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Glass offers a number of advantages as a dielectric material, such as a low coefficient of thermal expansion (CTE), high dimensional stability, high thermal conductivity and suitable dielectric constant. These properties make glass an ideal candidate for, among other things, package substrate and high-frequency PCB applications. We report here a novel process for the production of printed circuit boards and integrated circuit packaging using glass as both a dielectric medium and a platform for wiring simultaneously.
An ultrafast laser is used to etch away the desired pattern (pads, wires and vias) in the glass, and copper plating is “seeded” through the laser-based deposition of copper droplets. The seeded area is then plated using electroless plating followed by electroplating. Demonstrations of fine pitch wires, variable diameter through holes and blind vias, and a multilayer stack are shown. The deposits have a resistivity less than a factor of 1.5x that of bulk copper for 5-10 mm wires. Plated lines in borosilicate glass of 7-10 μm width and 5-20 μm depth and line spacing down to ~10 μm are demonstrated, as well as vias with a top diameter approaching 100 μm for 150 μm glass and 40 μm for 50 μm glass.
The process presents the potential for significant material savings in terms of base materials, process chemicals, and waste disposal/recycling costs (glass is on the order of 100-fold less expensive than some current high-frequency dielectrics, and wet processes account for a large part of standard PCB/substrate manufacturing). Additionally, the processes are amenable toward other dielectric materials such as FR-4, polyimide and PTFE-based materials.
Increased demand for high data transmission rates is driving the development of smaller PCB features. Electrical circuits are reaching the physical limitations of traditional PCB dielectric materials under which electromagnetic compatibility can be controlled.
To read the full version of this article which appeared in the August 2017 issue of The PCB Magazine, click here.