Bingfeng Guo

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.

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 the effects of Cu addition on the rapidly growth of Cu 6 Sn 5 at the Sn-base solder/Cu L-S interface during soldering heat preservation stage

Synchrotron radiation x-ray real-time imaging technology was used to study the interface reactions of Sn/Cu and Sn0.7Cu/Cu joints during reflow with soldering time of 1h under temperature of 300/350 °C. This experiment allows us in-situ observe the interfacial IMCs growth and completely avoids Cu6Sn5 precipitation from the solder in cooling stage to affect the morphology and thickness of interfacial IMCs. This work is a foundation work to establish quantitatively sufficient as well as accurate data for IMC growth under isothermal reflow soldering on one hand and be viewed as the non-destructively evaluated results on the other hand. It is in-situ observed that the IMC morphology is scalloped during the heat preservation stage at both two solders/Cu interfaces. Comparing with pure Sn solders, the Sn0.7Cu solders are observed with interfacial IMC of greater thickness, greater number of grains, smaller base width and more substrate consumption. During the heat preservation stage, all of the growth kinetic index (n) is close to 1/3 indicating that grain boundary diffusion determines the interfacial reaction. The mechanism of Cu addition to affect interfacial reaction by changing the microstructure of solder matrix is clarified, and then the morphology and thickness of interfacial IMCs can be controlled by alloy elements addition to improve the soldering reliability of electronic packaging.

In situ study of real-time growth behavior of IMC morphology evolution during the Sn/Cu soldering cooling stage

The morphology evolution of Cu6Sn5 intermetallic Compound (IMC) in cooling stage were studied. The IMC morphology is scallop-like with small plane observated by synchrotron radiation real-time imaging and high pressure air removing excess liquid solder cooling. But, the IMC morphology observated is prismatic with facet of interior portion and at exterior portion is scallop-like with small plane under air cooling and the cooling stage of synchrotron radiation real-time imaging condition. From that, it can be noted that the morphology evolution of IMC in the solder joint was greatly affected by the cooling phase of interfacial reaction, and IMC morphology partition phenomenon was found. With rise of the solder temperature, the thickness of IMC grows rapidly. At the same time, the thickness of the IMC in the inner region of the spherical solder joint reaches to 7.7245 μm, while the thickness of the IMC at the edge only reaches to 4.556 μm. In the analysis, the growth behavior of interface IMC in cooling stage is affected by copper precipitation in the solder. The precipitation of copper in the solder provides the necessary elements for the rapid growth of IMC in the cooling stage, resulting the IMC morphology transition during the cooling phase.

The study of the growth behavior of Cu 6 Sn 5 at the Sn/Cu interface during the heating preservation stage

In this study, Sn/Cu soldering interface and the growth characteristics of IMC grains were in-situ investigated. High-pressure air (0.8MPa) was used to blow away the residual liquid solder on the Cu substrate at different soldering temperatures (250 ° C, 275 ° C, 300 ° C) and different soldering time (10s,30s,60s,120s), thereby obtaining the unobstructed view for growth morphology of IMC in the heating preservation phase. The top-view and cross-section view of solder joints were observed under scanning electron microscopy (SEM). With these observations, in-depth analyses of the morphology and thickness of IMC were made. It was observed that IMC grains in heating preservation all presented scalloped or oval structured morphology with the increase of soldering temperature and soldering time. Scalloped IMC grains appeared at heating preservation, then became larger and the amount of IMC grains decreased with the increasing of soldering temperature and time, which can be explained in accordance to the ripening theory. The grain size in heating preservation illustrated a nearly uniform fashion, resulting from large swallowing up of small grains during the soldering process. By calculating the thickness of IMC layer in different soldering time, it was found that the increase of IMC thickness obeyed the t1/3 law, which means IMC growth is controlled by GB during heating preservation.

The morphology variation of IMC on the solder/bubble interface under different cooling rates and temperatures

As bubbles on the solder/copper interface can seriously decrease the joint reliability of lead-free soldering in electronic packaging industry, the research of interfacial bubbles is extremely imperative. In the local area around an interfacial bubble, the Cu atoms tend to migrate towards the gas/solid interface, providing the copper element to form the interfacial intermetallic compounds (IMCs). Because of the different thermal conductivity inside and outside the void (inside gas lower than outside solid), the bubble interface will have a disparate temperature gradient comparing with the solid area around the bubble. Based on this, the morphology variation of IMCs on the solder/bubble interface under various soldering temperatures and cooling rates was investigated in this study, using the scanning electron microscope (SEM) for the cross section each time after the soldering reaction part carried out in furnace with the soldering temperature of 250°C, 300°C, 350°C and holding time of 60s, followed by water, air, furnace cooling (WC, AC, FC). It was found that, IMCs are much more easily to exist on the bubble interface, especially the position including some small holes due to the higher concentration of metal atoms brought by them; On the other hand, most of the bubble interface will have no well-formed IMCs if no small voids sticking to when the cooling rate is high. In a certain soldering temperature, with the cooling rate declining (WC>AC>FC), the morphology of the interfacial IMCs gradually change from solid to hollow structure due to different growth rates of two vertical growing orientation inside the new formed phase. In addition, when the soldering temperature improved, the volume ratio of the cavity part in the hollow structure will reduce gradually.