Effect Of Diffusivity Anisotropy In ?-Sn Grain On The Electromigration Behavior Of Micro-solder Joints

With the demands of continuous miniaturization, high-performance and lead-free in microelectronics industry, the diameter of solder bumps is downsizing continuously and the current density through each solder bump increases sharply, causing electromigration (EM) to become a serious reliability issue. With the trend of continuously downsizing, the number of Sn grains in micro scale solder bumps is also decreasing to several or even only one. ?-Sn exhibits highly anisotropic behavior and such anisotropy is particularly pronounced for the diffusivity of Cu and Ni metal atoms. Therefore, it is vital to understand the effect of the anisotropy in ?-Sn grain on the EM behavior of micro bumps. To clearly reveal the dominant effect of anisotropy in ?-Sn grain on the EM-induced failure modes, cathode dissolution behavior and intermetallic compound (IMC) precipitation of micro-bumps, in situ observations of the Cu/Sn3.0Ag0.5Cu (SAC305)/Cu and Cu/Sn/Ni solder interconnects and Ni/SAC305/Cu flip chip solder joints were carried out in the present work.The main conclusions are drawn as follows:(1) For the EM behavior of Cu/SAC305 (two Sn grains)/Cu solder interconnects:when electrons flow from a small angle ??-Sn grain (between the c-axis of Sn grain and electron flow direction) to a neighboring large angle ?? -Sn grain, the failure mode is that the excessive dissolution of cathode Cu due to a larger Cu diffusion flux. The migration of dissolved Cu atoms toward the anode is retarded at the Sn grain boundary, resulting in massive precipitation of columnar Cu6Sn5 IMCs within the small angle ??-Sn grain. When electrons flow in the opposite direction, the failure mode is that void formation and crack propagation along the cathode IMC/solder interface, no Cu6Sn5 IMCs precipitate within the large angle ? ?-Sn grain.(2) For the EM behavior of Cu/Sn (single Sn grain)/Ni solder interconnects:Ni atoms show more obvious anisotropy in the ?-Sn grain. When the cathode Ni atoms diffuse along the [001] Sn grain, rapid dissolution of cathode Ni substrate and IMC occurs. However, when the cathode Ni atoms diffuse along the [110] Sn grain, the dissolution of cathode Ni and the growth of anode IMC are significantly retarded, which shows a higher EM resistance.(3) For the EM behavior of Ni/SAC305/Cu flip chip solder joints:when Cu acts as the cathode, two different EM-induced failure modes occur at the cathode, i.e., excessive dissolution of cathode Cu and void formation and crack propagation, which depends on the angle ? of Sn grain. The consumption of the cathode Cu follows a cosine function decrease as the angle ? increases. When Ni acts as the cathode, no evident dissolution of cathode Ni occurs, which is attributed to the protection of the stable interfacial (Cu,Ni)6Sns layer and the low solubility of Ni in ?-Sn. Instead, voids form and propagate along the Sn/(Cu,Ni)6Sns interface. Cu6Sn5-type IMCs selectively precipitate in small angle ? P-Sn grains or along the c-axis of ?-Sn grain. In addition, stress relaxation behaviors are observed, for example:the formation of hillock or whisker at the anode; the slight hollow of solder at the cathode; and the extrusion Sn hillocks besides the Cu6Sns-type IMCs.The different EM behaviors are well explained in viewpoint of atomic diffusion flux in ?-Sn.