How Compliant Should a Die-Attachment be to Protect the Chip From Substrate Bowing?

Abstract

The purpose of the analysis is to find out whether die attachment can be made compliant enough to protect the chip from excessive bowing of the substrate. We showed that in a typical situation, when the substrate (card) has a significantly larger flexural rigidity than the chip, the mechanical behavior of the chip-substrate assembly is governed by a parameter u=lK/4D14, where l is half the chip’s length, D1 is its flexural rigidity, and K is the through thickness spring constant of the attachment. We found that in order for a die attachment to have an appreciable effect on chip bowing, this parameter should be considerably smaller than 2.5. In the performed numerical example, for a 2 rail thick die attachment, this value corresponds to Young’s modulus of only 2300 psi. Therefore we conclude that conventional epoxy adhesives cannot provide sufficient buffering effect in this case, and, if such adhesives are used, the curvature of the die will be practically the same as the curvature of the substrate. In this situation, thinner dies will result in lower bending stresses. However, if low modulus die attachment materials, such as silicone gels, are considered, then employment of thicker dies might be advisable.