Optimisation of the Substrate Preheat Temperature for the Encapsulation of Flip Chip Devices

Flip chip on board (FCOB) technology involves the bonding of a bare die directly onto a printed circuit board (PCB). The interconnections are provided by solder bumps which are arranged in the area under the chip. Encapsulation is the preferred process to reduce the impact of the thermal stress that results from the mismatch in the coefficient of thermal expansion (CTE) between the silicon chip and the substrate. The length of time needed for the dispensing and capillary flow of the encapsulant is one of the important factors that inhibits the high volume use of FCOB technology. Preheating the substrate prior to and during the dispensing of the encapsulant enhances its flow characteristics by reducing its viscosity. This research addressed the feasibility of using a substrate preheat temperature which is higher than the manufacturer's recommendation in order to reduce the cycle time of the encapsulation process while still ensuring package reliability. Defects that may be caused by increasing the substrate preheat temperature are discussed and evaluated. A statistical model was generated to help determine the optimal substrate preheat temperature for given chip dimensions, standoff height, surface texture, etc. Using the substrate preheat temperature determined by the model, the flow time of the encapsulant could be reduced by more than a factor of two while still ensuring a 6σ quality level.