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

63Sn-37Pb solder bump was produced on Al terminal of Si. The bump pad dimension is 100 μm×100 μm while the pitch size is 250 μm. The bump pattern contains 20×20 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°C or 250°C. The reflow was conducted for 1, 5, and 10 cycles. Intermetallic compounds Ni₃Sn₂ , Ni₃Sn₄, and Ni₄Sn were formed between electroless nickel plating and solder when reflowed at 250°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 μm. No penetration of Cu occurs through a 10 μm electroless nickel during reflow. Sn and Al were not found to diffuse through the electroless nickel during reflow even for 1.8 μm of electroless nickel plating. The shearing strength of the solder bump reflowed at 210°C is 53.5±3.3 g/bump while the shearing strength is 62.9±2.8 g/bump when reflowed at 250°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±6.0 g/bump after ten cycles of reflow