| Ball grid array (BGA) solder interconnecting is one of the key technologies in electronic packaging and assembly. Legislation of lead-free process has made the application of lead-free solder become wider in electronic products. Compared to the lead-tin eutectic solder, the relatively higher melting points of most lead-free solders call for a higher reflow temperature. Thus conventional integral-heating process at an elevated temperature would induce severe warpage of components and substrates and reduce the in-service reliability of solder joint. In this dissertation, a new concept of selective heating was proposed to address the reliability issues of integral heating process in lead-free BGA interconnection. By using this concept a novel BGA interconnection technology, named as induction heating reflow, is developed with detailed discussions of size effect of induction heat, reflow processing parameters, thermal characteristics, solder joint shape control and interfacial reactions. The significance of this research lies in the potentials of acceleration of the lead-free process in electronic productions, promotion in the applicability of lead-free BGA packaging, and the enhancement of the reliability of BGA components.In this dissertation, the size effect of induction heating for film materials is investigated to guide the design optimization for its micro-scale application. And proper parameters such as frequency and magnetic intensity are obtained for BGA solder joints. In addition, the selective heating characteristics of this method for both the material and the position of solder ball are further discussed. The selective heating for the solder joint in the package solves the reliability problems of integral heating reflow process, and the selective heating for the specific position in the joint contributes to accurate shape control of solder joints. The prism-shaped Cu6Sn5 is found at the interface between the Sn3.5Ag lead-free solder and the polycrystalline Cu in the liquid-state interfacial reaction for the first time. Its formation mechanism, growth kinetics and evolution in the solid-state aging test are analyzed as well.The size effect of induction heating for films is discussed through classical electromagnetics theory, and verified by experiments and finite element method (FEM). The concept of limited scale is proposed in the application of induction heating in film components, which means that the film component will be heated by induction heating only when its size exceeds the limited scale. The induction heating effect increases and the corresponding limited scale shrinks with an increase in the area of the film surface, the thickness of the film, the frequency, magnetic flux density of the electromagnetic field and a decrease in the angle between the direction of the electromagnetic field and the film surface.According to the conclusion of size effect, the frequency 300 kHz is chosen for the reflow of the BGA solder joints. The joining processing parameters were obtained in the experiments. The temperature profiles tested by an infrared thermometer in real-time showed the characteristics of fast heating/cooling rate and selective heating of solder joint in a package. The feasibility for the application of this induction heating reflow in electronic packaging and assembly was verified by a series of electrical tests for the chips heated under electromagnetic field.The shape of solder joints can be controlled and the hourglass-shaped solder joint can be obtained by choosing an optimum frequency of electromagnetic field and a proper pad size, without any external surporting structures. The localized melting phenomenon was identified as the key for shape control, based on the results of experimental observation, theoretical analysis and finite element method, and a rational model was built, referring to the localized melting phenomenon, to explain the formation of the hourglass-shaped solder joints.Compared with conventional hot-air reflow, the thermal characteristics of liquid-solid interfacial reaction and solid-solid interfacial reaction, the formation of interfacial intermetallic compound (IMC) and their growth kinetics were all studied. The result shows that with the increasing of temperature, Jackson factor increases, thus Cu6Sn5 grains change from scallop-type into prism-type. The growth of scallop-shaped Cu6Sn5 grain is controlled by ripening flux, and the growth rate is lower. While the growth of prism-shaped Cu6Sn5 grain is controlled by interfacial reaction flux, and the growth rate is higher, because of the existence of conserved channel for Cu atoms diffusion between IMC grains due to the lack of ripening flux. In the solid-state aging process the evolution of interfacial IMC is merely controlled by the diffusion of Cu atoms from pad, and its thickness grows linearly with the square root of aging time.
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