C-axis Of Sn Grain Research Articles

Improvement of thermomigration resistance in lead-free Sn3.5Ag alloys by Ag interlayer

A significant improvement of thermomigration (TM) resistance in Pb-free Sn3.5Ag alloys was achieved by a Ag interlayer. Ag interlayer was used to form Ag3Sn intermetallic compound (IMC) during reflowing process, which can serve as a barrier to suppress thermomigration-induced Cu flux under a temperature gradient. Two different kinds of sandwich samples were designed and compared, Cu/Ag/Sn3.5Ag/Cu and Cu/Sn3.5Ag/Cu. A heat sink and a heat source were employed to establish a temperature gradient of 7000 °C/cm. The growth rate of IMCs at cold end in Cu/Ag/Sn3.5Ag/Cu was slower than that in Cu/Sn3.5Ag/Cu. Thermomigration-induced Cu fluxes were also effectively inhibited in Cu/Ag/Sn3.5Ag/Cu specimens, and the calculated value of JCu,TM was about one-third times as that in Cu/Sn3.5Ag/Cu specimen. In addition, the dissolution of IMCs and under bump metallization (UBM) at hot end was barely observed in Cu/Ag/Sn3.5Ag/Cu specimen. These results suggest that Ag3Sn layer can be a diffusion barrier to effectively inhibit the Cu flux from hot end to cold end under a temperature gradient. Furthermore, crystallographic orientation of Sn grains was affected by the Ag interlayer, and it leads to the c-axis of Sn grain being more vertical to the direction of JCu,TM for the samples with Ag interlayer deposited, which further enhances the effect of suppression on JCu,TM.

Effect of Sn grain orientation on growth of Cu-Sn intermetallic compounds during thermomigration in Cu-Sn2.3Ag-Ni microbumps

The effect of Sn orientation on thermomigration was investigated in Cu/Sn2.3Ag/Ni microbumps with diameter of 20 μm. Results showed that anisotropic diffusion of Cu in Sn grains affected both formation rates and locations of Cu6Sn5 intermetallic compounds (IMCs). Rapid growth of Cu6Sn5 IMCs at the cold-end interface was observed when the c-axis of Sn grains was parallel to the temperature gradient. In addition, diffusion of Cu atoms was blocked by a Sn grain with a high angle between the Sn c-axis and the thermal gradient, resulting in formation of Cu6Sn5 IMCs near the grain boundary inside the solder.

IMC growth behavior along c-axis of Sn grain under current stressing

Due to the asymmetric structure of Sn unit cell, the electromigration behaviors of Sn based solder joints were significantly depended on the crystal orientation of Sn grain. The present investigation illustrated the effect of Sn grain c-axis on the intermetallic compounds (IMCs) growth under current stressing. The Cu particles reinforced Sn3.5Ag solder joint was selected to obtain various observed surfaces of the sample after 46 days current stressing. The front side, back side, right side and anode side of the sample represented different phenomena, which reflected and proved the importance of c-axis orientation on Cu atomic migration in three-dimension space. Besides, another composite solder joint was polished severe times on the observed surface which was full of IMCs. It showed that the IMCs were covered on the Sn-based solder, and there was crack at the cathode underneath the front surface after 38 days current stressing. The IMC growth behavior would be well understood by systematic study in this work.

Effects of Sn grain c-axis on electromigration in Cu reinforced composite solder joints

Sn grain orientation in solder matrix has recently been considered as one of the principal failure contributions in lead-free solder joints. Since β-Sn exhibits high anisotropy, the differences of electromigration behavior of solder joints with various grain orientations are dramatic. With different angle (θ) between the c-axis of Sn grain and the direction of electron flow, the solder joints often demonstrate varying degrees of electromigration-induced damage. The present investigation illustrated significant differences in microstructural features on the surface of Cu reinforced composite solder joints, even with similar angle θ. Two Cu particles reinforced Sn3.5Ag composite solder joints with the similar angle θ of 35° and 40° were selected to investigate the effects of Sn grain c-axis on electromigration under high current stressing for 528 h. A large number of Cu6Sn5 compounds were observed and occupied nearly the whole surface with clear polarization effect in one sample. In contrast, very few Cu6Sn5 were found on the surface of the solder matrix in the other sample after current stressing, and the Cu6Sn5 which was formed after reflow disappeared with subsequent current stressing. Even though polarization effect was not obvious, and the cathode interfacial dissolution was observed on such sample. Systematic study revealed that the angle θ alone was not a sole factor to determine the electromigration damage. It has to be considered along with the coordinate system containing the orientation of the c-axis of the Sn grain, a more dominative factor for the diffusing species.

Effect of Sn grain orientation on formation of Cu6Sn5 intermetallic compounds during electromigration

The effects of Sn orientation and grain boundary misorientation on formation of Cu-Sn intermetallic compounds (IMCs) during electromigration were investigated. Significant anisotropic diffusion of Cu in Sn grains was observed. Interfacial Cu-Sn IMCs may grow rapidly, dissolve, or remain intact, depending on the angle of c-axis of Sn grains with the electron flow. In addition, grain boundaries did not play an important role in Cu diffusion because they are mostly cyclic twins.

Role of diffusion anisotropy in β-Sn in microstructural evolution of Sn-3.0Ag-0.5Cu flip chip bumps undergoing electromigration

Abstract The anisotropy of β-Sn grain is becoming the most crucial factor to dominate the electromigration (EM) behavior with the downsizing of solder bumps. When the c-axis of β-Sn grain is parallel to the electron flow direction, excessive dissolution of cathode Cu occurs due to the large diffusivity of Cu along the c-axis; when the c-axis of β-Sn grain is perpendicular to the electron flow direction, limited dissolution of cathode Cu occurs even in the current crowding regions. However, there is no evident dissolution of cathode Ni regardless of the orientation of β-Sn grains, due to the protection of a stable interfacial (Cu,Ni)6Sn5 intermetallic compound (IMC) layer and the extremely low solubility of Ni in β-Sn. Cu6Sn5-type protrusions selectively precipitated in specific Sn grains with small angle θ (between the c-axis of Sn grain and electron flow direction) but not in the neighbor grains with large angle θ or along the direction of c-axis of β-Sn. Sn hillocks are squeezed out to relieve the compressive stress generated by the formation of Cu6Sn5-type IMCs. The high diffusion anisotropy in β-Sn grains, which is calculated by a proposed model, accounts for the novel diffusion behavior of solute atoms, dissolution of cathode and consequent precipitation of IMCs in Ni/Sn-3.0Ag-0.5Cu/Cu flip chip solder joints.

Magnetic-field induced anisotropy in electromigration behavior of Sn–Ag–Cu solder interconnects

Abstract

Effects of crystallographic orientation of Sn on electromigration behavior

Electromigration behavior of SAC flip-chip joints with respect to the crystallographic orientation of Sn grains was investigated. The test sample had direct contact of Sn-3.0wt% Ag-0.5wt% Cu on Cu-OSP (organic surface preservative) and the applied current density was 15 kA/cm2 at 160 °C. Without current stressing, no microstructural change depending on the crystallographic orientation of Sn was observed. With current stressing, however, the bumps showed the substantial microstructural changes with respect to the crystallographic orientation of Sn. When the orientation of the current flow was parallel to the c-axis, fast failure of the bumps occurred due to the massive dissolution of the Cu electrode on the cathode side caused by the fast diffusion of Cu atoms along the c-axis of Sn grains while only a slight microstructural change was observed when the c-axis of Sn grains was perpendicular to the electron flow.