Energy based failure criterion for interfacial delamination [IC packaging]

Plastic integrated circuit packages can fail due to delamination between the interfaces of the different materials. The adhesion properties across the interface of the epoxy encapsulant and the substrate are important to package reliability. The button shear test is one of the standard tests carried out to evaluate the interfacial adhesion material properties. This paper presents a method to derive a criterion for interfacial delamination using the result of the button shear test. A series of button shear tests was conducted to evaluate the adhesion properties of epoxy molding compounds (EMCs) on copper substrates. In each of the tests, the critical load acting on the EMC of the button shear sample was measured at different shear heights and a finite element model was used to evaluate the stresses at the interface between the mold compound and the copper substrate. The distortional and hydrostatic strain energy densities across the interface were also calculated. The test was also applied to a range of EMC materials. The distortional energy was found to have a linear relationship with the square of the fracture load and the EMC´s Young´s modulus. The hydrostatic energy was also found to have a similar but marginally different relationship with those terms. The result of the energy densities across the interface at different shear heights was also reviewed. Based on these observations, a delamination criterion was developed which includes the ratio of the distortional and hydrostatic energy density. The delamination criterion is to be tested on packages under thermal loading