Fundamental study of the intermixing of 95Pb-5Sn high-lead solder bumps and 37Pb-63Sn pre-solder on chip-carrier substrates
Journal
Journal of Electronic Materials
Journal Volume
38
Journal Issue
11
Pages
2234-2241
Date Issued
2009
Author(s)
Abstract
This study investigated the intermixing of 95Pb-5Sn solder bumps and 37Pb-63Sn pre-solder in flip-chip solder joints. The reaction conditions included multiple reflows (up to ten) at 240°C, whereby previously solder-coated parts are joined by heating without using additional solder. We found that the molten pre-solder had an irregular shape similar to a calyx (i.e., a cup-like structure) wrapped around a high-lead solder bump. The height to which the molten pre-solder ascended along the solid high-lead solder bump increased with the number of reflows. The molten pre-solder was able to reach the under bump metallurgy (UBM)/95Pb-5Sn interface after three to five reflows. The molten pre-solder at the UBM/95Pb-5Sn interface generated two important phenomena: (1) the molten solder dewetted (i.e., flowed away from the soldered surface) along the UBM/95Pb-5Sn interface, particularly when the number of reflows was high, and (2) the molten pre-solder transported Cuatoms to the UBM/95Pb-5Sn interface, which in turn caused the Ni-Sn compounds at the chip-side interface to change into (Cu 0.6Ni 0.4) 6Sn 5. © 2009 TMS.
Type
journal article