A sachet of DP-50 developer is mixed with 1 litre of 25°C water. The exposed PCB is gently agitated in solution. The exposed areas of resist will dissolve into the solution, leaving the green resist in areas that were not exposed to UV light. If the resist does not fully remove from the exposed areas, the UV received was insufficient. Possible causes are: Insufficient exposure time; the UV source is old and not producing enough UV; incorrect UV frequency (use 350~450µm UV source); the artwork is