pcb-rnd knowledge pool

Abstract: A summary of the toner transfer method for home etching of boards designed in pcb-rnd.

  Prototype boards can be fabricated at home for fun, or if commercial fabrication is subject to minimum orders, is cost prohibitive, involves significant delays, or if the design is still experimental and undergoing frequent revision.

Commonly employed etchant solutions at home are ferric chloride (Fe2Cl3), or a chloride etch.

Ferric Chloride is effective but can stain materials it comes into contact with.

Chloride etches are an alternative to Ferric Chloride, which may be difficult to obtain or cost prohibitive.

Hydrochloric acid (HCl), also known as muriatic acid, is commonly available in hardware stores for cleaning cement and brickwork. Hydrochloric acid by itself will react quite slowly with copper after initially removing the top oxidised layer. Hydrogen peroxide, commonly sold as bleach, can be added to the hydrochloric acid to promote oxidation of the bright copper which then allows further reaction with the hydrochloric acid. In the absence of hydrochloric acid, acetic acid (vineger) and common table salt (NaCl) will still produce (rather dilute but nevertheless effective) hydrochloric acid sufficient to do the job.

Whichever etchant is used, the usual chemical precautions apply, i.e. wear safety goggles to prevent splashes getting into your eyes, do not dispose of the spent solution down a drain or sewer, and do not allow the chemicals to come into contact with food or be ingested.

Commercial fabrication typically employs ferric chloride, and the design is applied to the clean copper with a photosensitive layer which is then exposed with a suitable light source through transparencies of the design This leaves the photoresist on copper areas that are to be preserved, and allows the photoresist to wash off on copper areas which are to exposed and etched.

A similar effect can be achieved with the toner transfer method. As with photolithography, the copper to be etched is left exposed, and the copper to be preserved is temporarily coated with a layer of material that is impervious to the etchant.

A common home method involves the following steps:

1) export the finalised PCB layout in a format suited to printing on a laser printer, i.e postscript (ps) or portable network graphic (png), with mirroring if necessary for the reverse side of a board. For the ps export, use both mirror and auto-mirror. For two sided boards it's best to include align marks off the board edge.

2) obtain magazine or brochure paper stock with a shiny appearance, and prepare a sheet form printing on in the laser printer

3) print the design on the paper.

4) scrupulously clean the copper clad board with a stainless steel wool ("steelo") or similar "scotchbrite" pad to leave the copper bright and shiny.

5) carefully apply the printed design to the copper clad board and apply a clothes iron to heat the toner, which will preferentially adhere to the clean copper.

6) carefully place the copper clad board with heat bonded paper and toner in a warm water bath to allow the paper to be carefully removed with very gentle rubbing without harming the applied toner

7) dry the board, and inspect for any defects in the applied toner, fixing them with a wax crayon, or an acid resistant applicator such as a dalo pen.

8) place the board in your chosen etchant until the copper to be etched has been removed. Heating the etchant can speed the process, but carries the risk of staining utensils or kitchen equipment if used.

9) rinse the board, and inspect for any defects.

After this, the board can be drilled for through hole components, or tinned if desired with a soldering iron and solder.

Similar methods for home etching include modifications to inkjet printers that rely on the inkjet ink acting as a photoresist, or photolithography, where transparencies of the board artwork are used to expose the photoresist coated board (typically bought with photoresist already applied).

Double sided designs with associated registration of layer features are clearly more challenging with the toner transfer method, which is really best suited to single sided designs. Registration marks are useful in ensuring artwork remains aligned on top and bottom layers.

Etching time can be reduced, and etchant consumption minimised, if attention is paid at the design stage to minimising unnecessary areas of the PCB with no copper; i.e. fatter tracks and ground pours will reduce etching time and etchant consumption.