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| Isothermal fatigue-testing equipment |
ABSTRACT
There have been serious debates about whether Pb should be removed 6om 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-6ee alternative.
From the results obtained so h, it has been proven that the fatigue strength in lead-6% 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-ike solder joints during the retlow process, and the effects of void formation on the fhtigue strength of solder joints have attracted attention.
In this study, the relationship between formation of voids and fatigue 6acture 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 hcture 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.
KEYWORDS: void, Pb-free, solder joint, fatigue strength, mechanical fatigue test, FEM
INTRODUCTION
Electronic devices have been adopted in not only computers but also all fields of industry such as portable telephones, automobiles, industrial robots, and so forth. The tendency of the recent development of electronic components is characterized by miniaturization, performance enhancement, and the high pin count of the package.
At the same time, there have been serious debates about that Pb should be removed 60m solder joints, in view of environmental problem. 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.
Although the amount of Pb used in electric and electronic devices is small, there is still the possibility of it resolving in the underground water supply and badly influencing human health if printed circuit boards, abandoned as part of discarded electric and electronic components, are exposed to rain.
In particular, the movement that is debating the environmental effects of electronic equipment has been more rapidly accelerated by popularization of household electric appliances such as personal computers and portable telephones.
Therefore, many studies are aggressively being undertaken to develop technologies for replacing Sn-Pb solder with a lead- free alternative.
Thus, in this paper the authors examined the thermal fatigue life of Sn-37Pb eutectic solder and lead-6ee solder that has become important in recent years, using the mechanical fatigue test.
There is the serious problem that voids are easily generated in lead-6ee solder joints during the reflow process as compared with Sn-Pb eutectic solder. The reason why the voids are formed in the lead-6ee solder joints seem to be related to the rise and fall time of temperature during the reflow process, but the effect of voids on thermal fatigue reliability of lead-6ee solder joints is still unknown.
Therefore, the authors calculated nonlinear strain of solder joints with voids using FEM analysis, and examined the relation between formation of voids and thermal fatigue strength of solder joints based on the results.
Furthermore, mechanical fatigue tests of CSPs (Chip Size Packages) were carried out in order to compare the fatigue strength of solder joints in which voids exist and solder joints without voids.
COMPARISON BETWEEN THE FATIGUE STRENGTH OF SN-37PB EUTECTIC SOLDER AND LEAD-FREE SOLDER
In this study to compare the thermal fatigue strength of conventional Sn-Pb eutectic solder and lead-6ee solder we carried out isothermal mechanical fatigue test of the BGA package. Until now, the isothermal mechanical fatigue test had been proposed instead of the thermal cycle test by the group at Yokohama National University. The consistency of these two tests has sufficiently been accepted.
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