Origin of Primary Cu 6 Sn 5 in Hypoeutectic Solder Alloys and a Method of Suppression to Improve Mechanical Properties
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https://doi.org/10.1007/s11664-020-08428-9 Ó 2020 The Minerals, Metals & Materials Society
TMS2020 MICROELECTRONIC PACKAGING, INTERCONNECT, AND PB-FREE SOLDER
Origin of Primary Cu6Sn5 in Hypoeutectic Solder Alloys and a Method of Suppression to Improve Mechanical Properties S.F.N. MUHD AMLI,1 M.A.A. MOHD SALLEH ,1,6 M.I.I. RAMLI,1 H. YASUDA,2 J. CHAIPRAPA,3 F. SOMIDIN,1,5 Z. SHAYFULL,4 and K. NOGITA5 1.—Center of Excellence Geopolymer & Green Technology (CeGeoGTech), School of Materials Engineering, Universiti Malaysia Perlis (UniMAP), Taman Muhibbah, 02600 Jejawi, Arau, Perlis, Malaysia. 2.—Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. 3.—Synchrotron Light Research Institute, Muang, Nakhon Ratchasima 3000, Thailand. 4.—School of Manufacturing Engineering, Universiti Malaysia Perlis (UniMAP), Pauh Putra Campus, 02600 Jejawi, Arau, Perlis, Malaysia. 5.—Nihon Superior Centre for the Manufacture of Electronic Materials (NS CMEM), School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. 6.—e-mail: [email protected]
This study examines factor(s) behind the formation of primary Cu6Sn5 (in the bulk, rather than at the interface) in solder joints, even though solder alloys are hypoeutectic. To understand the contribution from copper (Cu) dissolution from the substrate a Cu-free alloy, tin-3.5 silver (Sn-3.5Ag), was used as a soldered-on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish substrates. Microstructure observations including in situ synchrotron were used to observe microstructure development real-time and confirm the time and location for nucleation of primary Cu6Sn5. High-speed shear tests were performed to determine the solder joint’s strengths. The results confirm that Cu dissolution during soldering is responsible for the formation of primary Cu6Sn5. The ENIG finish prevented Cu dissolution and the formation of Cu6Sn5 resulting in higher solder joint strength for the Sn-3.5Ag/ENIG solder joints. The findings can be used to understand the evolution of primary Cu6Sn5 and how it can be suppressed to improve joint strength. Key words: Soldering, intermetallics, surface finish, primary Cu6Sn5, synchrotron
INTRODUCTION In the electronic packaging industry, reflow soldering is used for surface mount assembly (SMT) to create excellent metallurgical bonds between the solder and substrate. During the soldering process, the tin (Sn)-containing solder will react with the copper (Cu) substrate to form copper–tin (Cu-Sn) intermetallic compounds (IMCs). An example of this
(Received May 10, 2020; accepted August 17, 2020)
is the Cu6Sn5 IMC forming as an interfacial layer, and upon solidification, the primary Cu6Sn5 in the solder matrix will form. Researchers have investigated the effects of using different surface finish substrates on the formation of intermetallics. Yoon et al. analysed the effect of electroless nickel immersion gold (ENIG) and copper
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