Jongmyung Kim

Eddy Current Induced Heating for the Solder Reflow of Area Array Packages

This paper presents a feasibility study of using eddy current induced heating for the solder reflow of area array packages. With a high frequency electromagnetic field, Sn 3.5% Ag lead-free solder balls were heated to melt and wet the solder pads on an organic substrate. The experimental results showed that such a solder reflow process could be implemented effectively in a wide processing window. With an infrared temperature sensor, it was found that the temperature difference between the solder balls and the FR4 board might reach 80 , indicating a rather localized heating mechanism. In addition, the heating and cooling rates during the soldering were found very high. This is another feature of the induction heating reflow process. It was also found that the generated temperature in the solder balls would depend on the size of the solder balls. This characteristic may be used to perform selective soldering of flip chip BGA packages. Furthermore, the developed soldering process was applied to the board level reflow of actual components. The result verified that the induction heating reflow is a feasible soldering process in actual applications.

Local melt process of solder bumping by induction heating reflow

Purpose – The purpose of this paper is to describe a local melt process of solder bumping employed in electronic packaging applications by an induction heating reflow method, for a combined numerical and experimental study involving a temperature measurement using an infrared thermometer during the reflow process and microstructural observations after reflow, which can be used to control the height and shape of solder interconnects.Design/methodology/approach – In the induction heating reflow process, the temperature distribution within the solder ball during the heating phase is of prime importance for the success of the process and the geometry control quality of final joints. This paper investigates the local melt process of solder balls reflowed onto Cu/Ni/Au pads, and focuses on the effect of the inductive heat on the thermal distribution during the melting process. A finite‐element model is applied to simulate the thermal field in a solder bump during the induction heating period in a reflow process...

Solid-state interfacial reaction of eutectic Sn3.5Ag and pure tin solders with polycrystalline Cu substrate

Intermetallic compounds (IMCs) that form at eutectic Sn3.5Ag/Cu and pure Sn/Cu interfaces during solid-state aging are comparatively studied in terms of their respective morphological formations, orientation evolution behaviors, and growth kinetics. During solid-state aging, all the interfacial Cu6Sn5 grains evolve into a layer-type morphology, except for select grains that experience abnormal growth. This abnormal growth is caused by the preferential growth of the Cu6Sn5 at the grain boundary in solder matrix. Meanwhile, textured growth occurs in the Cu6Sn5 layer formed at the eutectic Sn3.5Ag/Cu interface. The morphology of each texture is determined by the initial joint preparation conditions and affects the growth of interfacial IMCs. The results reveal that Sn diffusion occurs faster along the [0001] direction of the Cu6Sn5 crystal than along angles from 25° to 50° relative to the [0001] direction. Additionally, the effects of solder composition on the interfacial IMC growth are evaluated. The results indicate that Ag addition retards IMC growth upon aging by inhibiting diffusion of Cu.

Manufacture of hourglass-shaped solder joint by induction heating reflow

Induction heating reflow method, which can achieve the solder bumping and interconnecting process in a simple way, was studied to control the height and shape of solder interconnects employed in electronic packaging application. A solder joint model was built to investigate the temperature distribution in the joint during the whole induction heating process by ANSYS software. Based on the simulation, the localized melting phenomenon is defined and identified by scanning electron microscope (SEM) observation. In this work, the barrel-shaped solder joints with high heights and the hourglass-shaped solder joints can be obtained which is useful to increase the solder joint lifetime. The mechanism of solder joint height and shape control, which can be explained by the local melt phenomenon, is discussed and demonstrated by the different morphologies of Ag3Sn intermetallic compound (IMC). The findings of this paper will help to provide an understanding of the whole solder interconnecting process during induction heating reflow and the effects of electromagnetic field on solder interconnect shape controlling.

Twin-induced ultra-high thermal conductivity of sintering Ag nanoparticles for high power density electronic packaging

Rapid pressureless low temperature sintering of Ag nanoparticles for bonding is achieved. Organic shells adsorbing on the surface of Ag nanoparticles to stabilize them are thinned to be a sparse protecting layer. Numerous coherent twin boundaries formed in sintered Ag nanoparticles with a grain size of 21 nm induce ultra-high thermal conductivity (229 W/m·K), which overcomes the intrinsic defect that metals with nano-sized grain generally cause a pronounced decrease in thermal conductivity because of the grain boundary scattering effect.

Growth behavior of prismatic Cu6Sn5 and its effect on mechanical properties of eutectic Sn-Ag solder joints

The morphologies of Cu6Sn5 grains formed at the interface between Sn-3.5Ag (wt.% unless otherwise specified) and Cu substrates were studied in this work. Reflow experiments were performed for 60 s at peak temperatures of 513 K, 533 K, 543 K, and 553 K. Two morphologies of interfacial Cu6Sn5 grains were observed after the joints solidification in air: prism type, above 543 K, and scallop type, below 533 K. The crystal planes of the faceted surfaces of these prismatic grains were confirmed by electron backscattering diffraction (EBSD). To study the growth behavior of these prismatic grains, the morphologies of Cu6Sn5 grains formed at the interface during soldering were obtained by an experiment. The results show that all the interfacial grains formed during soldering displayed a similar scallop type, indicating that the prismatic grains were formed during solder solidification. In addition, significant differences in shear strength were observed for solder joints with different interfacial Cu6Sn5 morphologies in the case of a lower shear height. Joint strength is discussed in terms of the microstructure of the solder matrix and the morphology of interfacial Cu6Sn5 grains.

Nanoindentation characteristics of Cu 6 Sn 5 formed in lead free solder joints

A new structural effect that was developed based on the concepts of the cantilever beams has been proposed in order to illustrate the measurement divergence of nanoindentation test for Cu6Sn5 IMCs. Cu6Sn5 IMCs were fabricated in Sn3.5Ag/Cu solder joints by wetting reaction at 250 °C and subsequent thermal aging, and measured by nanoindentation. In this study, the extra displacement created by the cantilever-like beam structural effect were investigated and discussed through the experimental initial and boundary conditions of Cu6Sn5 IMCs. The results showed that the variations of the loading procedures, sample shape, indent positions and indent directions would change the strain gradients on the matrixes, and thus cause the extra displacement. Moreover, through the study of the two-dimensional finite element analysis, the data divergence is minor in the top-view direction. In light of the results obtained from this study, we believe that structural effect would influence measurement accuracy for nanoindentation and test in the top-view direction would have great potential for future applications.

Geometry control of solder interconnects via induction heating

Purpose – The aim of the paper is to control the height and shape of solder interconnects employed in electronic packaging applications by an induction heating reflow method, which can achieve the solder bumping and interconnecting process in a simple way.Design/methodology/approach – Through the application of a designed induction heating system, a new methodology was forced to exhibit its certain qualities of forming and geometry controlling in the process. The corresponding temperature distribution has been analyzed based on a discussion of skin effects in metal spheres. The local melt phenomenon is observed and identified via the microstructures taken by scanning electron microscope (SEM).Findings – In this work, barrel‐shaped solder joints with high heights and hourglass‐shaped solder joints can be obtained, which is good for improving the solder joint lifetime. The mechanism of solder joint height and shape control, which can be explained by the local melt phenomenon, is discussed and demonstrated v...

Lead-Free Soldering Technique by Using Medium-Frequency Electromagnetic Field

Area array soldering is one of the main interconnection technologies for the inter graded circuit (IC) packaging and assembly. The typical area array soldering processes include some conventional heating methods, such as forced convective hot-air reflow and infrared reflow, which have the merit of high throughput in mass industrial production and, however, sometimes may cause the warpage of components or printed circuit boards (PCBs) due to excessive heat supply and/or prolonged heating time. As such, lowering the temperatures of the component and the board during the reflow process for the lead-free solders will lead improvement in the device reliability. The localized heating methods have been developed and applied for electronic packaging to solve the aforementioned reliability problems. However, for most of the developed methods the equipment cost is tremendously high or the processes are rather difficult (if not impossible) to finish the board level soldering of area array packages. A novel lead- free interconnecting technique using medium-frequency (MF) electromagnetic field was investigated. The present study is a continuing effort to prove the feasibility of eddy current induced heating for the reflow soldering of area array packages. In this paper, experimental investigations and simulation on the heating effects of the MF electromagnetic field will be presented. The simulation was conducted to demonstrate the selective heating during the reflow process, which can also be proved by the experimental results. Comparison between the thermal characteristics and the interfacial microstructures of the lead-free solder joints on Ni/Au surfaces formed by a conventional thermal reflow process and MF inductive heating reflow process was conducted. The temperature profiles on solder balls and the PCB during the reflow were measured by infrared thermometer. The MF heating technique successfully created the lead-free solder joints through the intermetallic compounds (IMCs), while maintaining the substrate temperature as low as 180?C. This paper reports the soldering behaviors of the Sn-3.5 Ag joints on Cu and Sn surfaces by MF electromagnetic field. The solder joints formed by a conventional reflow process were compared with those soldered in the MF electromagnetic field, and their morphologies, the IMC formation and the joint formation were discussed.

Local Melting and Formation Steps of Solder Bumps via Induction Heating Reflow

This work focuses on the nonuniform melting process and its theoretical explanation. After induction heating for 0.8 s, 1.0 s, 1.4 s and 2.0 s, different welding state can be obtained, which gives a proof of the melting process. The experiment results demonstrate that the skin effect of induction heating forms the great temperature gradient in the solder bump. The computation results give a theoretical support of the local melting phenomena. The surface melting of solder ball can affect the height and shape of the solder bumps, which is an important factor of the thermal reliability for BGA.