Making printed circuit boards
A short, non-technical, description of the process of making printed circuit boards.
- Design the circuit board layout from the schematics and the specifications and dimensions of the components to be used. There are several tools to make the blueprint for the PCB layout, I use ARES Lite from Labcenter Electronics for advanced boards and DeluxePaint for simpler circuits.
There are a lot of conventions and considerations that apply when designing layouts like this. HF emission and susceptibility, wire capacitance and inductance, labeling conventions, wire widths for power and signal wires, grounding, physical component dimensions and so on. I’m not going to nag about that here, just keep it in mind because it will come into play especially at very high frequencies.
I have the habit of identifying the number 1 pins of the integrated circuits and resistor networks as squares to reduce the risk of making wrong connections since parts of this work is done mirrored and as seen from below unless you use surface-mounted components.
Another tip is to indicate the hole centers if you plan to drill the holes manually.
- Print the finished circuit layout mirrored on an overhead projector transparency film. Use a laser printer, not an inkjet - their ink does not cover ultraviolet well enough. Triple-check that the dimensions are correct after printing.
- The layout is now ready to be transferred to the PCB and the interesting part begins. Use PCBs that come pre-coated with photoresistive film, because the transfer is done through exposure with ultraviolet light.At this point, the board is completely covered by a thin copper film, and we are going to etch out the desired patterns later.
The photosensitive coating is sufficiently tolerant to be handled in regular light for short periods of time before exposure, but delay ripping off the protective film until the last minute of the preparations.
- Align the board and your template film accurately in the exposure bed and apply the vacuum to fix the alignment.
On this particular board, I am combining two circuits that will be cut into separate boards after etching.
This exposure bed is equipped for simultaneous treatment of both sides of a double-sided circuit board but can be configured for only one side also. Set the timer for 12 minutes or according to your equipment specifications.
- After exposure, develop the board using the appropriate chemicals for your film type. Clean it well with water, and rub gently with your fingertips to remove any unwanted remains of the film. The result will look like this.
The photoresistive film remains as a protective layer where is has not been exposed due to your transparent template.
- Put the board in the pre-heated etching machine before it is allowed to dry.
This machine keeps the acid heated to ~50°C for improved efficiency, and pumps air through it to help the process and keep the acid in motion.
- After etching, all the unwanted copper is gone and the board looks like this:
The precision of the exposure and etching process is incredible. These letters are 1.5mm tall!If you have problems obtaining this precision, make sure your etching acid is not saturated with copper.
- Wash the board thorougly. Any acid left will corrode the copper and make the board unusable after some time. Also, use a solvent or methylated spirit to wash off the remaining photoresistive film to make the copper solderable.
- Drill holes for the component legs. This little device drills the holes from below and is controlled with a funny little electric pedal. Take your time and be very precise, especially with the ICs.
I use a 0.8mm drill for resistors and ICs, 1.2mm for connectors and power semiconductors. The holes should be a little tight to prevent solder leakage and component displacement.
This is why you might like to indicate the hole centers in the layout.
- Then it’s finally time for the final assembly and soldering.
- Start with the smallest components first.
- Fix the component before soldering to prevent crystallization.
- Try not to let the solder and the soldering iron touch. Instead, heat the component leg and copper, and let the solder melt and flow directly on it.
- Do each soldering point as quickly as possible, use maximum 5s on a component or move on and retry after it has cooled down again.
- Use as little solder as possible.
- Enjoy your finished circuit board!
