You can print with either inkjet or laser.
I use a laser myself.
Just make sure the transparency is the correct type
for your printer.
You can get a resist developer that is sold as 'non-sodium-hydroxide', but I think it is quite corrosive in its own right. I use sodium hydroxide, otherwise known as 'caustic soda', a level teaspoon to a pint of warm water.
Caustic soda is one of the strongest alkali reagents known. If you get it on your skin in concentrated form it will burn you severely if it is not removed immediately, wash copiously with water or vinegar.
Caustic soda is commonly used to unblock drains, it dissolves fat (and flesh). The advantage to it is that it is readily available from hardware shops and it's cheap.
Another thing you need to know about caustic soda is that it gives off heat when diluted, so if you add the crystals to water, the water will get warm, even hot. If you add enough crystals you can get a boiling, spitting liquid which will damage your skin just as badly as a concentrated acid.
You have to be careful with caustic soda to re-seal the container it is in immediately you have poured out the amount you need. It's a fine white powder, usually supplied in grains of about the same size as granulated sugar. It's also supplied to chemistry labs as sticks or pellets. It absorbs water from the air (hygroscopic) if not kept in a tightly sealed container. Then you have a very concentrated and dangerous solution. The problem is that if you have a quantity of the concentrated solution and add water to it the water droplets can heat up rapidly and boil, causing spitting, so always dispose of concentrated solution by pouring into a large quantity of water.
This said, I use caustic soda myself, I've used it for years, and I've never had a serious incident with it. A level teaspoonful in a pint of water is sufficiently dilute so that a brief exposure will not burn the fingers of a person with averagely tough skin, like me.
Although caustic soda is considered a poison, and will kill you if drunk in concentrated form, it is not dangerous to life in dilute form in the way that dissolved copper is, and it can be allowed to go into straight into the sewage system (as in drain unblocker). You have to be careful to dispose of used etchant solution, ferric chloride, ammonium persulphate, hydrochloric acid + peroxide, which contains dissolved copper, by taking it to an approved disposal facility. Copper, a heavy metal, is highly inimical to marine life even in comparatively small concentrations, and used etchant should not be permitted to go into the sewage system.
What's the best way to do the two sides? Take the protective sticker of one side only, print one side, then develop. After developing one side, take the protective sticker of the other side, print and develop? Will it work like this, then etch everything in the end? I drilled the holes on the board through the board, I hope they match. I'm finding it quite hard to make both sides match exactly...
I exposed one side of the board, then the other.
Develop after both sides have been exposed.
I think if you expose and develop one side at a time,
the protective film may come off the board and wreck
the side that wasn't exposed.
Getting both sides to match exactly is pretty hard.
I find the printing process is not exact. That may
just be a problem with my printer...yours may
be better (or worse). Try and plan so that things
don't have to be exactly matched up. I make
my pads a little bigger so when I drill through
there is a bigger target.
If the transparencies line up well, then you should
be able to get good results with your boards.
I will try to do it tomorrow if I have time, if not on friday I will certainly do it. I'm just afraid that the holes and the circuit don't match and that I need to redo everything a thousand times.
You recommended developing after both sides are exposed, but won't that ruin the side I did first when I change the board to the other side?
A test print is necessary with any new setup to avoid wasting board. You make a test print by taking a small strip of resist covered board and exposing it through a piece of transparency with some features printed on it, a single line from one side to the other will do, but I usually use a fairly detailed board. All that's important is that the strip of board is fairly well covered with features.
Set up the board and transparency (mask). The printed side of the mask should touch the resist. This is why it's sometimes necessary to print the mask in mirror image. It's usual to hold the mask in place with a sheet of glass. The arrangement can be as in Avro_Arrow's case, but it's usual for a dedicated exposure box to have the lamp and reflector at the bottom with a sheet of glass a few inches above. The transparency goes on the glass with the ink side up, followed by the resist covered board, then there's a lid with foam on the bottom to press the sandwich together when it's closed. A doublesided box has a second lamp built into the (thick) lid, with some arrangement to hold everything still. A simple weight on top of the board is sufficient if doing singlesided without a lid, but make sure there is no airspace between the ink and the resist, as it will reduce the sharpness of the image.
When making the test print an exposure is made, could be 10 seconds, could be 100 seconds depending on the brightness of your light, we'll say 100 seconds. A piece of opaque sheet is put between the mask and board and the lamp, covering, say 20% of the area. A further exposure of 100 seconds is made. The sheet is moved in to cover 40% of the board. An exposure of 200 seconds is made. 60% is covered and a 400 second exposure and so on for 80% and 800 seconds. Now you have a board with exposures of 100, 200, 400, 800 and 1600 seconds.
Develop the board and select the best strip. A second test exposure can be made bracketing the best time in smaller steps.
Exposure can be quite critical, especially if using an inkjet printer, because the printed area is slightly transparent, laser toner is less transparent.
There are a few ways to manage doublesided.
As a rule both sides are exposed before developing. The first side should be in shadow when the second is exposed. Don't work in bright daylight or fluorescent light.
The boards I make at home now by this method are exposed one side at a time. I align them by cutting the board precisely to fit the board outline printed on the transparency. Even if the board size is slightly inaccurate, good alignment to 2 edges is sufficient, as long as which edge is which is remembered when the board is flipped.
Other methods of alignment include printing fiducials (crosses) on the masks and drilling through mask and board at the crossing point, or drilling through mask and board at a few chosen pad centres.
One method I have seen used effectively is to tape both masks to a piece of scrap board at the edges to make a kind of envelope. Using the piece of scrap board means that the masks don't slide relative to each other when the board to be etched is butted up to the piece of scrap.
The most important element in getting good doublesided boards is taking care...