Shearing strength and materials interaction during reflow of Al/Cu/electroless nickel/solder bump

Abstract

63Sn-37Pb solder bump was produced on Al terminal of Si. The bump pad dimension is 100 /spl mu/m/spl times/100 /spl mu/m while the pitch size is 250 /spl mu/m. The bump pattern contains 20/spl times/20 bumps. The UBM (under bump metallurgy) consists of Cu/electroless Ni. Cu was sputter deposited on Al film. The solder bump was electroplated on the electroless nickel followed by reflow at 210/spl deg/C or 250/spl deg/C. The reflow was conducted for 1, 5, and 10 cycles. Intermetallic compounds Ni/sub 3/Sn/sub 2/, Ni/sub 3/Sn/sub 4/, and Ni/sub 4/Sn were formed between electroless nickel plating and solder when reflowed at 250/spl deg/C. The interdiffusion behavior of the solder bump constituents within the solder bump was investigated by scanning electronmicroscope (SEM). Cu was found to penetrate the electroless nickel during reflow when the thickness of the electroless nickel plating is only 1.8 /spl mu/m. No penetration of Cu occurs through a 10 /spl mu/m electroless nickel during reflow. Sn and Al were not found to diffuse through the electroless nickel during reflow even for 1.8 /spl mu/m of electroless nickel plating. The shearing strength of the solder bump reflowed at 210/spl deg/C is 53.5/spl plusmn/3.3 g/bump while the shearing strength is 62.9/spl plusmn/2.8 g/bump when reflowed at 250/spl deg/C. The fracture after shearing test occurs within the solder. Repeating reflow tends to lower the shearing strength of the bump. The shearing strength of the bump becomes 37.6/spl plusmn/6.0 g/bump after ten cycles of reflow.