Lin Qu

In situ study on the effect of thermomigration on intermetallic compounds growth in liquid-solid interfacial reaction

Synchrotron radiation real-time imaging technology was carried out in situ to observe and characterize the effect of thermomigration on the growth behavior of interfacial intermetallic compounds (IMCs) in Cu/Sn/Cu solder joint during soldering. The thermomigration resulted in asymmetrical formation and growth of the interfacial IMCs. Cu6Sn5 and Cu3Sn IMCs formed at the cold end and grew rapidly during the whole soldering process. However, only Cu6Sn5 IMC formed at the hot end and remained relatively thin until solidification. The IMCs at the cold end were nearly seven times thicker than that at the hot end after solidification. The Cu dissolution at the cold end was significantly restrained, while that at the hot end was promoted, which supplied Cu atoms to diffuse toward the cold end under thermomigration to feed the rapid IMC growth. Moreover, the thermomigration also caused asymmetrical morphology of the interfacial IMCs at the cooling stage, i.e., the Cu6Sn5 IMC at the cold end transformed into facet ...

A numerical model for diffusion driven gas bubble growth in molten Sn-based solder

As soldering process is a high temperature phenomenon accompanied by surface reaction kinetics, the computational model for solder bubble growth can be a strong arena for describing those aspects which are otherwise difficult to be understood through the experimental methods. In this paper, the growth of gaseous bubbles in the molten tin solder has been modeled using finite element method. The advection-diffusion and lagrangian mesh adaptation equations have been utilized to obtain the numerical solution of concentration flux and radius change for a single spherical bubble. Utilizing the axisymmetric coordinate system (2D), the final bubble diameter has been obtained to be 31.06 μm at a working temperature of 350 °C. The experimental images of the voids have been obtained from Synchrotron Radiation Imagaing Technique and Scanning Electron Microscopy. Since the diffusion limited void growth is found to increase with the greater value of working temperature whereas decrease with the higher magnitudes of viscosity and surface tension of the solder alloys, these properties need to be addressed at high temperature applications. Future works in this area include the addition of the roles of the Intermetallic Compounds and bubble interface coalescences.

The study of cooling process' effect on the growth of IMC at Sn-3.5Ag/Cu soldering interface

As one of the important factors affecting the quality of soldering joints, the intermetallic compound (IMC) formed at the solder and substrate interface becomes more and more concerned by researchers. However, its difficult to study the effect of cooling process on the growth of IMC for the continuity and high temperature of the soldering process. This paper making use of high-pressure air to remove liquid solder in reaction to prevent IMC from continuing grow in the cooling process and achieve the purpose of rapid cooling, meanwhile retaining the growth morphology characteristics of IMC in the process of heating-heat preservation stage, consequently making it possible for studying the effect of cooling process on the growth of IMC independently in soldering process. By comparing the top-view and cross-section of the IMC growth morphology of soldering joints in general air cooling conditions with the high-pressure air cooled soldering at different soldering time (5 s, 10 s, 30 s, 60 s) and different soldering temperature (250 °C, 300 °C), obtained that the grains of IMC in high-pressure air cooled were all presented as scallop structure while the grains in general air cooling were faced, later even prismatic structure, which proved that Cu6Sn5 has a secondary growth in the cooling stage, and the Ag3Sn particles were also formed in cooling stage.

Positive feedback on imposed thermal gradient by interfacial bubbles in Cu/liquid Sn-3.5Ag/Cu joints

Synchrotron radiation x-ray imaging technique was applied for in-situ observation of interfacial bubbles in Cu/molten Sn-3.5Ag/Cu joint undergoing thermomigration. The heating plate temperature was maintained at 350 °C and reflow time of 1 h was considered. Interfacial bubbles at hot side, favoured by wetting transition for growth, increase the effective thermal gradient in the solder medium. The interfacial voids lower the resistance of Sn-based solder joints to thermomigration. The numerical model for effective thermal conductivity and heat transfer in the inhomogeneous medium was implemented using finite element method.

The effect of laser-soldering parameters on the Sn-Ag-Cu/Cu interfacial reaction

The effect of the fiber laser soldering parameters, laser-soldering output power and laser-soldering scanning speed, respectively, on the Sn-Ag-Cu/Cu interfacial reaction was investigated in this paper. When the output energy was very low, the Cu₆Sn₅ IMC was in scallop type with smooth surface; as the energy increased, the Cu₆Sn₅ grains showed the morphology of scallop type grains with faceted surface; further grew the energy, the scallop-type of Cu₆Sn₅ would be replaced by the prism-type Cu₆Sn₅; the Cu₆Sn₅ grains which were bottom closely arranged with a convex on the top can be found if the laser soldering energy was up again; when the laser soldering energy increased again, the plate-like Cu₆Sn₅ can be observed at the interface between the solder and Cu substrate.

The study on the rapidly-solidified Sn-0.7Cu lead-free solders and the interface reactions with Cu substrate

The rapidly-solidified lead-free solders had lower melting point, narrower melting interval, better wettability and improved mechanical performance because of the fine, uniform and small segregation microstructure compared with the as-cast lead-free solders. In this study, a rapidly-solidified Sn-0.7Cu eutectic alloy was used as a sample for investigating the microstructure, wettability and the Cu6Sn5 IMCs and their growth behavior formed between the solders and Cu substrate. In addition, an as-cast Sn-0.7Cu solder was prepared as a reference. The results of the study showed that the microstructure composition was more uniform, the wettability was much better, the melting rate was much faster of the rapidly-solidified Sn-0.7Cu solders than the as-cast solders; the morphology of Cu6Sn5 grains formed between solders and substrate was different between rapidly-solidified and as-cast Sn-0.7Cu solders in short soldering duration; the rapidly-solidified Sn-0.7Cu solders were suitable for soldering with low temperature and short duration.

In-situ study on the formation and evolution behavior of voids at the interface during soldering process by synchrotron radiation real-time imaging technology

In this paper, by using the synchrotron radiation real-time imaging, the growth of gas bubbles on solid-liquid interface during the soldering process was investigated. It was proved that the formation and growth of the bubbles was a process of heterogeneous nucleation. During the soldering heating insulation stage, the bubbles volume increased first and remained unchanged afterwards. When there were more than one bubbles, the annexation process was happened among the adjacent bubbles, the larger bubbles were pulled to the smaller ones. The bubbles volume remained unchanged during the cooling stage. At the same time, the dissolution of the Cu substrate and the growth of the IMCs were influenced by the bubbles.

Study of electrochemical migration based transport kinetics of metal ions in Sn-9Zn alloy

Abstract In microelectronic circuit exposed to humid environment, the growth phenomenon of dendritic deposits due to the electrochemical migration (ECM) of metallic ions, generates a serious reliability issue. ECM of Sn and Zn ions in deionized water with Sn-9Zn electrodes, has been in-situ studied at gap potentials of 3 and 5 V. At 3 V, tin ions (with 15.61 wt%) dominate the composition of metallic dendrites whereas Zn ions (with 20.99 wt%) show greater presence in the experiment with 5 V. The Nernst-Planck transport equation has been solved using finite element method in order to describe the kinetics of ECM of Sn2+ and Zn2+ ions. The composition of resultant dendrites is governed by the competition between advection rate, transport time scale and anodic surface concentration of metallic species.

In Situ Study on Current Density Distribution and Its Effect on Interfacial Reaction in a Soldering Process

The interfacial reaction in Cu/Sn/Cu solder joint during liquid–solid eletromigration (EM) was in situ studied using synchrotron radiation real-time imaging technology. The current density distribution in the solder joint was analyzed with the finite element method (FEM). The relationships among solder shape, current density distribution, Cu dissolution, and the formation and dissolution of interfacial intermetallic compound (IMC) were revealed. The current promoted dissolution of the cathode IMC and growth of the anode IMC and suppressed the dissolution of anode Cu. The change of interfacial IMC had little effect on the current density distribution; however, the dissolution of cathode Cu, which changed the solder shape, had a significant effect on the current density distribution. The dissolution of cathode Cu under forward current and cathode IMC under reverse current and the growth of anode IMC under forward current was faster where the current density was higher. The synchrotron radiation real-time imaging technology can not only in situ observe the change of solder shape, the dissolution and growth behavior of interfacial IMC and the dissolution behavior of substrate in a soldering process but also provide data needed for numerical simulation of current density distribution in a solder joint.

In situ study on growth behavior of interfacial bubbles and its size effect on Sn-0.7cu/Cu interfacial reaction

With the advent of lead-free soldering electronic packaging industry, there are growing concerns regarding the problems of voids in lead-free solder joints. Many researchers are eager to understand the growth, evolution and control factors of bubbles. Due to the restrictions in test conditions imposed by high temperature and opacity of soldering, one cannot directly observe and analyze the behaviour of bubbles. Moreover, bubble evolution and surface reaction effects go hand in hand with each other during soldering; so the latter should be interfering the pattern of the former and vice-versa. In this study, the growth behavior of interfacial bubbles and the effect of bubbles on interfacial reaction during a soldering process were in situ studied by the synchrotron radiation real-time imaging technology. In the scanning electron microscope, we can observe the bubbles size and its impact on the scope of IMC at Sn-0.7Cu soldering on 250°C. We found that bubbles effecting surface reaction have a the critical value and there is differentiate between different size of bubbles and its area of influence, the larger of bubbles, the greater effecting area. The closer to the bottom of the bubble, the less the Cu substrate dissolved. The effect of bubble on the IMC growth around the bubble was small, and the average IMC size decreased gradually with the increase of the distance and stabilized finally.