PCBs Fabrication of Exposure---WonderfulPCB(hk)Limited

The photo resist board needs to be exposed to ultra-violet light through the artwork, using a UV exposure box.

UV exposure units can easily be made using standard fluorescent lamp ballasts and UV tubes. For small PCBs, two or four 8 watt 12" tubes will be adequate, for larger (A3) units, four 15" 15 watt tubes are ideal. To determine the tube to glass spacing, place a sheet of tracing paper on the glass and adjust the distance to get the most even light level over the surface of the paper. Even illumination is a lot easier to obtain with 4-tube units. The UV tubes you need are those sold either as replacements for UV exposure units, insect killers or 'black light' tubes for disco lighting etc. They look white or occasionally black/blue when off, and light up with a light purple, which makes fluorescent paper etc. glow brightly. DO NOT use short-wave UV lamps like EPROM eraser tubes or germicidal lamps, which have clear glass - these emit short-wave UV which can cause eye and skin damage.
A timer which switches off the UV lamps automatically is essential, and should allow exposure times from 2 to 10 minutes in 15-30 second increments. It is useful if the timer has an audible indication when the timing period has completed. A timer from a scrap microwave oven would be ideal.

Short-term eye exposure to the correct type of UV lamp is not harmful, but can cause discomfort, especially with bigger units. Use glass sheet rather than plastic for the top of the UV unit, as it will flex less and be less prone to scratches.
If you do a lot of Multilayer pcb, it may be worth making a double-sided exposure unit, where the PCB can be sandwiched between two light sources to expose both sides simultaneously.

You will need to experiment to find the required exposure time for a particular UV unit and laminate type, expose a test piece in 30 second increments from 2 to 8 minutes, develop and use the time which gave the best image. Generally speaking, overexposure is better than underexposure.

For a single-sided PCB, place the artwork with toner side up on the UV box glass, peel off the protective film from the laminate and place it sensitive side down on top of the artwork. The laminate must be pressed firmly down to ensure good contact all over the artwork, and this can be done either by placing weights on the back of the laminate, or by fitting the UV box with a hinged lid lined with foam rubber, which can be used to clamp the PCB and artwork.

Ms. Jodie Zhou
Wonderful PCB
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