LED UV exposure box
Note: Don't look directly into the UV LEDs. It harms your eyes.
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.
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.
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.
I used nuts glued on the interior walls of the box
to hold the bottom 4 mm thick plexiglas panel. The
choice was simply of what was easily available:
Don't use normal glass as it blocks ultraviolet
light. There is no top panel mounted, because it is
part of the aligning procedure: I put the first film
on the bottom, then the board and the second film is
taped to a smaller piece of plexiglas. Aligning
means aligning the smaller piece of the plexiglas so
that the two layers overlap perfectly. Having the
film attached to it makes it easier. Extra markings
outside the board help this (ghost vias). Part two
should contain a more detailed tutorial on making
double sided boards.
The final assembly allows an area of about 10x15cm of board size, with the interior dimensions of the box being 18cm long, 11cm wide and 5.5cm deep for each of the two halves. Exterior dimensions will depend on the thickness of the material used. Each board contains 7×12 UV LEDs, with 3 and respectively 4 holes between them( the actual spacing between the center of the LEDs is the same). The distance between the top of the LEDs and the board is 3.5cm, including the 4mm plexiglas panel. Check out the detail below to see the LED arrangement on a normal 0.1″ or 1.27 spaced board:
Exposure results: I tested with a small board from 2 minutes up to 8 minutes. Starting with the minimum of 2 minutes proved not to be the best idea, but my long exposure time old setup made me choose this value.
can see in the picture above, from 2 to 7 minutes exposure if perfect.
At 7 minutes some over exposure starts to show up, only in large copper
areas, due to the fact that the film was printed with a laser printer.
This result is very different from what I have obtained when using a
fluorescent tube: below 12 minutes it was clearly underexposed and above
14 it was clearly overexposed, leaving only a small interval where it
So far I have made 4 boards with 2 minute exposure and they came out perfect. When free time will be available I will investigate what the results are for less than 2 minute exposure. As any time between 2 and 7 minutes seems to work, there is no immediate need for a timer. Still, part two should contain a basic circuit for this.
The finished panels:
Bottom plexiglas panel installed and a nut:
The safety switch:
UV on (switch is not used):
to build your own you first must decide what the
required size is for you. With my setup, boards up
to 10 x 15 cm may be exposed. For this, you will
need the following materials:
Continuing the development of my UV exposure box I
have designed and built a countdown timer.
The schematic shows, symbolically, the red and UV
LED arrays through the basic cell that is repeated on the panels: a
series of three LEDs and a resistor. As you can see, there is no way to
turn on the UV light when the lid is open, even if such a command would
come from the microcontroller. The microcontroller can turn the UV light
on via the MOS transistor only when the lid is closed. This is
controlled through software too. In order to sense whether the lid is
open or not, the microcontroller monitors the state of the cathode of
the red LED array. When the lid is open, this is shorted to ground and
the micro sees a low logic level. When the lid is open the red LEDs will
conduct current and force the microcontroller to see a high. This
current is small, limited by the 10K resistor and therefore the LEDs do
not appear to be on. I could have chosen to leave the red LEDs on all
the time as this does not influence the functionality of the exposure
box, but it adds extra consumption and more heat needs to be dissipated
by the lower panel.
The connection of the lid switch and LED arrays is shown below:
Because the box was functional, I decided to make the PCB a challenge for it: Small display, small SMD components. The finished PCB is shown below, the board is very compact and measures less than 6×3.5cm:
You may wonder why there is no programming connector. The reasons are two: I wanted to make it as compact as possible and the fact that it is the type of circuit that requires a single programming (For the end user). As all the pins required for programming are connected to the display it is easy to connect a few wires to the programmer, as you can see I did in the picture below:
The finished timer:
LM317 voltage regulator and heatsink also mounted on the box:
I've used 2 nuts on each of the mounting screws between the heatsink and the box to ensure good air circulation:
And, of course, a 3A protection diode, just in case:
The mounted timer:
The final result:
Checking the safety measures, red turns on, UV off, timer stops:
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