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 h as been investigated on the microstructure and the shear strength of EGA jo ints with Ni/Au electroplated Cu pad after reflow and the subsequent heat e xposure at 423 K. For joints using both Cu free Sn-Pb and Sn-Ag solder ball s, a metastable (Au, Ni)Sn-3 reaction layer was formed on the interface bet ween the solder and tile pad after reflow soldering and the subsequent heat storage at 423 K. while stable eta'-(Au,Cu,Ni)(6)Sn-5 reaction layer was f ormed for the joints using Cu containing solder balls. The growth rate duri ng 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. t he 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 ef fect was confirmed for the EGA joints using Cu cored solder bulls, since Cu dissolves into the solder during reflow soldering.