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In Pb-free solder joints formed by reflowing a bump of solder paste, voids are formed within the solder due to the residue of flux in the reflow process. These voids migrate toward the cathode contact during electromigration under current stressing. Accompanying the electromigration, resistance jumps of a few 100 mΩ were observed. It was postulated that a jump occurs when a void touches the cathode contact. This study investigated the effect of the void migration and condensation on the change in bump resistance using three-dimensional (3D) simulations and finite element analysis. It was found that there was negligible change in bump resistance during void migration towards the high-current-density region before touching the cathode contact opening. When a small void condensed on the contact opening and depleted 18.4% of the area, the bump resistance increased only 0.4 mΩ. Even when a large void depleted 81.6% of the opening, the increase in bump resistance was 3.3 mX. These values are approximately two orders of magnitude smaller than those reported in the literature for the change in resistance due to void migration in flip chips on flexible substrates. We conclude that the reported change in resistance was most likely that of the Al or Cu interconnection in the flip-chip samples. [ABSTRACT FROM AUTHOR]

Electromigration in Sn-8Zn-3Bi flip chip solder bumps on Cu pads has been studied at 120°C with an average current density of 4 x 10³ A/cm² and 4.5 x 104 A/cm². Due to the polarity effect, the thickness of the intermetallic compound Cu-Zn (γ-phase) formed at the anode is much greater than that at the cathode. The solder joint fails after 117 h of stressing at 4.5 x 104 A/cm², and void formation at the cathode can clearly be seen after polishing. However, it is the melting at the edge of the bump that causes the solder joint to fail. A simulation of the current density distribution indicates that the current density is not distributed uniformly, and current crowding occurs inside the bump. The results indicate that the increase of current density associated with Joule heating has affected melting and enhanced damage in the solder joint during electromigration. [ABSTRACT FROM AUTHOR]

Solid-state intermetallic compound (IMC) growth behavior plays an important role in solder joint reliability of electronic packaging assemblies. The directional impact of electromigration (EM) on the growth of interfacial IMCs in Ni/ SAC/Ni, Cu/SAC/Ni single BGA ball solder joint, and fine pitch ball-grid-array (FPBGA) at the anode and cathode sides is reported in this study. When the solder joint was subjected to a current density of 5,000 A/cm² at 125°C or 150°C, IMC layer growth on the anode interface was faster than that on the cathode interface, and both were faster than isothermal aging due to the Joule heating effect. The EM affects the IMC growth rate, as well as the composition and mechanical properties. The Young's modulus and hardness were measured by the nanoindentation continuous stiffness measurement (CSM) from planar IMC surfaces after EM exposure. Different values were observed at the anode and cathode. The energy-dispersive x-ray (EDX) line scan analysis was conducted at the interface from the cathode to anode to study the presence of species; Ni was found in the anode IMC at SAC/Cu in the Ni/SAC/Cu joint, but not detected when the current was reverse. Electron-probe microanalysis (EPMA) measurement on the Ni/SAC/Ni specimen also confirmed the polarized Ni and Cu distributions in cathode and anode IMCs, which were (Ni0.57Cu0.43)3Sn4 and (Cu0.73Ni0.27)6Sn5, respectively. Thus, the Young's moduli of the IMC are 141 and 175 GPa, respectively. [ABSTRACT FROM AUTHOR]