Feasibility and reliability study on the electroless nickel bumping and stencil solder printing for low-cost flip chip electronic packaging

We present a study on the feasibility and reliability of a low-cost flip chip technology using a combination of electroless nickel bumping and stencil solder printing. Circular aluminum bump pads of diameter 250 mum and pitch 400 mum with 200 mum openings in the passivation layer were patterned for the study. Since it has been suggested that phosphorous from the electroless nickel solution will degrade the adhesion of solder, three types of electroless nickel deposits were prepared as the under bump metallurgy (UBM) consisting of electroless nickel-high phosphorous (EN-HP), electroless nickel-low phosphorous (EN-LP) and electroless nickel-boron (EN-B). Eutectic lead-tin solder paste of composition 63Sn-37Pb was screen-printed on the electroless nickel UBM followed by reflow at a peak temperature of 230 degreesC. The final solder bump height achieved was 155 +/-8 mum. The shearing strength was found to decrease in the order of EN-B (185 +/-7.5 g/bump) > EN-LP (178 +/-8.7 g/bump) > EN-HP (170 +/-8.1 g/bump). Furthermore, multiple reflow caused a decrease of the shearing strength within 20%, indicating a satisfactory reliability of electroless nickel/stencil printed solder in wafer bumping. It was found by SEM/EDX analysis that the fracture occurred within the solder regardless of the type of electroless nickel. Further evaluation on bump height uniformity has indicated that the use of stencil printing technique for screen-printed bumps can be an effective low-cost method for fabrication of solder bumps on wafers.