Effect of Cu in Pb free solder ball on the microstructure of BGA joints with Au/Ni coated Cu pads

Effect of Cu in the eutectic Sn-37 mass%Pb or Sn-3..5 mass%Ag solder ball has been investigated on the microstructure and the shear strength of EGA joints with Ni/Au electroplated Cu pad after reflow and the subsequent heat exposure at 423 K. For joints using both Cu free Sn-Pb and Sn-Ag solder balls, a metastable (Au, Ni)Sn-3 reaction layer was formed on the interface between the solder and the pad after reflow soldering and the subsequent heat storage at 423 K. while stable eta'-(Au,Cu,Ni)(6)Sn-5 reaction layer was formed for the joints using Cu containing solder balls. The growth rate during heat exposure was much slower for eta' reaction layer than that for ( Au, Ni)Sn-3 reaction layer. All the joints after a heat exposure fractured on the lower shear load than as reflowed joints. but the shear strength ol. the joint using Cu containing ball was less degraded than that using Cu free solder balls, because eta' reaction layer grows slower and plays a roll as a good barrier layer to suppress the Ni-Sn interaction. Almost the same effect was confirmed for the EGA joints using Cu cored solder bulls, since Cu dissolves into the solder during reflow soldering.