Rollin' your own board - Techjistu #36

WARNING: The following tutorial involves the use of highly corrosive materials. At all times, use appropriate protective clothing, gloves and eyewear. We recommend that you use non-ferrous (plastic!) forceps when handling all materials in this article. Do NOT allow the materials to come into contact with foods, or food preparation surfaces. Please also be advised that the corrosive materials will destroy ferrous materials such as sink holes, taps and other fowler ware. Please also observe your local regulations in regard to disposal of toxic materials with heavy metal content.

AJB Publishing and Atomic claim no liability in your execution of or ability to follow these instructions nor is AJB Publishing or Atomic liable for any damages that may arise from the instructions contained within.

PCBs, or printed circuit boards, are the cornerstone of modern electronics, and in this issue of Techjitsu, we're going to show you how to roll your own. Forget silicon, without the PCB, the silicon is useless! PCBs are used because they provide a stable and robust substrate upon which we can mount components. Like electronic circuits, they vary in complexity. At the simplest level, PCBs just have tracks on one side and connect components together. However, the latest video cards feature PCBs that are as complex as six layers, essentially making them integrated circuits.

PCBs essentially comprise two bonded materials: copper is usually bonded onto an inert non conductive substrate such as fibreglass or polycarbonate. In manufacture, the copper is usually etched into shapes known as tracks or traces, which act like wire connections between the components. Copper is used in PCBs not only because it's the best conductor next to silver (which is sometimes used in critical scientific or audiophile circuits), but because it's easy to etch with chemicals such as ammonium persulphate and ferric chloride, and works well with conventional low heat 60/40 soldering (see Techjitsu, issue 35).

The downside to using copper is that it tarnishes easily. A simple fingerprint can tarnish a copper surface in under an hour, rendering the surface useless for electronics use. So, like soldering, the golden rule here is: cleanliness, cleanliness, cleanliness!

There are three main ways that PCBs are prototyped; however, the general aim is the same. Basically, 'artwork' featuring the tracks and pads of the circuit are laid down onto the copper with etchant-resistant material ('resist'), the PCB is then dunked into an etching agent, and any copper that is not protected by the resist material is eaten away. Usually a solution of ferric chloride or ammonium persulphate is used. The resist material is then removed, revealing the copper tracks ready for soldering.

Artwork is usually put onto prototype PCBs (read hobby or short-run stuff) in one of three ways: Dalo Pens, Press-n-Peel Blue or Riston. We'll concentrate on the first two in this Techjitsu.

Briefly, Du Pont Ristron is a preferred method used by commercial fabrication plants because of its excellent resolution for really fine work. The resist material is already fabricated onto the copper, and requires UV activation. Unlike Press-n-Peel, this requires that a negative artwork be made. In commercial environments, an actual film negative is produced, however most hobbyists usually use overhead transparency film. The board is then exposed to UV light, quenched in developing solution, then etched. Ristron has the benefit of being able to produce high-detail PCB work (essential with surface-mount circuits) and also allows for short production runs as the film negative can be reused. However, these PCBs cost the earth, the UV lights are hard to come by (although in uni we just counted on a harsh Adelaide 40ºC day, which seemed to work -- but you'd be stuffed in Melbourne or Tassie), the chemicals are also expensive, it takes time, and generally it's just too much of a pain in the arse to work with.

Whichever method you use though, the general process stays the same: prep the PCB, add resist, etch, drill component holes and protect the board.

Prep work
With all PCB prototyping, you need to 'prep' the board to make sure that the copper surface is free of contaminants and marks. From this point on, handle the PCB only from the edges, and preferably with gloves.

Bits:
-- Methylated spirits
-- Acetone
-- Isopropyl alcohol (Lab grade)
-- Scotch-Brite pad equiv to steel wool #0
-- Tissues



First of all, wash down the board with soap and water. Use the Scotch-Brite pad to scour the surface gently. Dry the board and the Scotch-Brite pad, then scour using methylated spirits until the copper surface shines up and looks bright. Next polish the surface with a tissue soaked in acetone. Let it air dry, and follow up with a final polish with isopropyl alcohol. Examine the surface. It's important to use good lab-grade isopropyl alcohol to finish off, as this will leave no residue. If you get residue, you need to go over again with the acetone, then buff off using a clean tissue. This residue is usually caused by oily contaminants in the processing of the solvent. This can lead to disaster by either preventing the resist from adhering to the copper or, worse still, preventing the etchant from working correctly.

Doodling away
Bits:
-- Dalo Resist Pen, 'Action Marker' Fine Line #33, PC-301 Blue
-- Copper laminated fibreglass PCB



The easiest, quick and nasty way to produce a 'quickie' PCB is by using a Dalo Pen. Basically, think a felt-tip pen loaded with resist material. These pens cost anywhere in the realm of $8-10, and are as easy as pie to use. Field engineers have been known to just 'doodle' a circuit freehand, however, when you're starting out, it's easier to trace the artwork onto the copper with a bit of carbon paper. The process of putting down the resist is about as low-tech as it can get -- just draw it on!



A few tips though: keep your pen strokes nice and even, and try and make your tracks one continuous movement. This avoids a problem where, if you lift up your pen, sometimes the resist will not seal over itself. Secondly, if you must retrace your tracks, or if you want to make larger tracks, build it up by 'layering' and overlapping previously drawn tracks. Another technique is to make your tracks by drawing them in small overlapping circles.

Finally, before etching, go over your PCB with a powerful magnifying glass and identify any potential breaks in the circuit. When that's all done, just whack the PCB into etchant, and then strip with acetone.

The Dalo Pen can also be used to 'touch up' resist artwork put down by other techniques, including Press-n-peel.

Press-n-Peel
Bits:
-- Press-n-Peel Blue resist film
-- Access to a photocopier and printer