Thermal stress analysis of direct chip attach electronic packaging assembly

Thermal stress analysis were carried out to study the residual bending stresses in a flip-chip-on-board (FCOB) assembly during the underfill encapsulation curing process, generated in the package due to mismatches in the coefficients of thermal expansion (CTE) of the various materials in the assembly. The solder joint stress-strain behaviour under thermal cycling loading were also investigated. In this paper, parametric finite element studies were conducted on 2D plane-strain models of the FCOB assembly to investigate the development of the curing induced bending stresses in the silicon die for a selected set of design parameters. In the first exercise, the parameters that were varied include the PCB thickness and die thickness. Results of die bending curvature were correlated to simple beam bending theory with reasonable accuracy. It was also verified that 2D plane strain FE simulation provides adequate results as compared to the more complicated 3D simulation. In the second exercise, the underfill encapsulant properties were varied for a specific geometry. Curing induced stresses on the die were then compared. Lastly, the elasto-plastic and creep deformation behaviour of solder was simulated under temperature cycling conditions to obtain the stress and strain results.