Glass Multilayer Lamination for PCB Manufacture using Pressure Assisted Low Temperature Bonding

Thin glass sheets may be a suitable alternative to FR-4 for the preparation of substrates for high density interconnect. However to build such substrates, the bonding of glass layers is necessary to create multilayer structures. In this work, a pressure-assisted low-temperature bonding technique for glass lamination was investigated. Glass, of thickness 100mum, was used as a substrate material and was selected as it has a coefficient of thermal expansion close to that of silicon. Cleaning and pre-treatment of the glass was critical to achieve good bonding at the interface. Three different cleaning solutions were tested: concentrated nitric acid, potassium hydroxide solution and Decon 90 solution, for which contact angle measurements were used to determine the hydrophilicity of the resultant surfaces. Obtaining perfectly clean surfaces was problematic due to the high surface energy of the cleaned glass. Experiments showed that the best way to avoid contaminants was achieved by assembling each pair of cleaned glass sheets together whilst in filtered deionised water. Treatment in a vacuum oven enabled a preliminary bond to be achieved. Final bonding was carried out by applying pressure to the samples in a clamping device, which was placed in a convection oven between 200 and 300degC for up to 48 hrs. Thermal cycling tests were used to evaluate the bonding. Permanently bonded samples were studied using a fatigue test to evaluate the resistance of the bonded sheets to delamination and/or fracture.