Assessment of Low Cost, Sustainable, Reflective Layers for Optical Mixing
The reflective properties of white solder masks are subject to this investigation. Selected materials were used to fabricate test boards with increasing solder resist thickness, ranging from 1 to 4 layers. Reflectivity and reflectivity loss were measured, and some visual and mechanical properties. Our main conclusion is that high reflective white solder masks appear suitable for optical mixing design-in for lighting systems. The main route seems to be to print a first layer of a white photosensitive solder resist, which will define the component footprint for soldering; followed by multiple layers of a screen printed, thermal curable white resist, with slightly bigger opening, which will give the needed reflectivity and stability. Reflectivity increases with increasing layer thickness from 87-90% for one layer of 20µm to 95% for four layers or 80µm. Reflectivity loss up to 10% is observed in the blue spectrum after heat treatment for 1000hrs at 100°C.