A Comparative Study on the Microstructure and Mechanical Properties of Cu 6 Sn 5 and Cu 3 Sn Joints Formed by TLP Solder
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INTRODUCTION
TO facilitate integration and miniaturization, the power density of electronic devices has been dramatically increasing, leading to increased junction temperatures. This is especially apparent in high-temperature power devices utilized in the fields of space exploration, petrochemical, electric power, and geological exploration, where junction temperatures resulting from power dissipation can exceed 300 C.[1–5] However, conventional bonding materials used in the packaging of electronic devices, such as Pb-free solders, typically have melting temperatures below 250 C, which can lead to solder interconnection failure if used in high-temperature power devices. Thus, the development of
H.Y. ZHAO, J.H. LIU, X.G. SONG, Y.X. ZHAO, and J.C. FENG are with the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China and also with the Shandong Provincial Key Lab of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China. Contact e-mails: [email protected], [email protected] Z.L. LI, H.W. NIU, H. TIAN, and H.J. DONG are with the Shandong Provincial Key Lab of Special Welding Technology, Harbin Institute of Technology at Weihai. Manuscript submitted December 14, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
suitable bonding materials or methods that can demonstrate reliable operation at high temperatures and in harsh environments remains a challenge. Recently, TLP soldering has been demonstrated as a potential interconnection method for fabricating joints with high melting points for application in the packaging of high-temperature power devices.[6–8] For Cu/Sn systems, the melted Sn interlayer reacts with Cu to form Cu-Sn intermetallic compounds (IMCs) (i.e., Cu6Sn5 and Cu3Sn with melting points of 415 and 676 C, respectively[9–11]). With the increasing reaction time, the Sn interlayer can be fully consumed and completely transformed to Cu-Sn IMCs. Unavoidably, the fabrication of complete Cu-Sn IMC joints is highly time consuming (> 60 minutes) in the TLP soldering process, which can introduce additional thermal stress, resulting in decreased reliability of packaging systems.[12] Therefore, the development of methods that can reduce the processing time for TLP soldering is desirable, not only for the reliability of packaging systems but also for economic viability of mass production. An effective approach to solve this challenge involves the use of ultrasonic waves during the TLP soldering process. Acoustic cavitation and streaming can occur when ultrasonic waves (USWs) are propagated in the liquid solder.[13–15] The solid–liquid interface reaction can be promoted by acoustic cavitation, leading to the
rapid formation of IMCs. The grain morphology and size of IMCs can also be modified. Therefore, it can be assumed that USWs strongly influence the growth mechanism of IMCs and consequently alter the microstructure of the resulting intermetallic joints, leading to mechanical property changes in the resulting joints. Clear exp
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