Origin of Primary Cu6Sn5 in Hypoeutectic Solder Alloys and a Method of Suppression to Improve Mechanical Properties

• Published in: Journal of electronic materials Vol. 50; no. 3; pp. 710 - 722
• Main Authors: Muhd Amli, S. F. N., Mohd Salleh, M. A. A., Ramli, M. I. I., Yasuda, H., Chaiprapa, J., Somidin, F., Shayfull, Z., Nogita, K.
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.03.2021; Springer Nature B.V
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Copper; Dissolution; Electronics and Microelectronics; Emerging Interconnection Technology; Instrumentation; Interconnect; Materials Science; Mechanical properties; Microstructure; Nucleation; Optical and Electronic Materials; Pb-free Solder; Preservatives; Shear tests; Solderability; Soldered joints; Soldering; Solders; Solid State Physics; Submerging; Substrates; Surface finish; Synchrotrons; Tin; Tin base alloys; TMS2020 Advanced Microelectronic Packaging; TMS2020 Microelectronic Packaging; intermetallics; primary Cu; Sn; synchrotron; Soldering; surface finish
• ISSN: 0361-5235; 1543-186X
• DOI: 10.1007/s11664-020-08428-9

This study examines factor(s) behind the formation of primary Cu 6 Sn 5 (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 Cu 6 Sn 5 . 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 Cu 6 Sn 5 . The ENIG finish prevented Cu dissolution and the formation of Cu 6 Sn 5 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 Cu 6 Sn 5 and how it can be suppressed to improve joint strength.