Effect of Trace Addition of In on Sn-Cu Solder Joint Microstructure Under Electromigration
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https://doi.org/10.1007/s11664-020-08602-z Ó 2020 The Minerals, Metals & Materials Society
TMS2020 MICROELECTRONIC PACKAGING, INTERCONNECT, AND PB-FREE SOLDER
Effect of Trace Addition of In on Sn-Cu Solder Joint Microstructure Under Electromigration MARION BRANCH KELLY,1 ARAVINDHA ANTONISWAMY,2 RAVI MAHAJAN,2 and NIKHILESH CHAWLA1,3,4 1.—Center for 4D Materials Science, Arizona State University, Tempe, AZ 85287, USA. 2.—Intel Corporation, Santa Clara, USA. 3.—Present address: School of Materials Engineering, Purdue University, West Lafayette, IN 47906, USA. 4.—e-mail: [email protected]
Recently In has been considered as an additional alloying element in Sn-rich solders primarily due to its abilities to decrease the solder melting temperature and to modify mechanical properties and microstructure. While In is an attractive candidate for addition to solder, its effect on solder microstructure is not well understood. In order to study the effect of minor In additions on Snrich solder alloys, solder joints were prepared using Sn-0.7 wt.% Cu and Sn0.7 wt.% Cu- < 1 wt.% In alloys. Thermal aging and electromigration testing were done, followed by post-mortem microstructure characterization including composition, morphology, and grain structure. The addition of In did not appear to affect the microstructure under thermal aging conditions, but slowed interfacial intermetallic growth under electromigration, particularly of the compound Cu6Sn5. Transmission electron microscope analysis revealed the formation of Cu7In3 IMC nanoparticles, which were semicoherent with the surrounding Sn matrix. Key words: Indium, lead-free solder, electromigration, TEM, intermetallics, thermal aging
INTRODUCTION In-containing alloys have gained popularity as a possible replacement for conventional Sn-Ag-Cu solders because of their low melting temperatures,1–5 high shear and bending strengths,6,7 greater hardness,1,2 greater creep resistance,1,8 and suppression of interfacial intermetallic compound (IMC) growth.7,9 Additions of In into Sn-Cu, Sn-Ag and Sn-Ag-Bi solders showed the improved behaviors listed above, but the precise mechanism behind how In causes these changes is not well understood. It has been shown that in eutectic SnIn, In segregates to the cathode interface under electromigration (EM) due to the back stress created by the Sn atoms migrating towards the cathode, in
(Received May 26, 2020; accepted November 3, 2020)
the direction of the electron flow.10,11 However, at lower concentrations In no longer segregates, instead forming IMC compounds such as Cu6(Sn, In)5 and Cu3(Sn, In) that are both at the solder/pad interfaces and throughout the bulk of the joint.1,6,7,10,12 At the interfaces, the addition of In has been shown to suppress the formation of Cu3Sn7,9 during thermal aging, while bulk IMC particles may impede Cu diffusion along grain boundaries.12 In additions have also been seen to cause grain refinement, creating a solder joint with a random grain structure and orientation.12 This randomized texture slows bulk diffusion
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