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The Process of Making PCB
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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
  • On a computer
  • On paper
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.

 

Getting a pattern on the board

From least to most desirable:

  • Masking tape
  • Drawing pattern directly on board using a resist pen
  • Drafting tape/patterns on board as resist
  • Photgraphic methods
  • Toner transfer systems
These things form the resist on the board to save what you don't want etched away by the chemical solution.

NOTE: BEFORE PUTTING DOWN ANY RESIST, CLEAN THE BOARD WITH AN ABRASIVE CLEANER LIKE AJAX OR PUMICE, SOAP AND WATER! Finger oils are acidic, and will discolor the board and keep resist/tinplate from sticking. Try not to touch the copper after the board's cleaned and dried.

Masking tape, drafting patterns, and resist pens are all manual methods, prone to misalignment and fraught with error. Also, board made like this are usually one-of-a-kind and not reproducable. One advantage to using premade rub-on patterns such as those available from Datak is that the spacing of the IC pins will be correct.

Photographic methods used to be more popular before toner transfer came along. Used to be, you exposed a sensitized board to a negative of your pattern in the darkroom, then processed the board like a photograph. Nasty chemicals, horrible odors, and errors in alignment characterized this process.

Recently, positive-image pre-sensitized circuit boards are now available that can be exposed in direct contact to your pattern printed as a transparency (traces black, unwanted copper as clear). Only one step is required to develop them, and only a simple #2 photoflood bulb is needed for exposure. I have used these, with good results.

Toner transfer sheets (like from Techniks ) are available that print useable resist right from your laser printer. (And, if you don't have a laser printer at home, usually a school or office supply store can print your layouts for you. I have used Kinko's a number of times to do this.) The only catch here is that you have to print the pattern as a mirror image. Most printers and layout software can do this.

One you have printed your mirrored pattern onto the toner transfer sheet, you carefully iron the pattern onto the board, ink side down. Use a household iron on the lowest setting, with a sheet of printer paper in between the iron and toner transfer paper. Let the toner transfer cool (I've dunked the whole thing in water to "anneal" the plastic) before you carefully, SLOWLY peel it off, leaving your resist behind.

Getting rid of excess copper
  • Milling / cutting (Xacto,dremel, mill)
  • Etching: Ammonium Chloride / Ferric Chloride
If you're cutting, be careful. We all tend to forget that those Xacto knives are very much like the surgical scalpels they were designed after! They can cut into the hand deeply and quickly, leaving a clean cut that bleeds profusely. BE CAREFUL!

In etching, the most important thing is to have HOT, CLEAN etchant. I dump etchant that's been used more than twice (down the sink is okay, so long as you rinse it with plenty of water). You want the etchant hot to the touch, like scorching hot bath water. Around 110-120 degrees is great. The hotter it is, the faster it etches, and the quicker you get that acid away from your precious traces! The slower it etches, the more undercutting you have, running the risk of obliterating narrow traces. I've seen it happen.

Use a Pyrex or plastic tray to etch in. DON'T USE ANYTHING METAL, EVEN IF IT'S COATED! A microscopic crack in a ceramic-coated metal vessel could allow the etchant to eat right through the wall. Cover the board in about 1/2" deep of etchant.

While it's etching, gently slosh it around, bringing fresh etchant into contact with the bare copper. Or, use a little motor with a magnet mounted on it, and a magnetic stirrer from a lab store. I used to do that when I was etching a lot.

Also, use work clothes, tongs, gloves and be careful not to get it on your skin. Ferric Chloride STAINS! I call it "Green Vulcan Blood" :-)

A 3"x4" board, with about half the copper taken off, should take around 30 minutes to etch. If it takes much longer, it's too cold or you're not sloshing it around!

 

Drilling
  • DON'T USE A DREMEL TOOL!
  • USE A DRILL STAND OR PRESS
  • Use only carbide drill bits
Usually, component leads go through the board, so we have to drill them out. A drill press or a cheap drill press stand for a portable drill is ideal. A Dremel tool isn't meant for precision drilling--since it does not have a real chuck on it, the drill tends to "precess", making the hole larger than the bit or causing the drill to break.

Sizes:
0.022 transistor leads, small holes
0.035-0.040 IC/resistor leads, general 0.046 large transistor leads

Finishing
  • Electroless tinplate solution makes a board easier to solder and keeps a good finish for a long time.
  • Solution is a powder which dissolves on hot water. Solution is reusable, but has a limited shelf life (about 6-8 mos).
I used to make boards without tinning them. In a day's time, fingerprint acids begin to discolor the board, and it looks horrible. Electroless tinplating is cheap, fast, and makes a board look professional. In addition, parts are easier to solder on, and if you need to rework the board later, tinning makes that much easier.

Did you clean the board before you etched it? Well, clean it again! This time, rub with steel wool and Ajax (or similar) abrasive cleaner. You must get off all the resist, all the fingerprint oils, and all etchant residue. AN OILY PATCH ON THE BOARD WILL NOT TINPLATE.

Like with etchant, use a Pyrex or plastic tray for tinplate (NOT the same one as the etchant--they are NOT chemically compatible!). Follow the directions on the tinplate package. Except for one point: I like to mix the tinplate crystals with HOT water, because they all dissolve that way. Be careful--there's an exothermic chemical reaction when those crystals hit that hot water! Use goggles.

Now, sit back, enjoy a beverage, and admire that nice-looking PCB for a bit before your put the components on it.

 

 

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