Feasibility study on replacing conventional epoxy dispensing with wafer back coating epoxy for QFN packages for discrete product

Abstract

With the advent of ultra portable consumer products, the challenge to attach a die as close to the package size as possible is becoming a major challenge. The current die-attach nozzle dispensing method requires a real estate around the die to be sufficiently large to ensure good epoxy coverage, epoxy fillet height and to prevent the epoxy from overflowing the edges of the DAP. Moreover, a thinner package will require a thinner die which, with epoxy dispensing, could result in die overflowing to the die surface. Such constraints takes time to set-up and a dedicated nozzle will be required to fit every die size combination which would end up with high nozzle inventory and maintenance. A solution is needed at die-attach to meet the new challenge where the die size is almost as big as the DAP and where the die thickness is thinner in order to achieve a lower electrical resistance and package height and, where current epoxy dispensing method is no longer viable. And the alternative now available in the market is highly electrically conductive epoxy wafer back coating. This process does away with epoxy dispensing and is printed directly over the back of the wafer and b-staged cured prior to wafer mounting and sawing. The epoxy thickness is directly controlled by the thickness of the electroformed stencil and mesh. Unlike non-conductive paste or lower electrically conductive epoxies, the epoxy does not “sag” after wafer back coating, hence, roughness of the printed epoxy is through squeegee type and the printing parameters. This study covers the feasibility of assembly from wafer preparation to test by using wafer back coating epoxy, its challenges and issues as well as comparing the electrical performance of the package to that of conventional die attach dispense epoxy.