Microstructure and shear strength evolution of Sn-Ag-Cu solder bumps during aging at different temperatures
Flip-chip devices with Sn-3.8Ag-0.7Cu solder on electroless Ni (EN) without immersion Au were studied after aging at different temperatures. The (Cu,Ni)6Sn5 intermetallics (IMCs) growth was volume diffusion controlled and Kirkendall voids were found in the Ni3P layer even at the initial stage of high-temperature aging due to the faster diffusion of Ni in the Ni3P layer via its column structure boundaries. The Ag3Sn IMCs were distributed in the bulk solder, existing as plate- or lamella-like phases or as small particles around the β-Sn dendrites, and the (Cu,Ni)6Sn5 IMCs existed as facet-like phases. The plate- and lamella-like Ag3Sn phases break up into small parts and these broken parts, together with small Ag3Sn particles, coarsen into pebble-like phases during high-temperature aging. Shear tests showed that all the solder bumps fractured in the bulk solder. The shear strength of solder bumps decreases at the initial stage of aging at 150°C and 175°C, and the strength degradation during aging may be caused by the coarsening of small Ag3Sn particles.