Increasing mechanical strength and refining grains of Cu-core solder joints with pressurized bonding
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Increasing mechanical strength and refining grains of Cu-core solder joints with pressurized bonding Rui-Wen Song1, Collin Jordon Fleshman1, Zih-You Wu1, Su-Yueh Tsai2, and Jenq-Gong Duh1,* 1
Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Sec. 2, Kuang-Fu Road, Hsinchu 30013, Taiwan 2 Precision Instrument Center, National Tsing Hua University, Hsinchu, Taiwan
Received: 17 September 2020
ABSTRACT
Accepted: 3 November 2020
For the advanced 3D-IC or 3D-packaging application, Cu-core solder joint is already a high reliability option for high power device and 3D packaging. However, with the harsher requirement for mechanical and electromigration reliability, it is imperative to further strengthen the solder joints. A method of pressurized bonding is proposed in this study to alter and improve the architecture configuration and grain distribution of the Cu-core solder joints. The correlation of strengthening process and microstructure was revealed by electron microscopy, elemental quantification, and grain analysis. Pressurized bonding results in a refined grain distribution, increased shear strength, and possibly stronger resistance to electromigration.
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Springer Science+Business
Media, LLC, part of Springer Nature 2020
1 Introduction For the advanced 3D-IC or 3D-packaging application, Cu-core solder joint is a high reliability option for heat dissipation in high power devices [1] or for prevention of bridging in fine-pitch bonding with heavy weight of components. Also, Cu-core joint has proven its higher resistance to electromigration [2, 3] and shear fracture [4], as compared to conventional solder joints. However, due to the configuration of Cu-core solders, the interfacial intermetallic compound (IMC) in the solder joints are typically (Ni,Cu)3Sn4 [3]. The impingement of Ni3Sn4 grains in confined space may lead to significant voids [5] and degrade the mechanical strength [6], and therefore, a
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https://doi.org/10.1007/s10854-020-04824-3
new method of pressurized bonding will be proposed to retain the fine-grain structure and the high fracture toughness of Ni3Sn4-based IMCs without formation of voids. Also, pressurized bonding would result in an architecture analogous to full-IMC joints, and therefore, the advantages of full-IMC joints, such as far better strength than conventional solder and electromigration resistance [7, 8], may exist in Cucore solder joints. In this study, mechanical pressure was applied during the bonding process to alter the grain and phase architecture of solder joints and to achieve higher mechanical strength compared to the regularly reflowed ones.
J Mater Sci: Mater Electron
2 Material and methods 300 lm Cu-core solder ball composed of Cu-core and electroplated Ni coating and then, subsequently, SAC305 solder shell was bonded with electroless nickel immersion gold (ENIG) surface finish for 5 min at 240 °C. The nominal thickness of Au in ENIG is 50 nm and the pad opening of ENIG is 25
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