Research Papers

High-Frequency and Low-Temperature Thermosonic Bonding of Lead-Free Microsolder Ball on Silver Pad Without Flux

[+] Author and Article Information
Fuliang Wang

State Key Laboratory of High Performance
Complex Manufacturing,
School of Mechanical and Electrical Engineering,
Central South University,
Changsha, HN Province 410083, China
e-mail: wangfuliang@csu.edu.cn

Junhui Li, Lei Han

State Key Laboratory of High Performance
Complex Manufacturing,
School of Mechanical and Electrical Engineering,
Central South University,
Changsha, HN Province 410083, China

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received September 11, 2012; final manuscript received February 12, 2014; published online May 5, 2014. Assoc. Editor: Kaustubh Nagarkar.

J. Electron. Packag 136(3), 031001 (May 05, 2014) (4 pages) Paper No: EP-12-1083; doi: 10.1115/1.4026877 History: Received September 11, 2012; Revised February 12, 2014

Lead-free solder balls are environment friendly; however, they require a high bonding temperature, which causes problems in the microelectronics package industry. To reduce the bonding temperature, a 60 kHz high-frequency thermosonic bonding method is proposed and realized using a lab bonder. Experimental results showed that this method could be used to bond a 300 μm-diameter Sn–Ag–0.5Cu microsolder ball onto a silver pad without flux at a low temperature of 160 °C in 3 s. A ball shear test showed that the high frequency led to a high bonding strength of 58.8 MPa, and a dimpled structure was observed at the bonding interface by SEM. Compare with the reflow method or laser soldering method, the proposed method requires a low bonding temperature and leads to a high bonding strength.

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Fig. 1

Schematic representation of the experimental setup

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Fig. 2

Solder bumping experimental process, (a) place ball on silver pad, (b) predeform ball by bonding force, and (c) bond ball with silver pad using ultrasonic vibration

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Fig. 3

Shear testing after bonding

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Fig. 4

Effect of bonding time and ultrasonic power on shear strength

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Fig. 5

Effect of bonding time on the deformation of solder ball, (a) solder ball before bonding, (b) after bonding (time = 0.5 s), and (c) after bonding (time = 3 s)

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Fig. 6

SEM image of bonding interface (bonding time = 1 s). (a) Corelike bond interface on pad, and (b) details of the square area on pad.

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Fig. 7

SEM image of bonding interface: (a) bonding time = 3 s; (b) bonding time = 5 s




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