The development of reflowable materials systems to integrate the reflow and underfill dispensing processes for DCA/FCOB assembly

Unique materials systems with sufficient fluxing activities to remove the metal oxides on the die and/or substrate bumps and assist in the formation of metallurgical interconnects were developed for the assembly of flip chip on board (FCOB). This materials system is referred to as a reflowable underfill. The experimental process flow was as follows: a finite volume of reflowable underfill was dispensed on the printed circuit board (PCB) at the die site, the die was aligned over the bond pads, and the die was placed into the material. Next, the FCOB assembly was transferred to a reflow furnace and subjected to a standard surface mount technology (SMT) eutectic Pb/Sn reflow profile, the solder was reflowed, interconnects were formed between the die and PCB, and the reflowable underfill was partially cured. The built FCOB assemblies were placed in liquid to liquid thermal shock and air to air temperature cycling. The assembly yields varied and continued development of systems with increased flux activity are warranted for the acceptance of this FCOB assembling technology. Promising accelerated life testing (ALT) results were obtained warranting further evaluation of the reflowable materials systems and process.