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

Abstract

Sn-Ag-Cu electronic solder joints typically solidify from a single nucleation event producing either a single βSn grain or twinned grains. With so few and variable βSn orientations, each joint is mechanically unique due to the anisotropy of tetragonal βSn. Here we explore the potential of increasing the number of βSn 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 μm 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 μm balls of Sn-3Ag-0.5Cu alloyed with Pt, Co, or Ti, compared with one independent grain in balls without nucleant particle additions.

Mechanisms of beta-Sn nucleation and microstructure evolution in Sn-Ag-Cu solders containing titanium

“…Ti 2 Sn 3 is as potent as βIrSn 4 , αCoSn 3 and PtSn 4 , since similar nucleation undercoolings have been measured in 550µm SAC305 freestanding solder balls with corresponding additions[24]. However, there are some interesting differences between the ORs involving Ti 2 Sn 3 and…”

“…Multiple independent grains correspond to multiple nucleation events, which indicates that Ti 2 Sn 3 particles triggered multiple nucleation during fast cooling. The mechanisms of grain refinement in SAC305 microalloyed with catalysts such as Ti, and the need for a high cooling rate, are discussed in more detail in references [3,24].…”

“…Microalloying has been extensively studied to suppress nucleation undercooling in SAC solders and joints. It has been shown that Co [3,[18][19][20][21][22], Zn [19,[22][23][24], Ti [24][25][26][27][28], Al [18,29], Pt [17,24,30], Pd [17,24,30], and Ir [17,24], all effectively suppress nucleation undercooling of SAC solders and joints. Among these, the underlying nucleation mechanisms of Co, Pt, Pd, and Ir additions have been identified in previous studies [3,17,30].…”

“…Although it has been shown that titanium additions can be as potent as Co, Pt, Pd, and Ir additions [24] [25][26][27][28] as indicated by the nucleation undercoolings in Table 1 and reports of grain refinement in reference [31], the mechanisms by which Ti has these effects are currently unclear. Past research has suggested that Ti 2 Sn 3 may be a nucleant [24,26] but this idea has not been tested.…”

On the 3-D Shape of Interlaced Regions in Sn-3Ag-0.5Cu Solder Balls

“…9,12 Electron backscatter diffraction (EBSD) techniques have been successfully used to identify crystal orientations and relationships between bSn grains, such as the common twinning axis. 1,7,13,14 A SAC solder joint with an interlaced microstructure, composed of many small bSn grains, shows increased mechanical properties, such as hardness and creep, 15 as well as better reliability compared to a beach-ball microstructure, by delaying crack initiation. 16 Furthermore, a solder joint of a single grain will have anisotropic mechanical properties, corresponding to bSn anisotropy, 17 and can result in early joint failure for certain grain orientations.…”