or other etchant substance
(transparency)
| Ferric cloride (FeCl3) or other etchant substance |
NaOH based solution (in water) | UVA lamps (blacklight) |
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| Tracing paper (transparency) |
Photo sensitive PCB | A drilling machine |
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| Safety glasses | Safety gloves | |
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First of all, you need a pair'o' clean pieces of glass. They must be bigger in surface than the circuit board, at least some centimeters by every side. You need them to toast the board along with the transparencies. The toast will be exposed to UVA light. Use a pair'o'clips to hold the pieces of glass tied together. | |
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Print the copper (or and component for a 2-sided board) artwork using a good quality printer (ie 600 laser printer) into tracing paper. If you are doing a double sided board you have to toast the artworks, with component side mirrored. The toner/ink side of the paper should be touching the board; not the glass for better results. | |
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Good quality of the printed artwork is critical. I mostly use cheap transparent paper (tracing paper) that can be printed in a laser printer. This is not the best paper for photo exposing, but works even for 0.2mm tracks with 0.1mm clearness. If you are going to do a lot of boards, or going to use smd components large in size like QFP80, you can print films in a printing house for better results, since printers (especially laser ones) strech things. | |
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Now your toast is ready to expose. Be sure there is no dust between pcb/paper/glass. You can clean the glasses with alcohol. | |
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Warm up the lamps for 5 to 10 minutes. Expose the artwok, and don't look at UVA lamps straight.
The lamps I use are the blacklight fluorecent (15W) lamps from Sylvania.
Any lamp that can light withing 350nm and 400nm [UVA] will work for you. Don't try UVC lamps cause they are dangerous and
won't work. If someone tries to sell you UVC lamps for UVA, just shoot him.
Exposing with four F15W/350 BL lamps takes about 150 seconds for each side. |
| UVA, UVB and UVC can all damage collagen fibers and thereby accelerate aging of the skin. In general, UVA is the least harmful, but can contribute to the aging of skin, DNA damage and possibly skin cancer. It penetrates deeply and does not cause sunburn. Because it does not cause reddening of the skin (erythema) it cannot be measured in the SPF testing. |  |
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Developing the exposed pcb: Development can be done using a solution of NaOH in water. A safe method is to use a commercial drain pipe cleaner. Mixing 60g of the NaOH based cleaner with 1 litre of water develops a medium size board in about 45 seconds. It's important to brush continuously the board within the solution with something soft like a soft sponge. Anything harder than a sponge might destroy the photosensitive layer of the board. When the pcb is developed successfuly, clean it with water. |
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The developed board ready for etching. |
| NaOH is caustic. May cause chemical burns, permanent injury or scarring, and blindness. Don't touch it with bare hands and be careful, it shouldn't get in contact with your skin or your eyes. If you have children, store it safe in a locker. Wear safety glasses except than plastic gloves. | |
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For etching I use FeCl3 (ferric cloride) with water [1gr FeCl3 : 2gr H2O]. Stir ferric cloride with cold water and then heat the solution to 50oC/122oF. Don't exceed this temperature. It's important to regularly raise the board out of the water to accelerate the process. Don't use anything metallic to stir the solution. When the solution becomes inactive (ie after etching 6-7 pcbs) discharge it to ground (ie your garden) and water down enough. There are also alternative substances for ferric cloride (perhaps safer). Ask your local electronics shop. |
| FeCl3 Ferric chloride is toxic, highly corrosive and acidic.. Don't touch it with bare hands and be careful, it shouldn't get in contact with your skin or your eyes. If you have children, store it safe in a locker. Wear safety glasses except than plastic gloves. | |
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The etched board. In order to remove the remaining photo resistive layer expose again the board to the UVA light for a little bit longer than the first time and redevelop it to the NaOH based solution. | |
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Now it's time to verify that the board is succesfully printed and there are no flaws.
Use a strong light and rotate the board to check the tracks for random cuts or other faults.
In this shot you can see the very important drill marks. The CAD program you use must be able to produce them. Drilling the board will be very difficult without them. |
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It's important to tin you board
because copper is sensitive of oxidation (in some months, bare copper gets rusty).
I use a soldering paste from Rothenberger
named Rosol 3. This contains S-Sn97Cu3 and you can transfer the
tin to your board by heat.
To tin your board apply the soldering paste to the board, and put it in an warmed up oven (about 200oC.
Actual time needed is something you'll find out after you try, since the timing depends on the oven temperature
totally. In a pre warmed up at 200oC oven, the process should complete in 120 seconds at most.
Don't leave the board in high temperature for more time, and don't try to cold the board with water.
The board will curve due to sudden change of temperature.
Let it cold by it self and then clean the residues with water and something soft like a sponge.
You might want to be carefull with this step, since in FR-4 datasheet (material the pcbs are made of), the pcbs are considered to be stable at 120oC. Thus without being sure, I suppose that 200oC for 120 seconds will be safe. Do it at you own will. |
| Don't touch the paste with bare hands | Prepare the oven at about 200oC. |
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| Cover the copper of your board with the tinning paste |
The board after heating. |
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| After cleaning with water, you'll see your tinned board. |
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| Rosol 3 is toxic. It can irritate eyes and skin. Don't touch it with bare hands and be careful, it shouldn't get in contact with your skin or your eyes. If you have children, store it safe in a locker. In case of contact rinse thoroughly with water. | |
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A cheap and easy way to apply silkscreen in you board is using a toner transfer paper (through heat). A paper that can be used to transfer your silkscreen artwork is cpm 6.1. To transfer the artwork, iron for about 14 minutes (again, timing is something you have to find out). Top layer silkscreen artwork must be printed in mirror. |
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The transfered artwork. To apply silkscreen in a double sided board can be done the same way, but: top layer isn't smooth because of the copper tracks on it and toner will be transfered only to the copper. Using a wet towel between your iron and the paper you might have good results. Another thing than a pro paper you can use, is magazine paper (again using a laser printer) but you have to try some times to get results. Actually, the toner transfer thing, can be used instead of a photo sensitive board for the step etching. Some people do it this way, but results are not as good as with photo etching. If you care about the quality of your board, don't waste your time in it. |
Drilling the board is something really boring especially if you do a lot of boards. People build home made cnc's to avoid it. With some patience and a stable table-drill you can do it without problems. Use a 0.6mm drill for vias, 0.7mm for small resistors and capacitors and a 0.8mm drill for large parts like DIP40, it will help desoldering the part if needed.
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| Top layer assembled | Bottom layer assembled | Another board, double sided, using smd parts like TQFP44. |
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