The Study Of Pulse Electroplating Of Au-Sn Bumps

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 t_on=3ms,t_off=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.