Non-Conductive Film (NCF) Underfill: Materials, Performance, and Evolution to Next Generation Devices

Abstract

As packaging technology continues to advance to smaller form factors, 3D chip stacking will become more of a requirement than an option and Non-Conductive Film (NCF) underfills will play a critical role in the assembly process. Capillary Underfills (CUF) have long been the standard method of protecting interconnected solder bumps from stress, moisture and contaminants. They are, however, becoming more problematic with the steady growth of fine pitch copper pillar interconnections. With CUF, there are difficulties related to cleaning flux residues. Handling thin die (<100 μm) before bonding has become increasingly difficult and CUF does not offer support because it is applied after the bonding is completed. There are also bleed issues associated with CUF resins that limit the die spacings that are possible with new designs. NCF underfills, applied as a film laminated to a wafer, offer significant advantages over CUF and other underfill technologies for fine pitch designs. Because NCF is applied via a lamination process to wafers prior to dicing, handling and dispensing of resins is eliminated from the assembly process. Additionally, the lamination of film materials allows for a precise, uniform placement of underfill. Since NCF is applied at the beginning of the assembly process, it is able to support thinned die after backgrinding. NCF's can be self-fluxing and the removal of flux residues is not necessary. Material flow can be precisely controlled during the lamination and bonding steps, thus allowing for tighter keep out zones and closer die spacings. There have been major advances in the development and mechanistic understanding of NCF technology over the past few years. A great deal of work was done in the first stages of the development which demonstrated the potential to achieve good interconnection and high reliability with low voiding on small test vehicles with larger pitch. The NCF approach has now evolved to multiple materials developed to accommodate varying design parameters. This paper will present the development and test results of initial NCF technology applied to a base test vehicle with 1,000 I/O copper pillars and the evolution to the next generation NCF materials for high I/O die assembly (36,000 I/O's). The optimization of the parameters critical to the development of a robust assembly process will be addressed. Specific interactions between the film properties, thermal profile for joining, and the force/bump ratio will be discussed in relation to solder joint formation.