Effect Of Sn Grain Orientation Research Articles

Role of Crystallographic Orientation of β-Sn Grain on Electromigration Failures in Lead-Free Solder Joint: An Overview

Due to the miniaturization of electronic devices, electromigration became one of the serious reliability issues in lead-free solder joints. The orientation of the β-Sn grain plays an important role in electromigration failures. Several studies have been carried out to investigate the effect of Sn grain orientation on electromigration. The efforts involve the influence of β-Sn grain orientation on the migration of Cu, Sn, and Ni atoms, on the morphology of the solder joint, and on the formation of Cu6Sn5 and (Cu, Ni)6Sn6 in the lead-free solder joint during electromigration. The current review provides a detailed review of past studies which were conducted to investigate the influence of β-Sn grain orientation on electromigration failures in lead-free solder joints.

Effect of Sn Grain Orientation on Reliability Issues of Sn-Rich Solder Joints

Sn-rich solder joints in three-dimensional integrated circuits and their reliability issues, such as the electromigration (EM), thermomigration (TM), and thermomechanical fatigue (TMF), have drawn attention related to their use in electronic packaging. The Sn grain orientation is recognized as playing an important role in reliability issues due to its anisotropic diffusivity, mechanical properties, and coefficient of thermal expansion. This study reviews the effects of the Sn grain orientation on the EM, TM, and TMF in Sn-rich solder joints. The findings indicate that in spite of the failure modes dominated by the Sn grain orientation, the size and shape of the solder joint, as well as the Sn microstructures, such as the cycling twining boundary (CTB), single crystals, and misorientations of the Sn grain boundary, should be considered in more detail. In addition, we show that two methods, involving a strong magnetic field and seed crystal layers, can control the Sn grain orientations during the solidification of Sn-rich solder joints.

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.

Effects of Sn grain size on intermetallic compounds formation in 5 µm diameter Cu/Sn pillar bumps

Uniform 5 µm diameter Cu/Sn micro-bump array has been fabricated by multilayer electrodeposition, and the effect of Sn grain orientation and size on Cu6Sn5 and Cu3Sn growth have been studied through FIB–SEM and electron backscatter diffraction. The solder layer in Ф5 µm Cu/matte Sn micro-bump we fabricated contains only four to five columnar Sn grains with no obvious preferred orientation and there is no significant difference in the growth of Cu6Sn5 in different oriented Sn grains. However, the Sn grain size has much more effect on intermetallic compound (IMC) growth. At 150 °C, the diffusion coefficient of Cu/bright-Sn system(~ 200 nm Sn) is calculated to be 13.09 × 10−17 m2 s−1, nearly three times that of Cu/matte–Sn system (2–5 µm Sn), about 4.51 × 10−17m2 s−1. Also, a simple model of boundary diffusion controlled IMC growth is proposed to explore the mechanism of grain size effect. Large Sn grain size will slow down the interfacial reaction in micro-bump due to low grain boundary density and large bulk diffusion distance.

Solid-State Diffusion of Bi in Sn: Effects of β-Sn Grain Orientation

Bismuth (Bi) is an alloying element being considered for inclusion in lead-free solder alloys to improve microstructure, properties, and reliability. It has been determined that aging a Bi-bearing alloy will preserve its strength, whereas the strength of SAC alloys is degraded. Examination of microstructure reveals that, over time, homogenization of Bi in the tin (Sn) matrix via solid-state diffusion occurs, which leads to stabilization of mechanical properties. In this study, the effect of Sn grain orientation on the diffusivity of Bi was analyzed. Sn was solidified slowly to produce coarse grains typical of grain morphologies in solder joints. Bi was subsequently sputtered onto the Sn, and samples underwent annealing at 125°C for 24 h. Electron probe microanalysis (EPMA) was utilized to collect compositional data, and diffusivities were extracted for several grain orientations. It was determined that the diffusivity of Bi in Sn has a low anisotropy ratio, with a difference in diffusivity of around an order of magnitude between Sn samples oriented with the concentration gradient perpendicular to the ‘c’ axis and those oriented parallel to the ‘c’ axis. As a result, low angle grain boundaries in the Sn samples can have a profound effect on the diffusivity and confound the orientation relationship.

Effect of Sn grain orientation and strain distribution in 20-μm-diameter microbumps on crack formation under thermal cycling tests

The thermo-mechanical properties of microbumps serving as interconnects between Si chips in three-dimensional integrated circuits were examined in this study. Arrays of SnAg microbumps were subjected to temperature cycling tests up to 2500 cycles. Extensive cracks formed in the microbumps, and some of them propagated across the entire microbump. A microstructural analysis by electron back scattering diffraction indicated that the cracks propagated along two paths: Sn grain boundaries with high misorientation angles and SnAg/Ni3Sn4 interfaces. The average Sn grain size was only 7.4 μm, implying a large proportion of grain boundaries per unit volume, which facilitated crack propagation across the microbump of 20 μm in diameter. Open image in new window

Effect of Sn Grain Orientation on Formation of Cu6Sn5 Intermetallic Compound Under Current Stressing

Solder joints with Cu/Sn-Ag/Cu structure and bump height of 15 μm have been used to investigate the electromigration phenomenon at different temperatures and current densities. Moreover, the grain orientation was analyzed using electron backscatter diffraction. It was found that the anisotropic properties of tin affected the formation rate of Cu-Sn intermetallic compounds (IMCs), and that the angle between the electron flow direction and tin grain orientation played an important role in the formation of Cu6Sn5 IMC. With changes in angle, the diffusion rate of copper atoms in tin also varied. When the c-axis of tin was parallel to the electron flux, copper atoms diffused rapidly, resulting in fast formation of Cu-Sn IMCs. On the other hand, if the angle between the c-axis of the grain and the electron flow direction was large, the tin grains were more resistant to Cu diffusion during current stressing, leading to a very slow IMC formation rate.

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.

The micro-mechanism for the effect of Sn grain orientation on substrate consumption in Sn solder joints

The reliability of flip-chip M/Sn solder joint strongly depends on Sn grain orientation, since it affects the dissolution of metal pads and the growth of intermetallic compounds. In this paper, the micro-mechanism of the effect of Sn grain orientation on the diffusion of M atom (=Cu, Ni, Ag, Au or In) in Sn solder joints was investigated by employing the density functional theory. The adsorption energy of M atom on Sn(100) and (001) surfaces, the energy of M atom penetrating from the Sn surface to Sn body (penetration energy, Epe) and the energy of M atom diffusing in the bulk Sn (diffusion energy) were analyzed, as well as the corresponding electronic structures for different steps. It is found that the M atoms were energetically favorable at the hollow sites of Sn(100) and (001) surfaces. Moreover, the adsorption energy of M/Sn(100) and M/Sn(001) were obtained. The M atom was more strongly adsorbed to Sn(001) than to Sn(100). The Epe for each M atom along a-axis of Sn grain was greater than the corresponding value that along c-axis. Moreover, the Epe increased gradually with the M atom changing from Ni to In. Electronic structure analysis shows that the Epe change can be attributed to three factors, including different atomic radius from Ni to In, difference in interatomic repulsive resistance between a and c-axis, and different hybrid orbitals between M 3d and Sn 2p from Ni to In. At last, the results show that M atoms could migrate easily in bulk Sn.

Effect of Sn Grain Orientation on the Cu6Sn5 Formation in a Sn-Based Solder Under Current Stressing

A SnAgCu-based solder stripe between two Cu electrodes is current stressed with a density of 5 × 104 A/cm2 at 393 K (120 °C). After current stressing for 24 hours, electromigration induces the Cu dissolution from the cathode-side Cu electrode, leading to the Cu6Sn5 formation in the solder stripe. Very interestingly, the Cu6Sn5 phase is selectively formed within a specific Sn grain. Electron backscattering diffraction analysis indicates the crystallographic orientations of Sn grains play an important role in the selective Cu6Sn5 formation.

Effect of Sn grain orientation on electromigration degradation mechanism in high Sn-based Pb-free solders

Electromigration induced damage strongly depends on Sn-grain orientation in Pb-free solders. Rapid depletion of intermetallic compounds and under bump metallurgy led to significant damages caused by the fast diffusion of Cu and Ni along the c axis of Sn crystals. When the c axis of Sn grain is not aligned with the current direction, electromigration (EM) damage is dominated by Sn self-diffusion, which takes longer to occur. This is a direct proof of the highly anisotropic diffusion behavior in Sn. Due to the presence of twin structures and stable Ag3Sn network, SnAg(Cu) solders are less susceptible to grain orientation effects and showed better EM performance than SnCu solders.