Why make a PCB?
- Convenience/looks good
- Alternatives: wire-wrap, dead-bug, point-to-point
- RF
- Analog
Usually, I want to make a PCB for a circuit just because the final package looks so neat. Other construction techniques don't seem to be nearly so neat or sturdy (with the possible exception of wire-wrap, used in the U.S. Navy and in military aircraft still today).
A very good reason to build a PCB is when a circuit has sensitive analog nodes or contains RF. On a printed circuit board, capacitance between traces can be made very low. Also, impedance to ground, which is crucial for high-speed analog and RF circuitry can be made very low.
For microwave circuitry that uses transmission line components, PCBs are a must. Well, we could use waveguide construction, but usually that's pretty bulky.
Designing PCBs
These days, when a useable computer can be had for less than $400, there really is no excuse for not using a computer to lay out a board. Especially when there is shareware software available that does both schematics as well as printed circuit boards.
If you're designing a circuit from scratch, computer programs that do schematic and PC board editing are very useful.
You can lay down "pads" and components on your "virtual board", then transfer the pattern to the PCB. I use Accel, which is a commercial board package and a bit expensive. There are shareware board design packages available, with even have libraries of commonly-used parts with them.
A computer can be used even if your circuit board layout comes from a magazine. By using a cheap flatbed scanner (mine was $39 from Office Max), you can get the circuit board layout into your computer. Then, you can use tools like Paintbrush, PhotoShop, or PaintShop Pro to edit the layout, adding your own logos or customizing the circuit to your likes.
The Basics
To create a circuit board, you must do one of three things:
- Lay down traces on a piece of non-copperclad board
- Etch, cut or mill away unwanted copper, leaving your traces
I have seen some microwave enthusiasts start with a piece of single-sides board, then use copper tape to lay down transmission lines and bias pads. A neat technique, but one with limited usability. Conversely, some people use an Xacto or similar razor knife to cut (isolate) traces from the surrounding copper. It's hard to cut straight, and curves are pretty much out of the question.
Where I work, we have something called a T-Tech mill that takes a file containing where traces are to go, and cuts (isolates) them from the surrounding copper. Optionally, the excess copper can be milled away (critical for microwave stuff). But, these machines are expensive to acquire (>$20,000) and run (mills are $220 per box, 5 in a box). They are mostly for engineering labs. But, if you are lucky to have the use of one...
That leaves etching. What we do there is put the traces we want on the board in the form of resist, a gummy chemical coating that makes the copper non-reactive. Then, we dunk the board in a solution that reacts with bare copper (it can be acidic or basic), until all the unwanted copper is removed. This is how 99% of commercial circuit board are produced.
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