Guangchen Xu

Joule heating effect on oxide whisker growth induced by current stressing in Cu/Sn-58Bi/Cu solder joint

The electromigration test was conducted in Cu/Sn-58Bi/Cu solder joint with high current density of 104 A/ cm2. Results showed that a large number of whiskers with natural weed appearance were observed at the cathode side in such a short current stressing time. Furthermore, some secondary whiskers were attached to the primary whiskers, which has never been reported before. We presumed the vapor-solid (VS) mechanism to explain the oxide whiskers growth, which was quite different from the traditional theory that the compressive stress took on the driving force. In conclusion, due to the over-Joule heating effect, the metal oxide whiskers were synthesized in bulk quantities by thermal evaporation of Sn oxide and Bi oxide.

Investigation of Stress Evolution Induced by Electromigration in Sn-Ag-Cu Solder Joints Based on an X-Ray Diffraction Technique

The critical current density and temperature endured by solder joints are identified as the two most important parameters that determine the occurrence of electromigration (EM). It is well known that movement of metal atoms/ions during current stressing can induce obvious stress concentrations at the anode and cathode interfaces of the solder joint. Therefore, the global average effects of EM on stress evolution at both the anode and cathode interfaces were investigated with different current densities by employing a nondestructive x-ray diffraction technique. The ultimate results indicated that the compressive stress increased gradually at the anode side under low current density. However, stress fluctuation could be observed under high current density due to an obvious Joule heating effect. For further understanding of the mechanism of stress evolution in the solder joint during current stressing, a four-stage model is considered. First, Cu expansion led to increase of the compressive stress at both electrodes. Second, stress relaxation compensated the compressive stress and caused it to decrease. Third, the EM effect promoted compressive stress and tensile stress at the anode and cathode, respectively. Finally, a steady state of stress was achieved at the anode, while the state of tensile stress at the cathode remained transient.

Bi Layer Formation at the Anode Interface in Cu/Sn-58Bi/Cu Solder Joints with High Current Density

Bi layer formation in Cu/Sn–58Bi/Cu solder joints was investigated with different current densities and solder thickness. Uniform and continuous Bi layers were formed at the anode interface which indicated that Bi was the main diffusing species migrating from the cathode to the anode. The electromigration force and Joule heating took on the main driving forces for Bi diffusion and migration. In addition, two appearance types of Bi layers, planar-type and groove-type, were found during current stressing. The morphology and thickness of Bi layers were affected by current density and current stressing time.

Retarding electromigration on the Sn-Ag-Cu solder joints by micro-sized metal-particle reinforcement

Electromigration (EM) has become one of the reliability concerns to the fine pitch solder joint due to its increasing capacity to bear the high current density The EM effects on microstructure evolution of Sn-3.0Ag-0.5Cu+XNi (X=0, 0.05, 0.45, 2.0wt. %) solder joint was investigated. Findings of this study indicated that an appropriate addition of Ni element and moderate ambient temperature can alleviate EM damages in solder joint. The intermetallic compound layer which held a planar type and appropriate thickness improved the resistance of EM effects by suppressing the polarity effect. Ag3Sn and the bulk IMCs also retard the effects from current stressing for the solder joint.

Electromigration in Lead-Free Sn3.8Ag0.7Cu Solder Reaction Couple

Electromigration is one of the severe reliability problems in IC progress. In this paper, the electromigration of eutectic Sn3.8Ag0.7Cu solder reaction couples were studied under high temperature (150degC) and high current density (5times103 A/cm2) in three days. An original design which could reduce the local Joule heating caused by current crowding was produced. Voltage change was also monitored during this experiment. Hillocks and valleys were found at the anode side and cathode side respectively. Unlike aging test, electromigration could promote the formation of intermetallic compound (IMC) at cathode side and inhibit the formation of IMC at anode side. Cracks also appeared along the cathode side after electromigration in three days, but they were not leading the solder reaction couple to failure.

Effects of micron-sized metal particles on the mechanical properties of In-Sn thermal interface materials

With the rapid increase of heat generation, thermal interface material (TIM) is becoming one of the most important materials of micro-electronic. In-based solder TIM has high thermal conductivity, but its high cost and low mechanical properties become the obstacles of its application in industry. In order to improve the shear strength of In-based solder-TIM, we add Al particle into melting InSn to produce InSn-Al TIM. At room temperature, the shear strength of InSn-Al composite thermal interface material (CTIM) was 14.85% higher than InSn TIM. This paper also investigates the microstructure, thermal conductivity and resistivity of above two TIMs. Our work developed a background for further study in the mechanical properties of metal particle enhanced InSn-based TIM.

Interfacial reaction between the thermopile materials and eutectic Sn-based solders

The indium-based solders and bismuth-based solders are commonly employed in soldering the thermopile. Since the wettability between bismuth telluride materials and low melting point alloy solders is poor, a nickel layer was employed on the surface of thermoelectric materials. In this study, in the atmospheric environment, the eutectic Sn-Bi and Sn-In solders spreaded on the p-type thermoelectric respectively without using soldering flux, the corresponding interfacial microstructures were indentified. The interfacial reaction is more sufficient under 300 °C when comparing to that under 250 °C. In addition, a thin nickel layer was deposited on the thermoelectric unit by vapor deposition facility. However, a thin Ni-plated layer on thermoelectric materials might induce severe formation of cracks.

In situ measurement of stress evolution in Sn-based solder joint under electromigration based on X-ray diffraction technique

The in-situ measurement of the stress evolution at the anode and cathode interfaces of the Cu/3.0Ag0.5Cu/Cu solder joints with current density of 4 × 103A/cm2 at room temperature by X-ray diffraction technique was investigated. The experimental results showed that the stress evolution at the interfaces was a very complicated process during current stressing, which owed to many influencing factors such as Joule heating effect, stress relaxation, electromigration force and so on. After current stressing, the whiskers and hillocks were formed at the interface of the anode side which was as a mode of relieving compressive stress. It was found that a uniform Cu6Sn5 intermetallic compound layer was formed at the anode interface after polishing the solder joint.

Thermoelectric coupling effect on the interfacial IMC growth in lead-free solder joint

This paper presents the morphology evolution of the interfacial intermetallic compound (IMC) in Cu/Sn-58Bi/Cu solder joint during current stressing and loading temperatures. The IMC growth kinetics and Bi migration towards the anode side was analyzed and discussed. One dimensional semicircular sectioned solder joint was fabricated and used to conduct the electromigration (EM) experiment, effectively eliminated the thermomigration effect induced by Joule heating accumulation. Results indicated that the solder matrix was melted due to the combined effect of electromigration force and high temperature. After cutting off the circuit, huge valleys were found in the solder joints. Furthermore, partial bulk solder extended upon the Cu substrates. For further detecting the microstructural evolution of the solder matrix, the tested sample was polished to reveal the interior microstructure. It can be noted that the eutectic microstructure coarsened slightly, but remained the original uniform distribution. Most important, the IMC layer formed at the anode interface and cathode interface displayed different thickness. To be precise, the IMC layer thickness formed at the cathode interface was larger than that of formed at the anode interface, which was contradictory with the traditional theory of EM effect on IMC growth in SnAgCu or SnAg solder joints. By calculation, the cathodic scallop-type IMC thickness can reach 34.3μm, however, merely 9.7μm-thick IMC layer was found at the anode interface. It should be noticed that a uniform 7.5μm-thick Bi layer was aggregated between the IMC layer and the solder matrix at the anode interface. The diffusion and migration path of the Sn atoms to the anode interface was blocked because of the compact Bi layer formation, which reduced the actual Sn reacted with Cu to form IMC. Accordingly, the growth rate of the IMC layer decreased, leading to the abnormal IMC growth at the interfaces.

Abnormal phase segregation induced by void formation in Cu/Sn-58Bi/Cu solder joint during current stressing

This work focused on the electromigration (EM) behavior of the Cu/Sn-58Bi/Cu solder joint affected by the large void formation. The as-reflowed one-dimensional solder joint was stressed with current density of 5×103 A/cm2 at 80°C continuously for 144h. The microstructural evolution was observed and analyzed by SEM. Results indicated that the abnormal Sn/Bi phase segregation was observed at the cathode interface of the solder joint owing to the early serious defects formation. Local current flow was aggregated surrounding the large void, resulting in the Joule heating accumulation there. Therefore, the original straight direction of the electrons was altered near the void region, leading to the unusual Bi migration.