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Glass as an HDI Substrate?

At the EY/IeMRC/ICT joint symposium, New Developments in PCB and Interconnect Manufacturing, held in Rotherham, UK on August 4, 2009, Dr. David Hutt of Loughborough University described work being carried out by his team at Wolfson School of Mechanical and Manufacturing Engineering to investigate the practicability of using glass as an alternative to organic substrates in the fabrication of multilayer interconnects for high-density devices. A space-grade glass, designated CMZ and available in thicknesses of 50 to 150 microns, with expansion coefficient close to that of silicon, had been chosen as the base material. The glass had been machined with a pulsed 248nm Krypton Fluoride excimer laser--using mask projection to produce fine grooves for conductors and waveguides, and microvia holes. Challenges included re-melt, debris, hole taper and microcracking--all minimised by careful process optimisation and the use of a protective film. Electroless copper and electroless nickel had both been successfully deposited onto smooth glass surfaces, and additional testing was ongoing to improve adhesion, using self-assembled APTS monolayers as adhesion promoters. Metallisation of laser-machined areas gave comparatively good adhesion and, if a photoresist was used during laser machining of the conductor pattern, this could act as a mask to define the initial APTS image so that catalyst was attracted selectively to enable full-additive circuit formation. Glass layers had been successfully laminated together without the use of adhesive, by pressure-assisted low temperature bonding, and multilayer interconnects were in prospect.

Click here to see Dr. Hutt's full presentation.