Study Of Electromigration Reliability Of Solder Bump Joints

The trend of electronic products (like notebook, cell phone) to high performace and miniaturization requires many tough improvements for integrated circuit. Bump-to-trace interconnect technologies, such as Flip Chip, Ball Grid Array, and Chip Scale Package, are the most important methods to achive the integrated circuit with denser characteristic and much more I/O ports. Alloy solder is the joining consumable used in bump-to-trace interconnecting process, and nowadays, the components of solder transfers from Sn-Pb alloys to lead-free Sn-based alloys. Traditional Sn-Pb eutectic alloy solders is sensitive to electromigration (EM) under current stressing, and therefore, EM failure has great threat to the realiability of integrated circuit. In fact, EM problem of alloy solders has been the hot issue for the past 10 years. The EM problem has still attracted a lot of attention even after development of lead free solders. Especially, since the dimensions of interconnects decrease continuously and current increases, the EM phenomenon can not be ignorated.In this paper, a Cu/solder/Cu sandwitch structure was designed, where, solder is Sn-37Pb, Sn-0.5Cu, Sn-3.5Ag and Sn-3.0Ag-0.5Cu eutectic or near eutectic alloys, respectively. EM accerated tests were conducted under 1×103 A/cm2 + 60℃. The EM process was monitored by using a resistance detector. After the tests were done, we analyzed the interfacial microstructures with scanning electronic microscope (SEM), and the products at the interface were indentified with EPMA. Electron wind is the main driving force to induce EM, so we simulated the current density distribution and concentration distribution of the solder bump, as well as the factors influencing current crowding with finite element method (FEM). Lastly, the effects of alloy elements on the EM process of Sn based solder were analyzed by employing the first principles method. Main conclusions are as follows,1. EM lifetime of Sn-37Pb solder joint is shorter than that of SnTM (TM=Cu or/and Ag) solder joint under same stressings and structure. After 187 h, there forms void with diameter of about 10μm in the former solder joint, and 320 h later, short circuit arises due to complete dissolution of Cu at cathode side. While, the resistances change little after 700 h for the latter solder joints.2. It is found that the current stressing has a polarity effect on the growth of intermetallic component (IMC) layer, i.e., growth rate of IMC layer at anode side is faster than that at cathode side. Growth of IMC at anode side is controlled by concentration of Cu element migrated into anode interface, the growth dynamic equation is w , obeys with parabola rule. While, growth of IMC at cathode side is governed by current density,growth also obeys parabola rule as the maximum current density in the solder bump is small relatively. However, as the current density increases, IMC at cathode side would dissolve, even lead nucleation of micro-voids.3. Nucleation rate of Kirkendall voids at Cu3Sn/Cu interface is accerated under current stressing relative to that under thermal aging. Formation of Kirkendall voids is also of polarity characteristic, density of Kirkendall voids at anode side is higher than that at cathode side. Microvoids coalesce into larger voids or cracks. Since the dimension of Kirkendall voids is much less than than formed at Cu6Sn5/Sn-37Pb interface at cathode side, failure caused by Kirkendall voids at Sn-37Pb solder joints can be neglect. However, for SnTM solder joints, Kirkendall voids should be attracted enough attention.4. Geometry structures and materials'properties are the main factor influencing the current density distribution in the solder bump according to finite element analysis. It is found that wider trace, thicker UBM, low resistivity trace and UBM, and solder with higher resistivity are beneficial in decreasing current crowding in solder bump.5. From first principles calculations, we found that the effects of Cu and Ag on the stability of the nearest neighbor (NN) Sn are more significant than those of Pb and Bi. The former alloying elements obtain charges from Sn, while the latter transfer charges. It can be predicted that the effects of electron flux on Sn atom can be scattered by Cu and Ag, the EM resistance of Sn is therefore improved.6. According to the calculated results of diffusion activation energy, it is found that Pb, Bi, Cu and Ag alloying elements are all increasing the barrier energy of NN Sn, and the effect from Ag and Cu is more significant. In the same system, the barrier energy for alloying element is lower than that of the NN Sn. Therefore, alloying elements would be the diffusing species. It seems that this conclusion is inconsistent with conclusion 5, however, experimental results all show that both Sn and Pb can be the dominated diffuser in Sn-Pb eutectic alloy solder, as determined by the environment conditions. Therefore, our results are reasonable.7. Both the diffusion energies of SnCu and SnAg sytems are higher than, while those of SnPb and SnBi systems are equal to that of pure Sn system. This result is another reason why Cu and Ag can improve the EM resistance of Sn based solder.