Microstructural evolution and change in macroscopic physical properties of microscale flip chip Cu/Sn58Bi/Cu joints unde
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Microstructural evolution and change in macroscopic physical properties of microscale flip chip Cu/Sn58Bi/Cu joints under the coupling effect of electric current stressing and elastic stress Shui-Bao Liang1, Chang-Bo Ke1, Cheng Wei2, Jia-Qiang Huang1, Min-Bo Zhou1, Xin-Ping Zhang1,a) 1
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China; and School of Civil Engineering and Transportation, South China University of Technology, Guangzhou 510640, China a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 2 March 2019; accepted: 24 May 2019
Severe phase coarsening and separation in Sn–Bi alloys have brought increasing reliability concern in microelectronic packages. In this study, a phase field model is developed to simulate the microstructural evolution and evaluate the change in macroscopic physical properties of the flip chip Cu/Sn58Bi/Cu joint under the conditions of isothermal aging, as well as the coupled loads of elastic stress and electric current stressing. Results show that large-sized Bi-rich phase particles grow up at the expense of small-sized ones. Under the coupled loads, Bi atoms migrate along the electron flow direction, consequently Bi-rich phase segregates to form a Bi-rich phase layer at the anode. The current crowding ratio in the solder decreases rapidly first and then fluctuates slightly with time. Current density and von Mises stress exhibit inhomogeneous distribution, and both of them are higher in the Sn-rich phase than in the Bi-rich phase. Electric current transfers through the Snrich phase and detours the Bi-rich phase. As time proceeds, the resistance of the solder joint increases, and the average von Mises stress of the solder joint decreases. The Bi-rich phase coarsens much faster under the coupled loads than under the conditions of isothermal aging.
Introduction In microelectronic products and systems, solder joints (interconnects) have been extensively used to act as physical, mechanical, and electrical connections [1]. Recently, there is an increasing trend to use Sn–Bi-based alloys as solders or as the important elements in Sn–Bi–X composition mixed solders to improve the microstructure and mechanical properties of solder joints [2, 3]. Sn–Bi-based alloys have salient features [4, 5, 6], such as lower melting temperature and lower coefficient of thermal expansion, in addition to the merits of lead-free and low cost. Moreover, owing to the relatively high processing temperature of mainstream lead-free solders (such as Sn–Ag– Cu and Sn–Ag-based solders) and increasing demand for multistep reflow soldering assemblies, in recent years there has been an increase in use of Sn–Bi-based solders in
ª Materials Research Society 2019
microelectronic packaging [5, 7]. In general, the eutectic alloys have features of low melting temperature and good liquidity; among binary Sn–Bi alloys, the eutectic Sn–Bi
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