Junhao Sun

Modeling the diffusion-driven growth of a pre-existing gas bubble in molten tin

Finite element method is utilized to solve the diffusion equation and model the diffusion driven growth of a pre-existing spherical gas bubble in molten tin at the solder/substrate interface for reflow time of 120 s and temperature of 250 °C. The gibbs free energy change required for determining the equilibrium concentration at liquid solder/gas bubble boundary was calculated using the thermodynamic polynomial coefficients. The rate of change of radius, as function of concentration flux, is calculated using the lagrangian mesh update methodology. With an initial diameter of 20 μm, the bubble growth is calculated as a function of contact angle. When the wetting angle is varied from a value of 30° to 135°, the numerical calculation has yielded the final sizes for the bubble to change from 62.87 μm to 82.8 μm respectively. The effect of wetting transition in the growth of bubble was studied by the in-situ observation of bubble dynamics through synchrotron radiation imaging technique. The scanning electron microscopy images of the morphologies of intermetallic compounds influenced by growing bubble in Sn/Cu solder joint and bubble pictures obtained through synchrotron radiation are utilized to get the experimental size of the bubble. The mean experimental bubble diameter has been obtained as 76.39 μm. The growing bubble inhibits the growth of intermetallic compound at its vicinity and thereby reduces the strength of solder joints.

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.

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 study of interficial reaction during rapidly solidified lead-free solder Sn3.5Ag0.7Cu/Cu laser soldering

Interfacial intermetallic compound (IMC) is a necessary condition for the reliability of solder connection. In this study, the fiber lasers were used to solder rapidly-solidified lead-free solder Sn3.5Ag0.7Cu and Cu substrate, investigating the influence of laser soldering process parameters on the growth of IMC at the solid/liquid interface and finding the optimum parameters of laser soldering process. To simulate the IMC growth under actual service conditions, the solder joints under the condition p=50w, v=140mm/min were chosen to age at 150°C. Scanning electron microscopy (SEM) and EDS were used to observe IMCs morphology and analyze the composition of IMCs. The results showed that when the laser power was 50w, the thickness of IMCs formed at the interface decreased with the increase of scanning speed. And the morphology of IMCs also changed with the scanning speed. In the aging process, the thickness of the IMCs increased with the aging time, and the morphology became relatively flat. In addition, the thickness of IMCs at the rapidly-solidified Sn3.5Ag0.7Cu/Cu, which was thicker before the aging process, was thinner than that at the as-cast Sn3.5Ag0.7Cu/Cu in the subsequent aging process. The distribution of Ag3Sn particles formed in the rapidly-solidified lead-free solders was more uniform, which suppressed the growth of Cu6Sn5 in the aging process better.

Effects of soldering temperature and cooling rate on the as-soldered microstructures of intermetallic compounds in Sn-0.7Cu/Cu joint

In this study, Sn-0.7Cu solder alloy, being selected as the research object, was allowed to react with polycrystalline Cu substrate at 250°C, 275°C and 300°C. After soldering reaction of 10 minutes, it underwent cooling in the three types of cooling medium: water, air and heating furnace, along with the simultaneous application of high pressure air for blowing away the liquid solder on the top of the intermetallic compounds (IMCs) of the specimens. Scalloped, faceted, prismatic and hexagonal shape Cu6Sn5 grains were observed at the solder/Cu interface, respectively. Scalloped shape grains were found at 250°C. At higher soldering temperatures such as 275°C and 300°C, faceted grains were dominant and the other two were mainly discovered under smaller cooling rates. With these observations, in-depth analyses of the morphology and size of Cu6Sn5 IMC as well as effect of brazing temperature and cooling rate on the microstructure of the joint were made. Moreover, synchrotron radiation real-time imaging technology, was utilized in observing the dynamic growth behavior of IMC during brazing process, thereby, providing direct evidence to the hypothesis regarding the IMC evolution pattern.

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.

The growth behavior of IMC on the Sn/Cu interface during solidification of multiple reflows

The appearance of IMCs on the Sn/Cu interface plays an important role in the joint reliability. Their growth behavior during solidification of multiple reflows was investigated. Compressed air was employed to blow away the residual liquid solder immediately after heating preservation so that the morphology corresponding to heating preservation would remain unaltered; and then by compared they with samples experienced air cooling, the growth of interfacial Cu6Sn5 grains could be investigated separately. The morphology of Cu6Sn5 grains remained a scallop-type during heating preservations of multiple reflows and they always converted to prism-type after air cooling. The dissolution and precipitation of Cu in liquid Sn with the temperature fluctuations should be responsible to the transformation. Moreover, scalloped Cu6Sn5 grains appeared at heating preservation became larger with the increasing number of reflows due to the ripening reaction. Their growth in dimension induced a change in appearance of those prismatic Cu6Sn5 grains formed on them. Shorter but larger main facets formed on those prismatic Cu6Sn5 grains with the increasing number of reflows.