Do-Seop Kim

Effect of Voids on Thermal Fatigue Reliability of Lead Free Solder Joint

There have been serious debates about whether Pb should be removed from solder joints, in view of environmental problems. These debates have now developed to the extent that a remarkable movement to establish regulations for the removal of Pb has emerged, especially in European countries and Japan. Therefore, Many studies have been aggressively undertaken to develop technologies for replacing Sn-Pb solder with lead-free alternative. From the results obtained so far, it has been proven that the fatigue strength in lead-free solder joints is almost equivalent to the fatigue strength of Sn-Pb eutectic solder joints. However, a new problem is that voids are very easily formed in lead-free solder joints during the reflow process, and the effect of the voids on the fatigue strength of solder joints has attracted attention. In this study, the relationship between the voids and fatigue strength of solder joints was examined using mechanical shear fatigue test and FEM (finite element method) analysis. Using the mechanical shear fatigue test, the effect of the position and size of voids on fatigue crack initiation and crack propagation has been investigated. And quantitative evaluation of fatigue life of solder joints with the voids has been enabled by Manson-Coffin’s law and Miner’ rule.Copyright

Fatigue Strength Evaluation for Sn-Zn-Bi Lead-Free Solder Joints

In this paper, the authors have investigated mechanical fatigue strength of Sn-Zn-Bi lead-free solder joints. The use of Sn-Zn-Bi solder is increasing for the advantage of low cost and low melting point. Therefore, it becomes important to ensure the fatigue strength of Sn-Zn-Bi solder joint. However, when the Sn-Zn-Bi solder was used as a solder material, there is serious problem that the fatigue crack is easy to generate at the interface between intermetallic compound layer and the solder matrix, and it makes the fatigue life of solder joint lower. Because the yield strength of Sn-Zn-Bi solder is high, and the difficulty of deformation causes high stress level concentrating at the corner of the interface between solder joint and substrate/package. It seems that the crack become easy to generate at the interface between intermetallic compound layer and solder matrix by this high stress concentration. The authors have found if Sn-Zn-Bi is used with another Pb-free solder material, a kind of composite structure can be built during the reflowing processes. In this study, the mechanical fatigue strength of this kind of Sn-Zn-Bi solder joint was studied. Based upon the results of mechanical shear fatigue test and FEM (Finite Element Method) analysis, it was found that if SnZn-Bi was used as reflow solder with Sn-Ag-Cu ball, the CSP solder joints are as reliable as thc pure Sn-Ag-Cu CSP. This is because Sn-Zn-Bi solder paste and Sn-Ag-Cu solder ball did not melt together completely and formed two-layer structure, and this two-layer structure reduces the stress concentration at the joint corners, and prevents successfully the occurrence of the interface cracks. As a result the fatigue life of Sn-Zn-Bi/Sn-Ag-Cu CSP is equivalent to that of Sn-Ag-Cu joints.Copyright

Influence of the Property of the Build-Up Package on Warpage at Flip Chip Assembly Process

High density and integrated packaging of electronic device requires fine pitch. This packaging causes reliability problem in electronic device. One of them, warpage of the package occurred at chip assembly process may affect reliability. Therefore, if the simulation at the time of a chip assembly process is possible, it will be able to evaluate warpage in advance. It is very effective for development of a new product. Then, in this paper, the build-up package is regarded as a single material and the simulation technique of accuracy warpage at the time of chip assembly is reported. Next it is investigated the simulation technique for package warpage at the chip assembly process. Finnaly, it analyzed about the properties which affect warpage.Copyright

Effect of Voids on Thermal Fatigue Reliability of Lead-Free Solder Joints

There have been serious debates about whether Pb should be removed from solder joints, in view of environmental problems. These debates have now developed to the extent that a remarkable movement to establish regulations for the removal of Pb has emerged, especially in European countries and Japan. Therefore, many studies are aggressively being undertaken to develop technologies for replacing Sn-Pb solder with a lead-free alternative. From the results obtained so far, it has been proven that the fatigue strength in lead-free solder joints is almost equivalent to the fatigue strength of Sn-Pb eutectic solder joints. However, the other problem is that voids are easily formed in lead-free solder joints during the reflow process, and the effects of void formation on the fatigue strength of solder joints has attracted attention. In this study, the relationship between formation of voids and fatigue fracture mode and fatigue strength of solder joints was examined using FEM (finite element method) analysis and mechanical shear fatigue test. From the results of FEM analysis, it has been found that the equivalent plastic strain and shear strain of solder joints with voids are not always larger than those of solder joints without voids and the magnitude of the strains relate to the position and size of voids in solder joints. However, the difference of the strains is not so great as to affect the fatigue strength of solder joints. It has also been proven from the mechanical shear fatigue test that the fatigue fracture mode of solder joints with voids is similar to that of solder joints without voids and fatigue strength in both cases is also almost equivalent.Copyright