Fan-Yi Ouyang

Effect of entropy production on microstructure change in eutectic SnPb flip chip solder joints by thermomigration

Thermomigration in flip chip solder joints of eutectic SnPb has been studied at an ambient temperature of 100°C. Redistribution of Sn and Pb occurs with Pb moving to the cold end. A stepwise concentration profile is observed. Significantly, the lamellar microstructure becomes much finer after thermomigration. Since the lamellar interface is disordered and is a fast path of diffusion, it indicates a high rate of entropy production in the thermomigration, in agreement with Onsager’s principle of irreversible processes. The effect of entropy production on microstructure change is shown here. The molar heat of transport of Pb has been calculated to be −25.3kJ∕mole.

Effect of current crowding on whisker growth at the anode in flip chip solder joints

Owing to the line-to-bump configuration in flip chip solder joints, current crowding occurs when electrons enter into or exit from the solder bump. At the cathode contact, where electrons enter into the bump, current crowding induced pancake-type void formation has now been observed widely. At the anode contact, where electrons exit from the bump, we report here that whisker is formed. Results of both eutectic SnPb and SnAgCu solder joints are presented and compared. The cross-sectioned surface in SnPb showed dimple and bulge after electromigration, while that of SnAgCu remained flat. The difference is due to a larger back stress in the SnAgCu, consequently, electromigration in SnAgCu is slower than that in SnPb. Nanoindentation markers were used to measure the combined atomic fluxes of back stress and electromigration.

In situ observation of thermomigration of Sn atoms to the hot end of 96.5Sn-3Ag-0.5Cu flip chip solder joints

In this study, we investigated the phenomenon of thermomigration in 96.5Sn-3Ag-0.5Cu flip chip solder joints at an ambient temperature of 150 °C. We observed mass protrusion on the chip side (hot end), indicating that Sn atoms moved to the hot end, and void formation on the substrate side (cold end). The diffusion markers also moved to the substrate side, in the same direction of the vacancy flux, indicating that the latter played a dominant role during the thermomigration process. The molar heat of transport (Q*) of the Sn atoms was 3.38 kJ/mol.

Concurrent electromigration and creep in lead-free solder

When electric current flows in a solder bump, electromigration generates stress, but creep relaxes it. After some time, the bump develops a steady-state stress field. We present a theory to show that the two processes — electromigration and creep — set an intrinsic length. When the intrinsic length is large compared to the height of the bump, electromigration is fast relative to creep and the steady-state stress field is linearly distributed in the bump. When the intrinsic length is small compared to the height of the bump, electromigration is slow relative to creep and the steady-state stress field nearly vanishes in the bump, except in a thin layer along the boundary of the bump. We further show that a critical electric current exists, below which the bump can sustain the steady-state stress field without forming voids. Theoretical predictions are compared with existing experimental observations.

Effect of electromigration in the anodic Al interconnect on melting of flip chip solder joints

Melting of eutectic SnPb flip chip solder joints has been observed to occur frequently in dc electromigration tested with current density above 1.5×104A∕cm2 at an ambient temperature of 100°C. It has been found that it is not due to the Joule heating of the solder bump itself, but rather due to the Joule heating of the on-chip Al interconnects. The current density in the Al approaches 2×106A∕cm2 and electromigration has especially generated voids at the anode of the Al. The damage has greatly increased the resistance of the Al, which produces the heat needed to melt the solder joint. The results of electrothermal coupling analysis show that when the resistance of the Al line changed threefold, the solder bump melted.

Electromigration induced failure on lead-free micro bumps in three-dimensional integrated circuits packaging

We report electromigration induced failure on lead-free micro bumps in three-dimensional integrated circuits samples with chip on chip configuration. Compared to flip chip solder joints, micro bumps of chip-on-chip samples exhibit better electromigration resistance and are able to withstand a higher current density. No exhibited electromigration-induced failure was observed when current density was below 2 × 104 A/cm2. A threshold current density to trigger electromigration in chip-on-chip samples was found to be 3.43 × 104 A/cm2. When current density was higher than 7.5 × 104 A/cm2 at an ambient temperature of 150 °C, no void propagation through whole bump opening was found; instead, electromigration induced voids were observed at the cathode side of Al trace.

Comparison of thermomigration behaviors between Pb-free flip chip solder joints and microbumps in three dimensional integrated circuits: Bump height effect

Packaging technology is currently transition from flip chip technology to three dimensional integrated circuits (3D ICs) to meet the requirements of consumer electronic products. Compared to flip chip technology, the dimension of microbumps in 3D ICs is shrunk by a factor of 10. In this study, the behaviors of thermomigration in Pb-free solders of flip chip and 3D ICs are presented. When the bump height is 100 μm in the flip chip samples, the Sn protrusion was observed at the hot end and voids formation at the cold end. However, when the bump height is reduced to 5.8 μm in the 3D IC samples, no significant microstructural evolution of Sn was found; instead, the dissolution of Ni under-bump metallization at hot end was observed. We propose that discrepancy between flip chip solder joints and 3D IC microbumps is mainly attributed to the effect of back stress and the presence of thicker Ni under-bump metallization in the 3D IC packaging. Moreover, the critical temperature gradient in terms of different bump ...

Thermomigration in SnPb composite solder joints and wires

Composite flip chip solder joints and composite solder wire samples were used to study thermomigration. The composite solder joint consists of 95Pb5Sn on the Si side and eutectic 37Pb63Sn on the substrate side. Composite wire samples were made by joining Cu (wire) - 95Pb5Sn (ball) - eutectic 37Pb63Sn solder (ball) - Cu (wire) together with the size of the wire and ball of 300 mum in diameter. We have observed thermomigration induced composition redistribution of Sn and Pb, in which Sn moved to the hot end and Pb to the cold end. A threshold temperature gradient, which is about 1000 degC/cm, is needed to induce the thermomigration

Electromigration and thermomigration of Pb-free microbumps in three-dimensional integrated circuits packaging

Packaging technology is currently transitioning from flip chip technology toward three-dimensional integrated circuits (3D IC) packaging. In 3D ICs, smaller dimensions of the microbumps, approximately 10 μm, are adopted. In such small size of the microbumps, the current densities applied in the microbumps will dramatically increase. Furthermore, due to intense joule heating and current crowding, current stressing may have caused a non-uniform temperature distribution in the microbumps, and thereby a thermal gradient can be established across them. The thermal gradient is expected to be larger in the 3D IC packages, because smaller dimension of Pb-free alloys are used. Thus, addressing the electromigration (EM) and thermomigration (TM) problems of the microbumps are critical for this newly developed technology. In this study, the EM of Pb-free microbumps in 3D IC packaging under electrical current stressing from 1×104 A/cm2 to 1×105 A/cm2 at 150 °C was investigated. The resistance evolution of the samples during current stressing was found to be correlated with microstructural observation of the microbumps. In addition, EM-induced damage was observed when current density was higher than 8×104 A/cm2 whereas no EM-induced damage on the microbumps was found after 14416 h when the current density was below 1.5×104 A/cm2. We proposed that the different behaviors of EM on the microbumps were mainly due to the effect of back stress. The theoretical critical current density to trigger electromigration of Sn in this study was calculated as 7.5×104 A/cm2. Furthermore, when a temperature gradient was across the microbumps, TM induced failure in the 3D IC sample was highly affected by dissolution of Ni UBM at hot end, resulting in the net flux of Ni toward the cold end accompanying with enhanced growth of Ni3Sn4 IMCs at lower temperature region.

Influence of purification process on incipient material corrosion in molten FLiNaK salts

Abstract This study investigated the incipient corrosion characteristics on Ni based and Mo based alloys at 600°C in FLiNaK molten salts regarding to different purification processes of FLiNaK molten salts. Ni based Hastelloys-N alloys exhibited relatively better corrosion resistances, while Mo based TZM alloys showed the poorest corrosion resistance. The mass loss of the tested alloys was strongly dependent on purification process of the FLiNaK molten salts; through preheating and stirring the FLiNaK salts at higher temperature during the purification process, impurities and residual moisture of the FLiNaK salts could be effectively removed, leading to the improvement of mass loss of the tested alloys. The results of microstructural characterisation revealed that Hastelloy-N tended to exhibit intergranular corrosion, whereas grains of Hastelloy-B3 were also attacked by the corrosive medium in addition to grain boundaries. The results of EDX mapping showed that both Cr and Mo were found to be dissolved into FLiNaK salts for Hastelloy-N and Hastelloy-B3. The Mo based TZM alloys exhibited general corrosion pattern.