Determination and Verification of Silicon Die Strength Using Ball-Breaker Test

Abstract

3D or stacked-die packages are becoming increasingly popular in the electronic packaging industry because of the current market demand for cheaper and smaller products with high performance characteristics. As a result, the IC silicon wafers have to be grinded through wafer-thinning processes to achieve greater packaging density. However, it is possible to induce crack of the chips during stacking process or in the use of the device. Therefore, this study aims to determine the die strength of (1 0 0) silicon which can provide to designers for reliability of the die. Several methods have already been adopted to determine the strength of silicon die. These methods include three-point bending test (3PB), four-point bending test (4PB) and ball-breaker test. However, 3PB and 4PB have difficulty for application not only in experiment set ups and silicon die sample preparation aspects but also in actual use because of their sensitivity to both edge and surface defects. Therefore, the ball-breaker test is then proposed in this study to measure the maximum allowable force of silicon die. Meanwhile, comparing with experiment data, the finite element method (FEM) analysis using commercial software ANSYS/LSDYNA3D® are introduced to determine the silicon die strength. Moreover, the 3D model of the ball-breaker test is verified through the Hertzian contact theory. The effect of the thickness on silicon die strength and the failure modes are also discussed in this study. As the applied force increases, the crack appears on the edge of the contact area on the top surface and flare out within the die. However, the radial crack occurs on the bottom surface while the bending effect on the bottom side of the test die has become significant as the die thickness decreases. The early failure may occur at the position and then crack through the top surface causing the die breakage. In other words, the determined strength in this experiment decreases as the thickness of test die becomes increasingly thin. Furthermore, the simulation results show that the allowable force of silicon dies increases as the softer foundation material is applied while the bending behavior is not significant. However, the breakage of the thinner test die placed on the softer material is much easier to happen because the tensile stress on the bottom surface resulting from the bending behavior increases rapidly and significantly influences the die breakage.