCuX CSP package

With the RoHS deadline approaching by July 1, 2006, many manufacturers are aggressively eliminating the use of lead in consumer electronic products. However, the development of lead-free products requires close cooperation between end-product manufacturers and component manufacturers because metallurgies, resin materials, reflows conditions and moisture resistance will be influenced. The increase in reflow temperature is generally considered to be the most difficult issue associated to the conversion to SnAgCu solder interconnect. But right now the mechanical drop performance in SnAgCu solder joint has been becoming another attractive topic even being industry headache due to brittle characteristics of SnAgCu interconnect. In the study, one 14 times 14 lead free CSP BGA with 0.3 mm/0.5 mm ball diameter/pitch was adopted as test vehicle. The intermetallic compounds morphology evolution which can be generated when using SnxAgCu and SnAgCuX solder ball in combination with electrolytic NiAu-plated substrates was investigated during 150degC thermal aging, such as single layer SnNi IMC and complex layer SnNi/SnNiCu IMC in the interface through top and cross-section view, respectively. In addition, the intensity of plate-like Ag3Sn IMC formation in the SnxAgCu solder bulk from 1 to 4%Ag was observed by SEM as well. The CSP BGA package with above combination was assembled to PC boards with OSP finish using Sn3Ag0.5Cu solder paste under 245degC peak temperature reflow. The test vehicle assembled was subject to mechanical drop test following JESD22-B111 to evaluate the solder joint integrity after zero and 150degC/250hrs thermal aging. The effect of interfacial IMC morphology evolution and Ag3Sn intensity in solder bulk by x (Ag percentage) and X (forth element addition) on the mechanical drop performance is concluded. Furthermore, one approaching in IMC microstructure control to overcome SnAgCu drop concern will be presented