Novel Pre-Applied Underfill Material Designed for Collective Bonding Process

Internet of Things (IoT) accelerated by the rapid progress of 5G is bringing a lot of challenges to semiconductor packaging technologies. In particular, flip-chip bonding technology is strongly required to realize ultrafine-pitch products, such as network, graphic, and artificial intelligence (AI) processors. Thermal compression bonding (TCB) with pre-applied underfill materials, such as nonconductive paste or nonconductive film (NCF), has been expected as one of the effective solutions for these applications for a long time. Nevertheless, the actual availability is still limited due to its high process cost. In other words, innovative breakthrough to achieve reasonable TCB cost is necessary to support the expansion of IoT. So far, some kinds of multiple die bonding processes represented by collective bonding have been proposed and demonstrated with conventional NCFs. However, it is becoming clear that the conventional NCFs optimized as fast cure type are very difficult to have enough processability for the multiple bonding processes due to mismatch between reactivities of the materials and bonding process conditions. That is why the new NCF dedicated to the multiple die bonding processes has been developed in this article. The developed NCF was designed to survive long-time thermal exposure on a bonding stage so that the collective bonding could be completed with reasonable process margin. To achieve target specifications, this article has started with the design of a new resin composition applied to the developed NCF. Then, basic properties required for NCFs, such as thermal stability and electrical insulation, were investigated in feasibility studies. Finally, TCB was demonstrated, and reliable solder joints were formed without any abnormality even after extended thermal exposure on the bonding stage. Moreover, it was found that the developed NCF might have the potential to achieve void-free encapsulation by pressure curing.