Pb-free Flip Chip Solder Joints Research Articles

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 heights is discussed, showing below which there will be no net effect of thermomigration of Sn.

Rapid diagnosis of electromigration induced failure time of Pb-free flip chip solder joints by high resolution synchrotron radiation laminography

We performed a rapid diagnosis of electromigration induced void nucleation and growth in Pb-free flip chip solder joints. Quantitative measurements of the growth rate of voids during the stressing by 1.0 × 104 A/cm2 and 7.5 × 103 A/cm2 at 125 °C were conducted by synchrotron radiation high resolution x-ray laminography. The results were analyzed by the statistical model of Weibull distribution function [W. Weibull, ASME Trans. J. Appl. Mech. 18(3), 293 (1951)] of lifetime data. The Johnson-Mehl-Avrami phase transformation theory is proposed to provide a physical link to the statistical model and to estimate the lifetime of the joints at early stages.

Electromigration and Thermomigration in Pb-Free Flip-Chip Solder Joints

Pb-free solders have replaced Pb-containing SnPb solders in the electronic packaging industry due to environmental concerns. Both electromigration (EM) and thermomigration (TM) have serious reliability issues for fine-pitch Pb-free solder bumps in the flip-chip technology used in consumer electronic products. We review the unique features of EM and TM in flip-chip solder bumps, emphasizing the effects of current crowding and Joule heating. In addition, the challenges to a better understanding of EM and TM in Pb-free solders are discussed. For example, the anisotropic nature of Sn microstructure in Pb-free solders can enhance the dissolution rates of Ni and Cu in solders driven by EM and TM.

Failure induced by thermomigration of interstitial Cu in Pb-free flip chip solder joints

Failure induced by thermomigration in Pb-free SnAg flip chip solder joints has been investigated by electromigration tests under 9.7×103 A/cm2 at 150 °C. The fast interstitial diffusion of Cu atoms from underbump metallization into Sn matrix caused void formation at the passivation opening on the chip side. The Cu diffusion was driven by a large thermal gradient and led to void formation even in the neighboring unpowered bumps. When the thermal gradient is above 400 °C/cm, theoretical calculation indicates that the thermomigration force is greater than the electromigration force at 9.7×103 A/cm2 stressing.

In-situ Study of Electromigration-induced Grain Rotation in Pb-free Solder Joint by Synchrotron Microdiffraction

AbstractThe rotation of Sn grains in Pb-free flip chip solder joints hasn't been reported in literature so far although it has been observed in Sn strips. In this letter, we report the detailed study of the grain orientation evolution induced by electromigration by synchrotron based white beam X-ray microdiffraction. It is found that the grains in solder joint rotate more slowly than in Sn strip even under higher current density. On the other hand, based on our estimation, the reorientation of the grains in solder joints also results in the reduction of electric resistivity, similar to the case of Sn strip. We will also discuss the reason why the electric resistance decreases much more in strips than in the Sn-based solders, and the different driving force for the grain growth in solder joint and in thin film interconnect lines.

Interfacial Reaction and Joint Strength on Cu UBM with Pb-Free Flip Chip Solder Bumps

The interfacial reaction and joint strength of the Pb-free flip chip solder joints were investigated. Microstructural investigations were conducted using scanning electron microscopy (SEM), and phase analysis of the intermetallic compound (IMC) was carried out using energy dispersive spectrometer (EDS). Cu6Sn5 IMC layer was formed at the as-reflowed state, and Cu3Sn was additionally settled at the interface between the Cu6Sn5 and Cu under bump metallurgy (UBM). Growth of the IMC layers followed the parabolic law, while the IMC layer growth was not highly affecting the mechanical properties of the solder joints. The shear force of the solder joints decreased because of the microstructural coarsening within the solder region.

Electromigration in Pb-free flip chip solder joints on flexible substrates

Flip chip on flexible substrate technology has attracted much attention in the electronics industry for portable applications because the reduction of packaging volume coupled with long-term reliability is a challenging issue. In this study, the electromigration behavior of flip chip Sn96Ag3.5Cu0.5 solder joints on flexible substrates has been investigated under a constant current density of 4×104A∕cm2 at room temperature. The in situ resistance change of the circuit during the electromigration test was measured and found to show stepwise changes before the final failure. Scanning electron microscopic images of the cross section of samples showed the existence of voids in the bulk of the solder joints before electromigration and the movement of these voids after electromigration. The void movement was matched with the resistance change during the electromigration test, and voids moved from within the bulk of solder bump to the cathode interface between Al interconnection and under bump metallurgy in the downward electron flow (from chip to substrate) joints. The mechanisms of electromigration-induced void migration and failure in the Pb-free flip chip on flexible substrates are discussed.