Epoxy Die Attach Challenges in Miniature and Compact DFN / QFN Packages

The global semiconductors market is still showing high demand growth in Dual Flat No-Lead (DFN) and Quad Flat No-Lead (QFN) packages since several years ago, and that involved a lot of applications. In mobile application, some DFN (or QFN) packages need to equip with multiple functions together with significant amount of miniaturization. Therefore, certain DFN packages were designed to be small and compact with multiple dice, and epoxy dispensing need to take place on narrow and flimsy bonding flag. The situation got worse when epoxy dispensing through tiny nozzle was exposed to capillary effect on roughen leadframe surface that intended for better package robustness. Whole dispensing process became very susceptible to quality issues such as epoxy inconsistency (insufficient or overflow), epoxy on die or lead, and even lifted die. In order to fulfill such stringent die bonding requirements, epoxy dispensing process need to have breakthrough design and optimization. A new nozzle design with proper material found to be effective in improving its life span several times and meantime providing ability to produce smaller but stable epoxy dots or lines. Some of the major design considerations were base on leadframe flimsiness and possibility of silver filler agglomeration inside the epoxy paste. In conjunction with the new nozzle, process parameters required changes through statistical study, together with development of special dispensing mechanism. New process window was tested on the most challenging dispensing process which happened with the smallest nozzle. It did not allow any significant amount of epoxy adhesion on nozzle tip, hence achieving long term cleanliness for consistent dispensing quality. On the other hand, epoxy material was found significant in causing setup issue, mainly due to its composition and mixture balancing between resin and silver filler. In addition to that, there were also some precautions on thermal expansion mismatch in between epoxy paste- and syringe inner wall during transportation or thawing process. If that happened, epoxy paste has trapped bubbles that could become disastrous to the stability of dispensing responses, regardless to their size and amount. At the die attach machines, new hardware modifications were focus on enabling robust handling of flimsy leadframe. Meanwhile, machine vision could be utilized in performing auto measurement on epoxy volume in real time. With proper machine configuration, epoxy dispensing process could be controlled statistically, hence maintaining robust and stable performance. The new improved epoxy die attach process which was established on extremely stringent requirements allowed smooth manufacturing process of other DFN/QFN packages without any quality risk.