Chip-on board technology for low temperature environment. Part II: Thermomechanical stresses in encapsulated ball-wedge bond wires

Wire-bonded chip-on-board (CoB) multi chip modules consist of die and bond wires that are encapsulated to protect them from mechanical and chemical damage. This paper describes a rapid-assessment model for the prediction of thermomechanical strains developed in the encapsulated ball-wedge bond wires due to thermal expansions experienced during curing or subsequent environmental changes. The wire profile is modeled using a piece-wise continuous polynomial function (cubic spline) with appropriate boundary conditions at the two bond sites. Plastic deformation is ignored in the current analysis as a first-order approximation. Then a 2D Raleigh-Ritz (RR) model is developed to estimate the thermomechanical stresses in the bond wire due to temperature cycling in the presence of an encapsulant. The purpose of the model is to provide a rapid ranking of the thermomechanical robustness of different wire-bond design options. Results are validated by detailed 2D finite element analysis (FEA) and are compared to fatigue failure data available from thermal cycling tests.