Chia-ming Hsu

Thermal Gradient in Solder Joints Under Electrical Current Stressing

The thermal gradient and temperature increase in SnAg3.5 solder joints under electrical-current stressing have been investigated by thermal infrared microscopy. Both positive and negative thermal gradients were observed under different stressing conditions. The magnitude of the thermal gradient increases with the applied current. The measured thermal gradients reached 365°C/cm as powered by 0.59 A, yet no obvious thermal gradient was observed when the joints were powered less than 0.25 A. The temperature increase caused by joule heating was as high as 54.5°C when powered by 0.59 A, yet only 3.7°C when stressed by 0.19 A. The location of heat generation and path of heat dissipation are believed to play crucial roles in the thermal gradient. When the major heat source is the Al trace, the thermal gradient in the solder bumps is positive; but it may become negative because the heat generated in the solder itself is more prominent.

Generalized phenomenological model for the effect of electromigration on interfacial reaction

Intermetallic compound (IMC) formation is important for the reliability of microelectronic devices. In this work, a generalized phenomenological model was developed to explain the effects of electromigration on IMC growth by considering reaction and diffusion of two species. When both reaction and mass transfer are important, the model predicts cathode enhancement and anode thinning if the electromigration effect on the dominant diffusion species is more pronounced. Cathode suppression and anode enhancement occur when the electromigration effect on the minor diffusion species is more pronounced. Simultaneous cathode and anode suppressions happen when there are two diffusion species and the diffusion and electromigration fluxes are comparable. Simultaneous cathode and anode enhancements occur when mass transfer is the limiting step and diffusion flux is negligible compared to electromigration. This model was found to be consistent with experiment data on IMC growth in the literature given the limited amoun...

Temperature and current-density distributions in flip-chip solder joints with Cu traces

Three-dimensional simulation was performed to investigate the temperature and current density distribution in flip-chip solder joints with Cu traces during current stressing. It was found that the Cu traces can reduce the Joule heating effect significantly at high stressing currents. When the solder joints were stressed by 0.6 A, the average temperature increases in solder bumps with the Al traces was 26.7°C, and it was deceased to 18.7°C for the solder joint with the Cu traces. Hot spots exist in the solder near the entrance points of the Al or Cu traces. The temperature increases in the hot spot were 29.3°C and 20.6°C, for solder joints with the Al traces and Cu traces, respectively. As for current density distribution, the maximum current density inside the solder decreased slightly from 1.66×105 A/cm2 to 1.46×105 A/cm2 when the Al traces were replaced by the Cu traces. The solder joints with the Cu traces exhibited lower Joule heating and current crowding effects than those with the Al traces, which was mainly attributed to the lower electrical conductivity of the Cu traces. Therefore, the solder joints with the Cu traces are expected to have better electromigration resistance.

Phase Equilibria of the Sn-Bi-Te Ternary System

Bi2Te3 is one of the most promising thermoelectric materials, and Sn is the primary constituent of most electronic solders. Knowledge of phase equilibria of the Sn-Bi-Te ternary system is important for thermoelectric applications. Twenty-seven Sn-Bi-Te alloys were prepared and equilibrated at 160°C and 500°C. The equilibrium phases were determined, and the isothermal sections were constructed based on three binary constituent phase diagrams and ternary phase equilibria results. No ternary compounds were observed in the Sn-SnTe-Bi region. The SnTe phase is very stable and has tie-lines with the Sn, liquid, and Bi phases at 160°C. At 500°C, in addition to the already known SnBi2Te4 and SnBi4Te7 ternary phases, two new ternary compounds (Sn2Bi2Te5 and SnBiTe2 phases) were found. SnTe and Bi2Te3 have significant mutual solubilities, and the ternary compounds SnBi2Te4, SnBi4Te7, and Sn2Bi2Te5 are all in the SnTe-Bi2Te3 pseudobinary section.

Phase diagrams of the Ag-In-Zn ternary system

There are 6 single phase regions, 9 two-phase regions, and 4 ternary tie-triangles in the isothermal section of Ag-In-Zn system at 500°C. The 6 single phase regions are Ag, β-AgZn, γ-Ag₅Zn₈, and ε-AgZn₃, liquid, and ς-Ag₃In. The calculated results using preliminary models based on the thermodynamic models of its three constituent binary systems are qualitatively in agreement with the experimental measurements. However, the significant ternary solubilities of the binary compounds, especially in the β-AgZn phase, need to be taken into considerations, so that quantitative correct modeling can be achieved.

Liquidus projection of Sn-Co-Ni ternary system

The preliminary liquidus projection of the ternary Sn-Co-Ni system is determined. No ternary compound is found. There are eight primary solidification phases: Sn, CoSn₃, CoSn₂, CoSn, (Ni,Co)₃Sn₂, (Ni,Co), Ni₃Sn and Ni₃Sn₄. More experimental measurements of the primary solidification phases and thermal analysis are needed.

Sn-Co-(Cu) / Ni solid/solid interfacial reactions

Soldering is an important jointing technology in the electronic packaging industry. It has recently gained much more attention due to the Pb-free requirements and various emerging techniques, such as ball-grid array (BGA), flip-chip and through-silicon via (TSV) [1–3]. Although Sn-Ag-Cu solders are the most popular Pb-free solders, there are still various problems which need to be overcome [4–6]. There are thus still a lot of continuing efforts to developing new solders and improving existing solders. Among the various Pb-free solders, Sn-based solders with added Co have been studied. It has been found Co additions are very effective in reducing the undercooling and refining the microstructures of solder joints [7–9].