There has been significant activity in recent years to develop Non-Conductive Films (NCF), which are also known as Pre-Applied Underfills (PAUF) and Wafer Level Underfills (WLUF), for use in the High Volume Manufacturing (HVM) of 2.5D and 3D packages. They are essentially underfills in laminate film form. Like other underfills, they ensure the integrity of the electrical interconnects in a package by mitigating stress, acting as an adhesive to bind the package together, and encapsulating to protect against moisture and other unwanted materials that can compromise electrical connectivity. NCF's are seen as an alternative to the more traditional capillary underfills, especially in devices with finer pitch, smaller gap, and larger size. Since the NCF can be applied to multiple devices simultaneously, this new technology has an advantage over capillary underfills in HVM. These film-type underfills can be applied to wafers (PAUF, WLUF, NCF) or substrates (PAUF, NCF). Two key attributes of any underfill are no voiding and high reliability. No voiding is an essential requirement for high reliability. Voiding in NCF materials can result from the lamination and thermocompression bonding (TCB) processes. Voiding under the die can manifest from a variety of causes, including some of the following: (1) volatilization of materials in the coating, (2) dimensional changes within the coating during processing, (3) poor fundamental conformation to surface topography during film lamination and (4) ineffective air release during TCB due to un-optimized material flow. Unwanted issues such as reduced adhesion, solder shorting, increased moisture uptake and reduced stress mitigation can result from the presence of voids in the NCF. Pressure cure after joining is one method for eliminating voids in NCF materials. An earlier version of the NCF discussed in this study required pressure to eliminate voids after TCB. Pressure curing, however, adds process steps and is not accepted by all manufacturers. An improved NCF formulation and bonding process has resulted in a void-free NCF that does not require pressure cure for void elimination. This improved version not only addresses voiding after bonding, but also it has been proven to be very reliable in the presence of extreme temperatures, high humidity and temperature cycling. No one test is sufficient to adequately determine the long term reliability of NCF materials so a battery of tests was run to conduct a comprehensive assessment. The reliability evaluation results demonstrate that this newly developed NCF exhibits not only no voiding after TCB without the need for pressure cure, but also high reliability to various forms of temperature and humidity stress testing.

Current demands of the industry on performance and cost has triggered the electronics industry to use high I/O counts semiconductor packages. Copper pillar technology has been widely adopted for introducing high I/O counts in Flip Chip and 3D Chip Stacking. With the introduction of flipchip technology new avenues have been generated involving 3D chip stacking to expand the need for high performance. With the increase in the demand for high density, copper pillar technology is being adopted in the industry to address the fine pitch requirements in addition to providing enhanced thermal and electrical performance. For this study, Copper pillars and SnAg were electrolytically deposited using Dow's electroplating chemistry on internally developed test structures. After plating, wafers were diced and bonded using thermocompression bonding techniques. Copper pillar technology has been enabled to pass reliability requirements by using Underfill materials during the bonding. Underfill materials assist in redistributing the stress generated during reliability such as thermal fatigue testing. Out of the several Underfill technologies available, we have focused on pre-applied or wafer level underfill materials with 60% silica filler for this study. In the pre-applied underfill process the underfill is applied prior to bonding by coating directly on the whole wafer. Pre-applied underfill reduces the underfill dispense process time by being present prior to bonding. In this study, we have demonstrated the application of wafer level underfill for fine pitch bonding of internally developed test vehicles with SnAg-capped copper pillars with 25 μm diameter and 50 μm bump pitch. This paper demonstrates bonding alignment for fine pitch assembly with wafer level underfill to achieve 100% good solder joins after bonding. Wafer level underfill has been demonstrated successfully to bond and pass JEDEC level 3 preconditioning and standard TCT, HTS and HAST reliability tests. This paper also discusses defect mechanisms which have been found to optimize the bonding process and reliability performance. Alan/Rey ok move from Flip Chip and Wafer Level Packaging 1-6-12.

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.