Temperature Gradient Induced Orientation Change of Bi Grains in Sn-Bi57-Ag0.7 Solder Joint

doi:10.1007/s40195-021-01357-4

Abstract

Abstract:

Sn-Bi-X solders are widely used in electronic packaging industry. However, thermomigration (TM) behaviors of Sn-Bi-X solder joints and the orientations change of Bi grains under the temperature gradient are rarely reported. In this study, Sn-Bi57-Ag0.7/Cu solder joints were used to conduct a TM test under a temperature gradient of 625 °C/cm for 400 h, and an isothermal aging test at 85 °C was also conducted for comparison. The microstructural evolution of Sn-Bi-X solder joints after reflow, TM and isothermal aging were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA). The results indicated that the Sn/Bi areal ratio after TM did not change significantly whether at the hot end (from 46.78%/52.12% to 50.90%/48.78%) or at the cold end (from 50.25%/49.64% to 48.71%/51.16%) compared with that of as-reflowed samples due to the insufficient thermal energy. The thickness of intermetallic compound (IMC) after TM at hot end (2.49 μm) was very close to that of the IMC at cold end (2.52 μm), which was also close to that of the aged samples. In addition, the preferred orientations of Sn and Bi grains in different Sn-Bi-Ag solder joints resulting from different conditions (reflow, TM and isothermal aging) were characterized by electron backscatter diffraction (EBSD). The obtained results demonstrated that both Sn and Bi grains had no preferred orientation whether after reflow or isothermal aging, while the orientation of Bi grains of the sample after TM changed from random direction to c-axis ([0001] direction) parallel to the heat flow. Ag₃Sn could hinder the change of orientation of Bi grains under the temperature gradient, and the corresponding mechanism was also systematically illuminated. This study firstly revealed the orientation change of Bi grains under the temperature gradient, which would have a profound guiding significance for enhancing the reliabilities of Sn-Bi-Ag solder joints.

Key words: Sn-Bi-Ag solder, Grain orientation, Temperature gradient, Aging, Ag₃Sn

Yinbo Chen, Zhaoqing Gao, Zhi-Quan Liu. Temperature Gradient Induced Orientation Change of Bi Grains in Sn-Bi57-Ag0.7 Solder Joint[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(7): 1184-1194.

Figures/Tables 13

Table 1 XRF results of Sn-Bi57-Ag0.7 solder alloy (wt%)

Ag	Sn	Bi	Co
0.679	41.943	57.376	0.002

Table 2 Test conditions

	Initial reflow	Test conditions
Thermomigration (TM)	180 °C for 30 s	Hot end: 110 °C for 400 h
Thermomigration (TM)	180 °C for 30 s	Cold end: 60 °C for 400 h
Isothermal aging	180 °C for 30 s	85 °C for 400 h

Fig. 1 Schematic diagram of TM setup and the sandwich-structure Sn-Bi-Ag sample was inserted under the temperature gradient

Fig. 2 Simulations of a the temperature distribution and b the temperature gradient from COMSOL

Fig. 3 a EPMA characterization of Sn-Bi-Ag solder joint, b-e distributions of Sn, Bi, Ag, Cu elements in the solder joint, f TEM selected area electron diffraction spectrum (TEM-SAED) of Cu6Sn5

Fig. 4 Sn-Bi-Ag solder joint after reflow at a the cold end and c the hot end, Sn-Bi-Ag solder joint after TM under 110 °C to 60 °C for 400 h at b the cold end and d the hot end

Fig. 5 Distributions of a β-Sn phase, b Bi phase in as-reflow and TM-tested solder joint near the cold end and the hot end

Fig. 6 Sn-Bi-Ag solder joint after reflow at a the cold end and d the hot end, Sn-Bi-Ag solder joint after isothermal aging under 85 °C for 400 h at b the cold end and e the hot end, Sn-Bi-Ag solder joint after TM under 110 °C to 60 °C for 400 h at c the cold end and f the hot end

Fig. 7 Changes in thickness of the IMCs in as-reflow, TM tested and aged Sn-Bi-Ag samples

Fig. 8 a OIM characterization of as-reflowed Sn-Bi-Ag sample and IPFs of b Sn grains and c Bi grains by EBSD analysis, d OIM characterization of the aged under 85 °C for 400 h solder joint and IPFs of e Sn grains, f Bi grains by EBSD analysis

Fig. 9 a Orientation and b IPF of Sn grains of the TM-tested samples, c orientation and d IPF of Bi grains of the TM-tested samples, e 0001 pole figure of Bi grains without Ag3Sn nearby

Fig. 10 a Orientation and b IPF of Sn grains of the TM-tested samples, c orientation and d IPF of Bi grains of the TM-tested samples, e 0001 pole figure of Bi grains near large lath-like Ag3Sn

Fig. 11 Schematic drawing illustrating the change of Bi grains in Sn-Bi-Ag solder joints after TM under 110 °C to 60 °C for 400 h: a without Ag3Sn nearby, b near lath-like Ag3Sn particle

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