Grain refinement of electronic solders: The potential of combining solute with nucleant particles

摘要

Sn-Ag-Cu electronic solder joints typically solidify from a single nucleation event producing either a single beta Sn grain or twinned grains. With so few and variable beta Sn orientations, each joint is mechanically unique due to the anisotropy of tetragonal beta Sn. Here we explore the potential of increasing the number of beta Sn orientations in solder joints by promoting heterogeneous nucleation during solidification. It is shown that large (60 g) samples can be grain refined effectively by combining growth restricting solute with potent heterogeneous nucleant particles introduced by alloying additions of Zn, Ti, Co, Ir, Pd, or Pt. These mechanisms are discussed with reference to the grain refinement of structural casting alloys. In 500 mm solder balls, these grain refinement approaches were less effective and a relatively high cooling rate of 17 K/s combined with solute and nucleant particles were required to trigger multiple nucleation events. With this approach, up to 12 independent grains formed in 500 mu m balls of Sn-3Ag-0.5Cu alloyed with Pt, Co, or Ti, compared with one independent grain in balls without nucleant particle additions. (C) 2017 Elsevier B.V. All rights reserved.