A low cost method of solder bumping wafers is described using a solderable electroless nickel film plated over aluminum. Flip chip test die were then assembled to ceramic substrates using eutectic solder attachment. Similarly prepared assemblieswere made with an optimized sputtered trimetal under bump layer as a control group. Accelerated reliability tests were used to examine the impact of under bump metallurgy on product life. Electroless nickel parts exhibited manageable film stress,excellent solder wetting and a slow growth rate of intermetallics. In all cases, flip chip joints failed by metal fatigue of the solder, independent of the under bump metallurgy.
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