Variable Frequency Microwave Heating Research Articles

Chemical recycling of polymer composites induced by selective variable frequency microwave heating

AbstractThe application of variable frequency microwave (VFM) heating to achieve rapid thermal depolymerization of polymer composites is reported for the first time. The thermal and chemical influence of composite additives on polymer decomposition has been studied for a set of chemically recyclable polymers, including polyphthalaldehyde, polypropylene carbonate, two polyhydroxyalkanoates, and nylon 6. Carbon‐based additives, specifically nanocarbon particles, were used as effective microwave absorbers to controllably degrade the surrounding polymer matrix into valuable, monomeric compounds. Depolymerization and byproduct confirmation were quantified by gel permeation chromatography and nuclear magnetic resonance spectroscopy. Synergistic effects can be leveraged from pre‐treating composite samples to further reduce the total microwave energy required to depolymerize composite samples. This study establishes the use of VFM heating for chemical recycling of polymer composites that can be leveraged toward a plastic circular economy.

Reduced graphene oxide catalytically enhances the rate of cyanate ester curing under variable frequency microwave heating

AbstractThe curing of Lonza Primaset PT‐30 novolac cyanate ester resin and EPON 826 bisphenol‐A diglycidyl ether were investigated using convective thermal heating and variable frequency microwave (VFM) heating. The addition of 1 part per hundred reduced graphene oxide (r‐GO) to PT‐30 novolac cyanate ester increased the VFM cure rate compared to thermal heating. Curing it at 160°C for 240 min with VFM heating resulted in a 55% degree of cure compared to a 26% degree of cure with thermal heating. This observed VFM rate enhancement is due to selective microwave heating of the r‐GO particles in the resin resulting in increased r‐GO catalytic activity toward cyanate ester curing. It is both a thermal and catalytic effect, the latter of which is absent when r‐GO is added to a bisphenol‐A diglycidyl ether resin with o‐phenylenediamine hardener. Impurities present in the PT‐30 matrix do not appear to contribute to its overall cure kinetics, nor do they participate in the observed VFM rate enhancement.

Lead-free interconnect technique by using variable frequency microwave

A novel lead-free interconnect technique using variable frequency microwave (VFM) was investigated. The lead-free solder interconnection between the component chips and the metal pads through VFM was first demonstrated. Comparison between the microstructures of the lead-free solder joints on Cu and Sn surfaces formed by a conventional thermal reflow process and VFM was conducted. The VFM heating technique successfully created the lead-free solder/Cu and Sn joints through their intermetallic compounds (IMCs), while maintaining the substrate temperature as low as 210°C.

Low Temperature Rapid Curing of Polymer Dielectrics on Metallized Organic Laminates by Variable Frequency Microwave Processing

The curing of polymer dielectrics on organic laminates by variable frequency microwave (VFM) processing was studied. Polymer dielectrics, such as polyimides, normally require long cure times at elevated temperatures resulting in processing incompatibility with epoxy-based laminates due to the low thermal stability of the organic laminates. In this study, VFM processing was investigated as an alternative, low temperature technique for curing polyimide and benzycyclobutene (BCB) on metallized bismaleimide triazine/epoxy (BT) board. The VFM heating characteristics of the metallized (BT) boards were studied. The VFM heating rates of the BT boards strongly depended on the configuration, size, and shape of the copper patterns on the boards. The VFM curing of polyimide and BCB on the metallized BT boards was also performed. The curing condition was optimized to minimize board degradation. At the same board degradation level, the chemical, thermo-mechanical and electrical properties of the VFM cured film were compared with a film thermally cured using a conventional convection furnace. A higher degree of curing and lower dielectric constant was achieved for benzocyclobutene (BCB) films on BT boards processed using VFM curing than was obtained using conventional curing. VFM cured polyimide (PI 2611) showed a similar degree of curing and Young's modulus to conventionally cured PI 2611. However, the VFM cured PI 2611 showed a higher residual content solvent and a higher dielectric constant than conventionally cured PI 2611.

Wafer bonding using microwave heating of parylene intermediate layers

This paper describes a novel wafer bonding technique using microwave heating of parylene intermediate layers. The bonding is achieved by parylene deposition and thermal lamination using microwave heating. Variable frequency microwave heating provides uniform, selective and rapid heating for parylene intermediate layers. The advantages of this bonding technique include short bonding time, low bonding temperature, relatively high bonding strength, less void generation and low thermal stress. In addition, the intermediate layer material, parylene, is chemically stable and biocompatible. This bonding technique can be used for structured wafers also because parylene provides a conformal coating. Therefore, this is a very attractive bonding tool for many MEMS devices. The bonding strength and uniformity were evaluated using diverse tools. Fracture mechanisms and the effects of bonding parameters and an adhesion promoter were also investigated. The bonding with a structured wafer was also successfully demonstrated.