Reliability and die stress measurements in flip chip assemblies with carbon fiber core laminate substrates

In this work, we report on our efforts to develop high reliability flip chip on laminate assemblies for deployment in harsh thermal cycling environments characteristic of ground and aerospace vehicles (e.g. -55 to 150 degC). Reliability enhancement has been achieved through the use of a novel low expansion, high stiffness, and relatively low cost laminate substrate material that virtually eliminates CTE mismatches between the silicon die and top layer PCB interconnect. The utilized laminate features a sandwich construction that contains standard FR-406 outer layers surrounding a low expansion high thermal conductivity carbon fiber-reinforced composite core (STABLCORreg). Through both experimental testing and modeling, we have demonstrated that robust flip chip assemblies can be produced that illustrate ultra-high solder joint reliability during thermal cycling and extremely low die stresses. Liquid to liquid thermal shock testing has been performed on test assemblies incorporating daisy chain test die, and piezoresistive test chips have been used to characterize temperature dependent die stresses. In both sets of experiments, results obtained using the hybrid PCB laminate with FR-406 outer layers and carbon fiber core have been compared to those obtained with more traditional glass-epoxy laminate substrates including FR-406 and NELCO 4000-13. Nonlinear finite element modeling results for the low expansion flip chip on laminate assemblies have been correlated with the experimental data. Mechanical testing of the carbon fiber-reinforced laminate materials was used to demonstrate its high elastic modulus over a wide temperature range. In addition, unconstrained thermal expansion measurements have also been performed on the hybrid laminate materials using strain gages to demonstrate their low CTE characteristics