In most cases we do not lay out a PCB for early proof-of-principle models - we either use plug-in prototyping boards or hand-wired veroboard. But there are situations where a real PCB is more convenient. One is where we need to keep the board small to make it mechanically fit in whatever model we're building. Another is where track layout is functionally important, such as in high-frequency or high-impedance work (as in a proof-of-concept model where we needed to measure photocurrents below 1pA). Yet another is convenience when the key components are only available in an SMT package. In short: there are plenty of reasons why we use PCBs for prototypes.

Although we generally have boards made by specialist vendors - we've used all of the ones mentioned in the sidebar - I sometimes etch boards in-house because I can get from layout to finished board in less then an hour. Examples on the left: The photodiode detector board mentioned earlier and an adapter board to go from 0.1" header to 0.5mm pitch flex connector.

In-house etching

Board lay-out

For simple layouts that will not be subsequently made externally I tend to use the free ExpressPCB. It can't export Gerber files, but I print the top and bottom copper layers to a PostScript file. For more complex layouts that we intend to have externally made after testing a first version we use Protel from which we output Gerber file data. I then use the freeware ViewMate Gerber file viewer to again print to PostScript. In both cases I use Adobe Illustrator to clean up the PostScript file and print the artwork on inkjet film.

We can reliably etch PCBs for 0.5mm pitch components as shown right, but there are some design limitations for our shop-made PCBs. The biggest is that we can't make plated-through holes, but have to solder component leads on both sides which is not always possible (e.g. on connectors). Vias are also awkward. This means we need to keep tracking fairly simple and use 0-ohm links or wire bridges to cross tracks - this also reduces the time it takes to make the layout in the first place. The left board in the top picture shows such a "hybrid", where the analogue connections are made by PCB-tracks and the digital connections are wired.

Expose and develop photoresist

I've made my own UV-exposure units from an old A4 scanner and some insect-zapper tubes. I always use pre-sensitised boards and have no experience with spray-on photoresist. With these boards from RS I get good results with an exposure time of 2 minutes per side.

Once exposed I develop the boards with RS universal developer. For the RS boards the developer is used at half the concentration stated on the jar. Timing is not that critical - it typically takes 30-60 seconds - but make sure the developer is at room temperature: If it's too warm it eats away your layout in seconds. As I don't make boards that often and developer degrades over time I just make a fresh solution every time - and my 500g jar of developer still lasts forever.

Etching, stripping and tin-plating

I etch with hot (45°C) concentrated FeCl₃ solution agitated with an airstone and an aquarium pump. Wear old clothes and use a drip tray as etchant stains are almost impossible to remove. Etching typically does not take more then 10 minutes if you've flooded the layout with ground plane to minimise the amount of copper that needs to be etched away.

After etching rinse the board with water and strip of the solder resist - I use a Seno applicator - I plate the bare copper with a thin layer of tin using Seno Immerse Tin (halfway down the page).

Drilling, cutting and routing

In my layout I change all drill hole sizes to 0.3mm, as this gives me just enough of an etched hole to centre the drill. With the small solid carbide PCB-drills with 1/8" shaft it's necessary to use a drill stand as even the smallest lateral movement will snap the drill. I've got a 32,000rpm spindle attached to my mill and can use the digital position read-outs to accurate drill holes and route the board outline. It will even be better once it's converted to CNC! It looks a bit disproportionate though seeing a 160kg milling machine holding a 0.5mm drill...

PCB-assembly

We do most prototype PCB assembly in-house. We either hand-solder or put down solder paste using a PET-film stencil and reflow in the desktop oven I bought on Ebay from this vendor. A steady hand and magnifier are required to place parts. For very accurate placement I've made an accessory to my mill so I can use it as a manual pick-and-place machine using the digital position readouts and a webcam - see the sidebar.