Research On The Fabrication And Reliability Of Alloy Bumps Based On Multilayer Electroplating

Nowadays the requirement for electronic packaging industry is higher and higher since the development of electronic products is lighter, thinner and smaller. In order to meet the demands, three-dimensional(3D) packaging and flip chip technology are developed. Both of them require fine and reliable solder bumps to achieve mechanical joining, electrical interconnection and heat dissipation. There are several methods to form solder bumps, but as the pitch is decreasing, electroplating becomes the most suitable way to fabricate solder bumps. Because it could fabricate bumps with small size, fine pitch and low cost. However, it’s difficult to control composition precisely through alloy plating in one single bath due to the relatively large gap in standard electrochemical reduction potentials between different elements. Therefore, multilayer electroplating method, instead, is proposed to form alloy bumps with precise composition.Multilayer electroplating method suggests that each alloy element is electroplated separately and sequentially with settled proportion, and then form uniform alloy bumps after reflowing. In this study, Sn-3.5Ag and Sn-9Zn alloy bumps were formed through multilayer electroplating. They were examples of alloy bumps to verify that multilayer electroplating was a suitable method to make alloy solder bumps. Meantime, the effect of electroplating layer structure and reflow time on shear properties and interfacial microstructure of alloy bumps were investigated.The main results are summarized as follows.(1) Sn-3.5Ag solder bumps with smooth surface were obtained successfully using multilayer electroplating method. The shear strength of Sn-3.5Ag solder bumps peaked at the reflow time of 60 s, and then decreased with further increasing reflow time. As the reflow time increasing, the grain size of Cu6Sn5 increased while the morphology of Cu6Sn5 IMC changed from scallop-like type to the hexagonal and rod-like type.(2) The bumps with Sn/Ag structure displayed the highest shear strength among different electroplating layer structures. For Sn/Ag structure, the morphology of Cu6Sn5 was fat, rod-like and continuous. While when there was Ag electroplated between Sn and Cu(Ag/Sn and Ag/Sn/Ag structures), the morphology of Cu6Sn5 was thin, hexagonal and scattered.(3) Sn-9Zn solder bumps were obtained through multilayer electroplating method, but the surface of bumps was not smooth with lots of oxide. For Sn/Zn electroplating layer structure, the shear strength of Sn-9Zn solder bumps was decreasing with increasing reflow time, and the fracture mode was ductile. In the contrast, for Zn/Sn structure, there was no obvious changing trend for shear strength with increasing reflow time, and the shear strength was far lower than that of Sn/Zn structure. The fracture happened at the interface with lots of IMC and voids on the fracture surface.(4) For various electroplating layer structures, the composition of IMC at the Sn-9Zn/Cu interface was different. But for all the structures, the thickness of IMC increased with increasing reflow time. When the reflow time increased to 20 min, the thickness was dropped a little and its morphology became more fluctuated and loosen.