Lead Free SnAg Solder Bumping with Size sub 100 Microns

Abstract

Sub 100 micron lead-free solder bumping and its related interconnection & reliability is becoming one of the important issues in today's electronic packaging industry. In this paper, a SnAg solder bumping area-array of 13 times 13 with the bump size less than 100mum on the TiW/Cu UBM was investigated. Bumps with smooth and shiny surface of 70mum in the height 90mum in the diameter were fabricated and reliability of SnAg solder bumps were evaluated by shear tests and multireflows. The microstructure and the microvoids at the interface of the solder bumps as a function of the reflow cycles were studied. Results show that Ag element uniformly distributes in SnAg solder bumps containing 2.98 wt% Ag. The total thickness of IMC layer slightly increases and the microvoids grow up with reflow times going on. The intermetallic compound (IMC) of Cu3Sn/Cu6Sn5 with the ratio of 22.2/77.8 forms between Cu and Cu6Sn 5 after 13 times reflow treatment. It is found that the growth of the IMC layer is determined by reaction diffusion at the initial reflow. There are two kinds of sizes of voids observed in solder bump with 13 times reflow. One with size of 1~3 mum mainly exists in Cu6Sn5 or at the interface between Cu6Sn5 and solder due to congregation of kirkendall voids driven by stress caused by phase transformations; Others with size less than 300 nm mainly existing at the interface between Cu6Sn5 and Cu due to the consumption of Sn when forming Cu3Sn, then leaving the microvoids. Therefore, we can infer that the void growth can be controlled by adjusting reflow process, such as decreasing the reflow time when the temperature higher than 150 degC, or annealing at a certain temperature after intial reflow. The average shear strength of solder bumps on TiW/Cu UBM is about 80N/mm after initial reflow. The shear strength increases by about 35 % keeping up to 108.16N/mm2, and shear plane of bumps is a ductile deformation after 13 times reflow