Thermo-mechanical modelling and thermal performance characterisation of a 3-D folded flex module

This paper reports the initial investigation into the thermo mechanical stress and thermal performance of a folded flexible substrate module. The module consists of flip chip bare die silicon onto a flex and interconnected via conductive adhesive and folding the flex to obtain the final module. From the FEM analysis it was concluded that the stresses in silicon and conductive adhesive are not significant to cause potential reliability problems. The model showed that for 525 and 250 microns thick silicon chip the stresses do not vary notably with change in polyimide or copper thickness but vary significantly below this critical value. This shows that silicon is the dominant material in the stack when the chip thickness is above 100 microns but below this thickness, stress is more dependent on copper and polyimide thickness. Thermal performance characterisation showed that for the current module set-up total power to the module rather then power to the individual chip is an important criterion for the selection of thermal management scheme. Changing the thickness of polyimide and copper has little effect on module temperature. Decreasing the chip thickness increases the thermal density and thereby increases the total temperature for the stack for any given power.