Mechanical failure in COB-technology using glob-top encapsulation

There are several reasons for mechanical stresses in “globbed” assemblies. For example: cure shrinkage; thermal gradients; and moisture diffusion. Thermal mismatch between substrate, silicon die, and encapsulant represent a main reason for thermally induced stresses and is investigated in detail. The filled polymers used have shown temperature, time, and moisture dependent mechanical characteristics. The viscoelastic properties of the encapsulant below the glass transition temperature (T_g) are represented by a Prony series sum of exponentials with three terms, while the temperature dependence is included by a temperature time shift formula. Finite element (FE)-simulations including the creep characteristics allow the evaluation of thermally induced stresses. The calculations show the major stress concentration to occur at the inner edge between die/adhesive/encapsulation and the ceramic board. Local stress concentrations arise at the upper interfacial edge die/encapsulation and at the outer border of the glob-top. Furthermore, the theoretical results are compared with observations of thermally cycled hybrids made by scanning acoustic microscopy (SAM) and metallographic investigations. In this way not only the influence of thermal cycling, but also humidity exposure on the glob-top´s integrity was evaluated. The typical failures were shown to be delaminations at the epoxy encapsulant interface