What is the Poisson’s ratio of a typical die attach adhesive?
Poisson’s ratio of a material is the ratio of transverse strain (lateral strain) to longitudinal strain (axial strain) when it is stretched or compressed. Most materials will have a Poisson’s ratio between 0 and 0.5. A material that shows no dimensional change in the lateral axis, such as cork, has a Poisson’s ration of 0. Since the greatest effect on stress of a molded package are the CTE and Young’s modulus of the molding compound, the Poisson’s ratio of the die attach adhesive is of little importance when modeling stress of a package. Using a number between 0.3 and 0.49 will give a satisfactory result. The following paper, which is available through Ablestik, offers reference to the above. B. Romer and H. Pape, “Stress Effects of Package Parameters On 4Mega DRAM With Fractional Factorial Designed Finite Element Analysis”, Electronics Components Conference, 1989.
