The light surface should be downward, because when making exposure, the downward side of the light is used to transmit light. The light surface can make the light pass through the ultraviolet light more evenly and improve the exposure quality;
The rough surface should be close to the PCB circuit board. In order to better adhere to the Mylar film, air extraction and vacuum should be used. If this surface is also smooth, bubbles will be formed between the Mylar film and PCB, affecting the exposure effect.
The function of rough surface is to form a certain gap between Mylar film and PCB, so that the air can be pumped clean when pumping.
Mylar has stable size, straightness and excellent tear strength, heat and cold resistance, moisture and water resistance, chemical corrosion resistance, super insulation performance, excellent electrical, mechanical, heat resistance and chemical resistance.