Wafer-Level Film Selection for Stacked-Die Chip Scale Packages

Abstract

Wafer-level dicing tape format die-attach (DA) film and the corresponding lamination method provide a suitable solution for handling thin-wafers. However, such die-attach films typically have a Young's modulus less than 10 MPa at soldering reflow temperature (e.g. 260degC). This introduces a new failure mode, i.e., cohesive failure in the DA film. Through extensive experimental DOE studies described in this paper, it has been observed that some CSP packages with such film are very sensitive to substrate thickness and reflow profiles. In this work, a fundamental understanding of failure mechanisms was obtained through comprehensive finite element simulation and material characterization. It was noted that there might be a risk of cohesive failure with low-modulus die-attach film during reflow. Further, several types of die-attach films were evaluated based on full stack and discrete packages. Experimental results showed that not all die-attach films with very low modulus are sensitive to reflow profiles with cohesive failures. A general methodology for selecting die-attach film for ultra-thin stacked-die packages was developed based on the advanced measurement techniques and finite element simulation. If the film is not sensitive to reflow profile, even though the die-attach film has very low modulus, the film modulus, moisture diffusivity, and saturated moisture concentration will not be critical the parameters for screening the DA films. In this case, interfacial adhesion and the film voids become the key modulators. Since the stress state in the film is hydrostatic in a confined constraint condition, the effective hydrostatic stress in the film is not as high as vapor pressure, the low-modulus die-attach film can be used without cohesive failure. On the other hand, when cohesive failures are present, the integrated modeling approach with material characterization can be applied to design a package without failure.