PCB fabrication (homebrew)

PCB is an abbreviation for printed circuit board, the copper clad boards used for mounting and connecting electronic circuit components. Originally PCBs had holes for component leads, pins, wires, etc. which passed through and were soldered to the copper tracks on the underside. From the 1980s surface mount components have been used increasingly instead of through-hole components. Double-sided or multi-layer boards use plated-through holes, called vias to connect the traces on different layers. The most common PCBs are composed of either FR4 (glass fiber and epoxy), or the cheaper and easier to cut and drill but more brittle SRBP (Synthetic Resin Bonded Paper).

This article aims to be a guide to quickly and efficiently producing consistently high quality homebrew PCBs. The cost of materials is of secondary importance.

PCB layout design

Wikibooks' Practical Electronics PCB Layout and Trace Current Capacity.

PCB fabrication

Making PCBs at home is much quicker, with results the same day instead of waiting weeks for a commercial fabricator, and cheaper. However it involves working with messy chemicals, drilling will be tedious and the overall quality wont be as good.

Etching and printing

Photo resist

How to make really really good homemade PCBs by Mike Harrison

Toner transfer method

This is using a laser printer or copier (not inkjet) to print an image of the PCB tracks on glossy inkjet paper. Laying this printed side onto the de-oxidised and de-greased copper laminate and using an iron at a high temperature, on the rear of the paper to transfer the toner from the paper onto the copper. After soaking in water to remove the paper, the toner now transferred to the copper laminate acts as an etch resist.

Cutting boards

With SRBP use a craft knife and straight edge. Score it deeply on both sides. Clamp one or both sides between wood to prevent the board being scratched. Then snap it off. A craft knife works especially well for perforated prototyping board. Score down the center of a row of holes, line the score line up with the edge of the table, and press down quickly on the overhanging piece.

FR4 boards are much harder to cut and very hard on tools. A PCB guillotine is the right tool for the job. Failing that use a bench shear or an office guillotine that can cut through more than 2mm of paper. Use a nibbling tool for non-straight cuts. If there is going to be any dust a conventional dust mask might be insufficient protection as the dust is extremely fine. Use a vacuum cleaner to catch the dust at source. Safety glasses should always be worn.

Alternately take a straight edge and secure it along the line to cut. Then take a chisel (one set aside for this), and run one of the corners along the straight edge. First pass go lightly to just get a bit of a groove dug in for the tool to follow so it wont jump out on subsequent passes. Next pass or two press hard, and dig in for a deep groove. Score on both sides. Secure the board and snap the piece off. To finish the edge make a pass or two along it with a fine file.

Tinning

If you don't tin the board, coat the copper laminate with rework flux to prevent it oxidising.

Tin-plating a PCB makes it a lot easier to solder. Use room-temperature tin plating crystals e.g. Seno Immerse Tin Crystals, these produce a good finish in a few minutes but can be expensive. Only make enough tinning solution to cover a PCB in the tinning tray. Keep the solution in a concertina-type bottle to exclude air. Also avoid contamination with metals other than copper. Thoroughly rinse and dry the PCB before tinning. Use a separate tray and pair of tongs specifically for tinning, and rinse them after use. If the solution stops tinning, discard it, clean & rinse the tray, and make up a fresh solution. A cool tinning solution will usually prevent tinning so ensure the temperature of the tinning solution is at least 25ºC, but not more than 40ºC, put the bottle in hot water to warm it up.

Strip the etch resist thoroughly. Rub the copper surface with a plastic scourer until it is bright and shiny all over, wipe with a paper towel and immediately immerse the board in the tinning solution. Take care not to touch the copper surface after cleaning, as fingermarks will impair plating. Within about 30 seconds the copper should turn a silver colour, leave the board for about 5 minutes, agitating occasionally. For double-sided PCBs ensure the solution can get to both sides. Rinse the board thoroughly, and dry with paper towel to remove any tinning crystal deposits, which can spoil the finish. If the board isn't going to be soldered for a day or two, coat it either with a rework flux spray or a flux pen.

External links

• Printed_Circuit_Boards at Open Circuits

Further reading

• The Circuit Designer's Companion by Peter Wilson, Newnes, 2012, ISBN 0080971385