Experimental and numerical reliability investigations of FCOB assemblies with process-induced defects

To develop comprehensive design guidelines, models and experiments cannot overlook process-induced imperfections in the flip chip on board (FCOB) assemblies. The following items were noted as being significant factors which were used for modeling and by thermal cycling tests: (a) varying standoff-heights and alternative bump sizes, (b) underfill-particle settling, (c) underfill-void effects, (d) underfill-to-bump coverage, (e) asymmetrical fillets vs. symmetrical fillets. A detailed numerical and experimental reliability study of perfect and imperfect flip chip assemblies has been completed. Experimental studies of the failure modes and of the mean cycles to failure are in good agreement with the failure modes and life time, as predicted by FEM for the different technological variants. A hierarchy of influences was worked out in three levels (important, medium, negligible). Most important imperfections resulting in a strong reliability decrease are particle settling, asymmetrical fillets and small and big voids