This tutorial on circuit board manufacturing is the first in a series of presentations on technical topics that will appear on our web page. Future updates will cover structural analysis of the car and eventually the most of the electrical and mechanical system of Monsoon.
On the Arizona Solar Racing Team, we make all our own circuit boards (at least for prototypes), mostly due to the cost of pro-made boards. However, it is pretty convienent to make them yourself, because you can have a working board in a day. I've been refining the method we use for more than 6 years, and it is very reliable and easy. This method uses laser printed transparencies as a mask on pre-sensitized positive acting circuit boards (from GC Thorsen). Line widths down to less that 12 mil can be made this way (this is good enough for SSOP packages).
Creating the pattern
The first step is to create your circuit pattern, either with a commercial program made for this purpose, or simply with a drawing program like Adobe Illustrator or even something like Microsoft Paint (though making the printed version the correct size may be difficult). I've make boards both ways, but of course, the professional board programs make it much easier.
As a simple example, here is an adaptor so you can use the surface mount package SO-8 in a normal breadboard (the picture is not actual size, but the pdf link will print actual size.
The next step is to print it on a laser printer on a transparency. The key to this technique is to print it twice, to make the dark areas dark enough. So print your pattern again, using the same transparency. Try to align the transparency in the printer exactly the same way both times. A misaligned printout will look like this:
It typically takes me two or three tries to get one that is perfect. I've had good success with the HP LaserJet 6MP and the LaserJet 4MP. The LaserJet 3 almost never let me print twice with good alignment. I would guess any 600dpi laser printer will give good results with some practice. I've found it helps to let the transparency cool before printing on it again.
Aligning the pattern
The next step is to tape your transparency onto a piece of glass. It is important to make sure the right side of the transparency is facing the glass, otherwise your pattern will be mirrored on the board. It is helpful if you print some text on or near the pattern so it is easy to tell which side is which.
In a room that's not too brightly lit, peel off the protective sheet from the circuit board, and align it with your pattern. Place something flat on the back side of the board (like another piece of glass) and clamp it together.
I use large paper binder clips for this (using these has an advantage you'll see later). Double check your alignment and make sure that the pattern is not mirrored. Now you are ready to expose! I keep the whole thing shielded from light until I am outside and ready to go. I usually just put it inside a newspaper or whatever's handy while I get setup outside. It is best to do your exposure around noon, on a cloudless day. If it is partly cloudy, or much earlier or later than say, 11 AM-2 PM, you will have to experiment with the exposure time
Exposing the board
Now expose it to the sun for 1.5 to 2.5 minutes (depending on the light conditions, how thick your glass is, weather, latitude, etc). Fortunately, this process is not too sensitive to exact exposure time. Here in Tucson, we're known for our strong sun and cloudless days. Two minutes works well for me. You can use the levers on the binder clips to get your board aligned perfectly normal to the sun by making the shadows of both clips line up. When you are done exposing, wrap it up, and take it inside for development.
Developing the pattern
The developer for the GC Tech positive acting boards I use is part number 22-226.
This is basically a sodium hydroxide solution (no pun indended), so you should keep it in a tightly closed container when you are not using it to prevent the pH from changing. If you keep the developer in a tightly closed container it will develop a lot of boards and last a long time.
Be sure you read, understand and follow the instructions on both the developer and the etchant solutions before you begin.My instructions here are just a guideline, and are not intended to be a substitute for the label instructions.
Place your exposed board in a plastic tray and pour in enough developer to cover the board. You can speed things up and get a better pattern by gently rocking the tray so fresh solution is washing over your board.In just a few seconds you will see the pattern start to form, and within a couple minutes the exposed areas will be completely washed away. You don't want to overetch, but it is better to be sure the exposed regions are completely rinsed away. You can take the board out and wash it carefully with water then give it a close up inspection to be sure. Be careful not to touch the pattern with your fingers or anything else. It is easy to scratch it and that will cause problems when we etch. When the pattern is finished developing, pour your developer solution back into its container.
Etching the pattern
Before you start etching the board, it is a very good idea to put on some old clothes, or wear an apron and gloves. The board etchant contains ferric chloride which will permanently stain your clothes, or almost anything else it touches, like your countertop.
The etchant works faster if it is warm. You can warm the solution by flowing warm water over the bottle for a few minutes before you use it (be sure to loosen the cap a little). It will still work fine if you don't warm it though. Place your developed board into a plastic tray and pour in the etchant. You will want to gently rock the tray frequently during the etching to speed things up. Depending on how fresh your solution is, how warm it is and how much you agitate the solution, the etching will take from 10 minutes to more than an hour. You will have to check the progress of your board. It is done when the copper is completely gone from the exposed areas. It is helpful to take the board out of the etchant and rinse it with water for better inspection. When the board is done, rinse it thoroughly with plenty of water.
Finishing touches
Now you can start drilling all the holes for your leaded parts. It is easiest to use a dremel for the small holes. The size of the holes can be measured from the actual components, or read off the datasheets for that part. Be careful not to drill larger than it needs to be. This can make soldering the lead to the pad a little difficult.
Tips and tricks
Make your circuit traces as wide as possible. Though I've had success makes traces as narrow as 0.012", there are often pinholes in the traces than can cause open circuits. This is especially true if the board is overexposed. It is important to experiment with exposure time. Too much exposure causes the minor defects in the transparancy to show up in your pattern. Of course, not enough exposure and the exposed areas will not be completly removed by the developer, so the etching will not be able to remove the copper completely. This will cause short circuits at worst, and large leakage currents as best.
I cannot emphasize enough the utility of placing text on your patterns. After your pattern is sandwiched between the glass but before you go outside to expose, LOOK at the pattern and verify it is on correct. It is way to easy to put the wrong side of the transparency towards the board, which would make a mirror image of your pattern.
After you develop your pattern, inspect it carefully with a magnifying glass before etching. If there are scratches or holes in the resist pattern, you can touch it up with a permanent marker. Permanent marker will resist the acid etch remarkably well.
Don't forget to wear clothing protection, as well as eye and hand protection. The board etchant has ruined more than a few of my shirts before I wised up.
After the etching is complete and you have rinsed and dried the board thouroughly, you can double check that all the copper is removed by checking continutity between closely spaced parts of the board with a multimeter. If there is no residual copper still on the board, you will see resistances greater than 30 Mohms (the most I can measure with my meter) between closely spaced points on the board. This is only a fair test if the board is dry.
The green photoresist can be easily removed with acetone (nail polish remover) or less easily with rubbing alcohol. There is no need to buy the expensive resist stripper you may see in stores.
It is useful to make the hole sizes of your component pads the actual size you will be drilling them, or a little smaller. This helps guide the drill bit when you drill so your holes will be better aligned. You'll want to buy a wire drill index with sizes 61-80.
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