High Performance Insulating Adhesive Film for High-Frequency Applications

Abstract

In recent years, continuing enhancement of highly-functional electronic devices, such as mobile terminal devices, has significantly increased the volume and speed of data transmission. This made high-frequency communication for data transmission between electronic devices essential. Thus, device component suppliers must offer products with low transmission loss in high-frequency range. An insulating adhesive film used for a semiconductor package substrate requires component materials which provide low dielectric property and low linear expansion coefficient to achieve higher data transmitting signal, eliminate chip delamination during the mounting process, and prevent internal copper wiring from breaking. Meanwhile, a recent trend of thinner, lighter electronic devices such as smartphones and tablets leads to more consideration of potential solutions including the use of: flexible printed circuit with softness and excellent flexibility, microwiring to better work for high-density wiring on circuit substrate, and a multilayer-thin-film substrate. Providing optimal low linear expansion coefficient is acknowledged as an additional requirement, by which reliable adhesion between layers of different materials in multilayer substrates is ensured. Previously, such insulating adhesive film had not been developed. We have successfully developed an insulating adhesive film for high frequency applications to ensure the following properties to meet the demand. Through our investigation, we attained low dielectric properties, dielectric constant (Dk) of 3.0 and dielectric loss tangent (Df) of 0.0025, by selecting the resin with low polarity molecular structure. Optimization of additives to resin ensured the new film to provide high peeling strength (7 N / cm with copper film) and low modulus (1 GPa or less). Low expansion coefficient (α1: 25 ppm, α2: 100 ppm) was also achieved through selection of a suitable inorganic filler, optimization of filler particle size, and dispersing filler uniformly. Thus, the highly uniform film thickness of the new film enables a multilayer-thin-film structure and also facilitates impedance matching. The developed film facilitates flexible wiring design owing to stable dielectric properties at a high-frequency range (1 to 20 GHz), lower transmission loss, and a lack of directional dependence of material properties. The film also adheres to a smooth copper conductor. This makes microwiring possible and lowers conductor loss (i.e., surface effect). In addition, the film's lower linear expansion coefficient ensures reliability of adhesion between layers of different materials in the package.