Improved thermal fatigue reliability for flip chip assemblies using redistribution techniques

The bond pad design on a chip can be reconfigured to a new pad design using a redistribution layer, based on multichip module-deposited (MCM-D) technology. The new pad configuration can be used for flip chip mounting. The thermo-mechanical reliability of these redistributed flip chip structures is in particular determined by the visco-plastic deformations of the solder joints and by the stresses in the photosensitive BCB redistribution layers. In this paper, the influence of this redistribution layer on the solder joint reliability is investigated. Also the induced stresses in this redistribution layer may not exceed the ultimate stress level. Three different redistribution processes are considered. Finite element simulations and Coffin-Manson based reliability models are used to compare the thermal cycling reliability of redistributed and standard flip chip assemblies. The existence of a photosensitive BCB redistribution layer on the chip influences the thermal fatigue of solder joints. The largest reliability improvement using redistributed chips is achieved by moving the solder joints from the perimeter to the interior of the die resulting in an area array flip chip.