Microstructure and Damage Evolution During Thermal Cycling of Sn-Ag-Cu Solders Containing Antimony

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https://doi.org/10.1007/s11664-020-08507-x Ó 2020 The Author(s)

TMS2020 MICROELECTRONIC PACKAGING, INTERCONNECT, AND PB-FREE SOLDER

Microstructure and Damage Evolution During Thermal Cycling of Sn-Ag-Cu Solders Containing Antimony S.A. BELYAKOV ,1,5 R.J. COYLE,2 B. ARFAEI,3,4 J.W. XIAN,1 and C.M. GOURLAY1 1.—Department of Materials, Imperial College, London SW7 2AZ, UK. 2.—Nokia Bell Labs, Murray Hill, NJ, USA. 3.—Binghamton University, Binghamton, NY, USA. 4.—Ford Motor Company, Palo Alto, CA, USA. 5.—e-mail: [email protected]

Antimony is attracting interest as an addition to Pb-free solders to improve thermal cycling performance in harsher conditions. Here, we investigate microstructure evolution and failure in harsh accelerated thermal cycling (ATC) of a Sn-3.8Ag-0.9Cu solder with 5.5 wt.% antimony as the major addition in two ball grid array (BGA) packages. SbSn particles are shown to precipitate on both Cu6Sn5 and as cuboids in b-Sn, with reproducible orientation relationships and a good lattice match. Similar to Sn-Ag-Cu solders, the microstructure and damage evolution were generally localised in the b-Sn near the component side where localised b-Sn misorientations and subgrains, accelerated SbSn and Ag3Sn particle coarsening, and b-Sn recrystallisation occurred. Cracks grew along the network of recrystallised grain boundaries to failure. The improved ATC performance is mostly attributed to SbSn solidstate precipitation within b-Sn dendrites, which supplements the Ag3Sn that formed in a eutectic reaction between b-Sn dendrites, providing populations of strengthening particles in both the dendritic and eutectic b-Sn. Key words: Pb-free solder, accelerated thermal cycling, electron backscatter diffraction, thermal fatigue

INTRODUCTION Accelerated thermal cycling (ATC) test programs have shown that Sn-Ag-Cu solders can outperform the Sn-37Pb solder they were designed to replace.1 However, various applications now require solder joints to operate under harsher conditions, cycling through wider temperature ranges, with higher maximum temperatures and/or longer dwell times. It has been found that the beneficial effects of Ag on ATC reliability diminishes as the severity of the thermal cycle increases,1 making Sn-Ag-Cu solders unsuitable for some emerging applications in, for example, the automotive, aerospace and military sectors. To address this, a third generation of Pb-

(Received July 14, 2020; accepted September 21, 2020)

free solders is under development where the alloy design approach has been to provide additional strengthening mechanisms, including solid solution strengthening and improved precipitation strengthening.2–4 Many of the third-generation Pb-free solders are complex multicomponent alloys based on near-eutectic Sn-Ag-Cu compositions with significant additions of Bi, Sb and/or In, often with a combined Bi + Sb + In content of 3.5–6.5 wt.%.2 In this paper, we focus on the influence of a 5.5 wt.%Sb addition to a near-eutectic Sn-Ag-Cu solder to build the understanding of its effect on m