Non-Conductive Film (NCF) with No Voiding and High Reliability

Abstract

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.