Improving plated copper adhesion for metallisation of glass PCBs

Glass is a promising substitute material for use as a high density substrate for electronic packaging due to its low cost, stability, similar coefficient of thermal expansion to silicon and transparency for optical interconnections. Electroless plating has been applied to deposit conductive tracks on this insulating substrate; therefore improving the plated layer adhesion is one of the most important considerations for development of the technology. Randomly rough surfaces do show improved plating retention compared to smooth untreated glass. However, it seems likely that controlling the surface texture would improve adhesion still further. Many studies have covered the relationship between surface roughness and adhesion performance, but few of them have considered the detail of surface topography in any depth. This paper reports on the use of excimer laser machining to produce controlled texture glass surfaces to improve the adhesion of electroless copper plating for interconnection tracks. As a major tool for the study the textures were characterised using non-contact areal/3D surface topography measurement, coupled with the new ISO 25178 areal surface texture parameters. The areal parameters in principle capture much more of the complexity of surfaces than traditional profile parameters such as Ra. Adhesion of the plated copper was quantified using scratch testing. The development of the experimental techniques for the study is described. From the initial results some potentially useful areal parameters for plating adhesion such as Sal, Str, Sdq and Vw have been identified. Their predictive value needs to be confirmed using statistical hypothesis testing with larger sample batch sizes.