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The Study of Pulse Electroplating of Au-Sn Bumps

Author: GaoJieZuo
Tutor: HuangMingLiang
School: Dalian University of Technology
Course: Materials Science
Keywords: Flip Chip Au-Sn Bump Pulse Electroplating Au Electrodeposition Sn Electrodeposition Microstructure
CLC: TQ153
Type: Master's thesis
Year: 2009
Downloads: 128
Quote: 0
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Abstract


With the advantages of energy saving,environmental protection and long life,LED has developed rapidly theses years.Due to the invention of HB-LED,it has been entered lighting field.LED will gradually replace incandescent lamp and fluorescent lamp to be the next generation of light source.In practical applications,the main development trend is the replacement of low-power LED by high-power LED.In order to solve the problems of heat dissipation and luminescence efficiency of high-power LED,flip chip（FC） technology has become one of the most popular methods in the package of LED.Bumping is a key step in FC technology.Au-20wt.%Sn eutectic solder was chosen for the bumps material and sequential electroplating of Au and Sn was adopted to manufacture the bumps.This paper optimized the pulse electroplating parameters of Au and Sn in order to obtain fine deposits.Au/Sn dual-layer film was electroplated on Si wafer by pulse electroplating of Au and Sn sequentially.The solid-solid interracial reaction during aging and the eutectic reaction during reflowing were investigated.The results showed that:（1） The Au film electroplated by direct current had high porosity and bad uniformity.The pulse electroplating parameters were optimized to obtain grain-refined and homogeneous Au film.When ton=3ms,toff=3ms,if the current density varied from 4 mA/cm2 to 10mA/cm2,the grain size of the Au film firstly declined and then rised.There was a linear relationship between electroplating time and the thickness of Au film under the optimum electroplating condition.（2） Compared to pulse current,periodically reversed pulse current could improve the property of Sn films.There was a linear relationship between electroplating time and the thickness of Sn film under the optimum electroplating condition.（3）After storage at room temperature for one week,the reaction zone of Au/Sn（10μm/10μm） film was about 5μm thick.Three phases,AuSn,AuSn2 and AuSn4,were sequentially formed in the reaction zone from the Au side to the Sn side.Firstly,AuSn4 was formed at the Au/Sn interface,and then AuSn and AuSn2 were formed at the Au/AuSn4 interface.After aging at 150℃,the layered structure of AuSn/AuSn2/AuSn4 was observed, too.Due to the faster diffusion of Au to Sn,all of Sn was consumed after 15 hours and AuSn4 layer gradually transformed into AuSn and AuSn2 layers.After reflowing at 280℃for 10 seconds,the reaction zone of Au/Sn（9μm/6μm） films had a bamboo-shoot-like microstructure of Au5Sn/AuSn/AuSn2.After reflowing at 280℃for 60s,Au/Sn（9μm/6μm） films formed a typical two-phase（Au5Sn and AuSn） eutectic microstructure.

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