Materials to integrate the solder reflow and underfill encapsulation processes for flip chip on board assembly

Materials systems were developed to integrate the reflow and underfill dispensing processes required for flip chip onboard (FCOB) assembly. FCOB underfill formulations with intrinsic 'flux-like' properties capable of assisting in the reflow of an oxidized metallurgical body and the formation of an interconnect are referred to as fluxing underfills. These materials will reduce the cycle time, pulse rate, line length, and cost of the present qualified surface mount technology/flip chip on board (SMT/FCOB) assembly line. A cost benefit analysis showed a 20% reduction in the total cost per product unit. Also, in an attempt to identify the critical materials rheological properties, a theoretical model was developed. Studies were conducted on a prototype SMT/FCOB assembly line and it was shown that a fluxing underfill could be dispensed on the substrate prior to die placement without affecting interconnect formation. The preferred fluxing underfill was characterized and a design of experiments (DOE) was performed to identify the window of processability. In addition, FCOB assemblies were built and placed in reliability testing: air to air temperature cycling (AATC), liquid to liquid thermal shock (LLTS), and 85° %/85% RH exposure prior to LLTS. Assembly yields of 100% were achieved and the FCOB assemblies built with the preferred fluxing underfill were able to survive more cycles of reliability testing than required for some consumer products. The results from this study suggest that the proposed FCOB assembly process should be evaluated further.