Influence of Polymer Viscoelasticity on a Bending Bilayer

The bending bilayer technique is commonly employed to provide an empirical assessment of the often significant thermal stresses that develop in thin polymer films used in microelectronic packaging. Polymers are known to exhibit time-dependent (viscoelastic) behavior that is greatly accelerated at elevated temperatures. The solution for a bilayer beam with a linear viscoelastic, thermorheologically simple film bonded to a linear elastic substrate is developed. A wide variety of assumed viscoelastic material properties are considered along with properties for several specific electronic polymers in order to illustrate the general effects of polymer viscoelasticity upon the thermal stress in a polymer film. Practical methods for deducing the relative significance of polymer viscoelasticity from empirical bending bilayer results are emphasized. Comparisons with experimental data for two polymides and one epoxy molding compound are also provided.