Lead-Free Soder Alloys

A Compatibility Evaluation of Lead-Based and Lead-Free Solder Alloys in Conjunction with Electroless Nickel/Immersion Gold Flip Chip UBM

It has been previously demonstrated that the utilization of electroless nickel/immersion gold under-bump-metallurgy (UBM) in conjunction with solder paste stencil printing provides a reliable, cost effective wafer bumping technology capable of high volume production. Furthermore, the combination of electroless nickel UBM with stencil printing allows for an efficient means of evaluating and bringing to production various solder alloys with the same equipment set. In the current investigation two test vehicles have been successfully bumped with both 63Sn/37Pb and 90Pb/10Sn lead-based solder alloys, as well as the 95.5Sn/3.8Ag/0.7Cu lead-free alloy. In order to evaluate the compatibility of these alloys with the electroless nickel UBM, solder bump shear tests were performed as a function of number of reflows. Results show no degradation in shear load or failure mode among all three of the alloys tested, indicating no critical UBM consumption (i.e., excessive intermetallic growth) during reflow. Additional tests were performed comparing nickel UBM thicknesses of 1, 2 and 5 ìm. Again, no critical UBM consumption was detected.

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