Microwave assisted processing of metal loaded inks and pastes for electronic interconnect applications

Isotropically conductive adhesives (ICAs) and inks are potential candidates for low cost interconnect materials and widely used in electrical/electronic packaging applications. Silver (Ag)filled ICAs and inks are the most popular due to their high conductivity and good reliability. However, the price of Ag is a significant issue for the wider exploitation of these materials in low cost, high volume applications such as printed electronics. In addition, there is a need to develop systems compatible with temperature sensitive substrates through the use of alternative materials and heating methods. Copper (Cu) is considered as a more cost-effective filler for ICAs and in this work, Cu powders were treated to remove the oxide layer and then protected with a self-assembled monolayer (SAM). The coating was found to be able to limit the re-oxidation of the Cumicron particles. The treated Cu powderswerecombined with one of two different adhesive resins to form ICAs that were stencil printed onto glass substrates before curing. The use of conventional and microwave assisted heating methods under an inert atmosphere for the curing of the Cu loaded ICAs was investigated in detail. The samples were characterised for electrical performance, microstructure and shrinkage as a function of curing temperature (80–150°C) and time. Tracks with electrical conductivity comparable to Ag filled adhesives were obtained for both curing methods and with both resins. It was found that curing could be accelerated and/or carried out at lower temperature with the addition of microwave radiation for one adhesive resin, but the other showed almost no absorption indicating a difference in curing mechanism for the two formulations. [Continues.

Laser processing of printed copper interconnects on polymer substrates

With the increasing demand for integration of electronics embedded within devices there has been a consequent increase in the requirement for the deposition of electrically conductive materials to form connecting tracks on or within non-traditional substrate materials, such as temperature sensitive polymers, that may also have non-planar surfaces. In this work, micron scale copper powder based materials were deposited onto acrylic and glass substrates and then selectively laser processed to form electrically conductive copper tracks. Before deposition, the copper powder was chemically treated to remove the surface oxide and subsequently protected with a self-assembled monolayer coating. The copper was then patterned onto the substrate either as a dry powder confined within pre-formed grooves, or was combined with a binder to be printed as a paste. A CO2 laser was then used to heat the copper powder in air, leading to tracks that showed good electrical conductivity. At low laser power levels, the tracks appeared largely unchanged from the original material, but showed measureable conductivity. With higher laser power levels the tracks showed evidence of partial melting of the surface layers and further reductions in resistivity, to values approximately 30 times those of bulk copper, were obtained

Three dimensional printed electronic devices realised by selective laser melting of copper/high-density-polyethylene powder mixtures

A manufacturing process with the capability to integrate electronics into 3D structures is of great importance to the development of next-generation miniaturised devices. In this study, Selective Laser Melting (SLM) was used to process copper/high-density-polyethylene (HDPE) powder mixtures to build conductive tracks in a 3D circuit system. The effects of copper/HDPE volume ratio, laser input power and scanning speed on the resistivity of CO 2 laser processed tracks were investigated. The resistivity of the tracks decreased from 26.6 ± 0.6 × 10 −4 Ωcm to 1.9 ± 0.1 × 10 −4 Ωcm as the copper volume ratio increased from 30% to 60%. However, further increasing the copper ratio to 100% resulted in poor conductivity. The lowest resistivity was achieved with an input power of 20 W and scanning speed of 80 mm/s. Additionally, processing using single-track-scanning and raster-scanning programs was compared; the overall energy distribution on the surface was more uniform using a raster-scanning program, which further reduced the resistivity to 0.35 ± 0.04 × 10 −4 Ωcm. Based on the results, a 3D multi-layered circuit system was manufactured with the HDPE as the substrate/matrix material and copper/HDPE mixture as the conductive-track material. This circuit system was successfully manufactured, demonstrating the possibility of using SLM technology to manufacture dissimilar materials towards 3D electronic applications