Investigation On Thermal And Bending Perfomance To CoF Package By Simulation

With the demand of high density and small form factor of electronic products, substrates become commonly used in integrated circuit packages. Among them, the flexible package technology has been developing rapidly in the 3-D(Three-dimensional) package and CoF(Chip on Flex) package with the significant advantages such as light, thin, soft etc. But with the increase of power and the number of stacked chips, thermal performance has become an important issue. As bending radius reducing, the CoF package is expected to be applied in flexible electronic products, bending perfomance has become a growing concern.In this thesis, a finite element method has been used to estimate the thermal resistance and the bending perfomance of CoF package. And the thermal simulation has been verified by a thermal resistance test.In the thermal perfomance study, considering the influence of routing layer distribution to thermal conductivity of the flexible substrate, this paper used two equivalent methods — the length-weighted method and the image-recognition method to obtain accurate thermal conductivity. Then thermal resistance of a CoF package was simulated by ueing the two equivalent thermal conductivities, respectively. The simulated thermal resistance based on the length-weighted method is 1.653 K/W, while the value based on the image-recognition method is 1.911K/W. The device was measured by tester, and its thermal resistance was found to be 1.812K/W. So the error between the real value and simulation based on the length-weighted method is 8.8%, and the error between the real value and the simulation based on the image-recognition method is 5.5%. Therefore, both methods can produce effective simulation results, and the image-recognition method is more accurate. Finally, two improved structures were proposed based on the temperature distribution analysis.For the bending perfomance, maximum bending and bending fatigue life of a CoF package are studied. In this paper, we use the distance between two ends of substrate to reflect the degree of bending. For maximum bending, safety coefficient is considered as 1.5 on the basis of maximum tensile stress of chip and maximum shear stress of gold bump bonding surface by measured. The maximum bending is that the distance between two ends of the substrate should not be less than 3808μm by simulation. For bending fatigue life, a prediction equation was established that suited to room temperature and high frequency cyclic strain to estimate fatigue life of solder. And an existing equation was used to predict the fatigue life of the substrate. By comparison, the fatigue life of substrate is much smaller than the solder, so the bending fatigue life of CoF package is mainly desided by flexible substrate. Finally, the influence of chip thickness and substrate thickness on bending perfomance had been studied, and optimal chip thickness of 40μm and substrate thickness of 55μm were obtained.Through the above research, simulation method for the thermal and bending perfomance of CoF packags had been demonstrated, including calculation of the equivalent thermal conductivity of the substrate, the prediction of solder fatigue life under room temperature and high frequency cyclic strain, the maximum bending and bending fatigue life of CoF package.