Nucleation, grain orientations, and microstructure of Sn-3Ag-0.5Cu soldered on cobalt substrates

Abstract

The potential of cobalt substrates to control the microstructure of 550 μm Sn-3Ag-0.5Cu ball grid array (BGA) joints is explored. It is shown that cobalt substrates give a small and reproducible nucleation undercooling for tin and prevent the formation of large primary Ag3Sn blades and Cu6Sn5 rods. βSn dendrites grew from the CoSn3 reaction layer and the βSn grains are shown to inherit their orientation from the CoSn3 layer due to heterogeneous nucleation with the following orientation relationship: (100)Sn||(100)CoSn3 with [001]Sn||[001]CoSn3. Changes in the dendrite and eutectic microstructure are shown to be caused by the reduced nucleation undercooling for βSn and the altered solidification path due to cobalt substrate dissolution

Nucleation and twinning in tin droplet solidification on single crystal intermetallic compounds

“…. Many of the droplets on these IMCs formed microstructures that are analogous to those widely reported in Sn-Ag and Sn-Ag-Cu solder joints on Cu or Ni substrates[1,[3][4][5][6][7][8][56][57][58][59][60][61]. For example, the single-grain, cyclically-twinned and interlaced-twinned structures in the droplets inFigure 4,Figure 6, andFigure 8.…”

“…This commonly generates βSn cyclic twins involving {101} or {301} type twins with 57.2° and 62.8° rotations around <100> respectively [1]. In the case of Sn-Ag and Sn-Ag-Cu, solidification twinning creates beachball and interlaced βSn microstructures [1][2][3][4][5] whereas, in Sn-Cu and Sn-Cu-Ni, twinning usually creates βSn plate structures [1,3,[6][7][8][9]. Similar twinning also occurs in Sn-Zn solders [10].…”

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

“…For example, the coefficient of thermal expansion 2,3 and solute diffusivity 4 vary between the a-direction and the c-direction of bSn. In leadfree solder joints, bSn usually presents only a few grain orientations, with single grain joints [5][6][7] and three grain morphologies 1,5,[8][9][10][11] frequently reported in Sn-Ag-Cu (SAC) solders. The three orientations of bSn are related via cyclic twinning around a common h100i direction, 1,6 and seem to come from a single nucleation event.…”

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

“…Sn-Ag-Cu (SAC) solders are common alternatives to eutectic Sn-Pb solders and have been intensively studied in recent decades [1][2][3][4][5]. A concern of SAC solders is the large melt undercooling prior to β-Sn nucleation (~10-40 K), which induces large primary Cu 6 Sn 5 rods [6] and Ag 3 Sn plates [7], that can lead to poor mechanical behaviour of solder joints [8][9][10].…”