LED UV exposure box




Introduction

I have been using the toner transfer method for about 9 years with great results. Occasionally I would need a board with finer traces and I would use UV method. My exposure setup allowed only a single sided board to be made and it was composed of a 30cm UV tube, some holders, a table, board and film. Exposure time was rather long as the tube was far away from the board.

A few months ago I was looking for some LEDs at one of my suppliers and found that the also had UV LEDs. My mind went directly to upgrading my not so great setup, so I began looking on the internet for info. It turns out some other people have done it, and this instructable confirmed it was really possible. A LED UV box has a clear advantage over a tube one: it can be made in any size, depending on the needs.

I bought 25 LEDs and began to experiment. This way I was able to find a compromise between LED density (spacing), distance between LEDs and board and cost. I wanted something compact, I don’t make large PCBs and those 30cm tubes were not right for this job.

LED UV exposure box

Note: Don’t look directly into the UV LEDs. It harms your eyes.

The LEDs

The LEDs are OSSV53E1A from OptoSupply and they have an 140° angle. This means that the first plane with uniform density can be obtained at a shorter distance from the LED, for a given spacing between LEDs which means smaller height for the board. Their peak wavelength is 405nm, which is higher than recommended by the boards’ manufacturer. Results showed they emit sufficient UV light in the responsive spectrum of the photo resist to allow for a perfect exposure.

I wanted to use a common voltage so 3 LEDs in series with a 91Ω resistor are connected to 12V to ensure a 20mA current.




Schematic LED UV exposure box

The final setup will contain a LM317 for stabilizing as I don’t plan on using my variable regulated supply all the time. There is another reason for this, due to the non linear nature of the LEDs, varying 12V with +/-5% creates much larger current variations which influences the required time. Putting a regulator on the box simply allows more flexibility in choosing the power supply. 

On the bottom board I have added red LEDs. These LEDs help me align the top and bottom films. I have used low brightness, cheap, matte LEDs as I have to look into them when aligning the films. Connection is similar, 3 LEDs in series with a 220Ω resistor, except for the last row which contains 2 LEDs and a 330Ω resistor. These are not really necessary, aligning the films can be done in a dim ambient light.

For safety reasons and extra functionality I added a switch to turn off the UV light when the box opens, turn on the red light and stop the timer. This makes the box more high tech and gives it a professional behavior.

For more detail: LED UV exposure box




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