Bubble Tank
One of the more annoying things about making PCBs at home is that the normal method is to use developing trays to hold the ferric chloride and these trays are gently agitated during the etching process. This process can take up to 30 minutes depending on the temperature. There are other methods to etch printed circuit boards, one such method is by the use of a bubble tank. This is a tank which holds the ferric chloride and also it contains a plastic tube into which air is blown which creates a stream of bubbles which agitate the ferric chloride, this greatly speeds up the etching process. Commercial bubble tanks are available but they do tend to be expensive. Cheaper versions are available via the Internet, however it is just as easy to manufacture your own bubble tank, and this is what I did.
I decided to use a rather thick acrylic Perspex to construct my tank. I used 10 mm thick material. There are several companies on the web that will provide this material and also cut it to size which saves an awful lot of work I decided on a size which was slightly larger than an A4 page with a width of 40 mm I also decided to double the thickness of the Perspex by bonding two layers together to create the two sides and base this was purely to give more glue area. In hindsight this was probably overkill. Perspex requires a special glue, and the gluing process is often referred to as a solvent welding. The adhesive is called TENSOL 12.
I ordered the Perspex from Barkston www.barkstonltd.co.uk in the following sizes:
300 mm x 230 mm, x 2 : 300 mm x 40 mm x 4, : and 300 mm x 60 mm x 4.
In order for the glue to bond correctly the edges of the Perspex must be smooth. Deep cut marks are not acceptable, fortunately the cutting of the Perspex was a very good and only a light sanding was required to smooth the cut marks. You must not bevel the edges during sanding because any gap will prevent the glue from forming a proper bond. As a "belt and braces" approach, I also ran a bead of translucent silicone sealer along the inside joints of the tank. This was difficult to do, and made quite a mess.
The PCB holder is a simple folded plastic cage made from a sink drainer, and secured with a few cable ties. The ends of the cage are open so that you just slide the PCB in.
In order for the etching process to proceed quickly ferric chloride should be heated. In order to achieve this I used a 300 W aquarium heater, this is the tall tube you can see on the left-hand edge of the tank. To produce the bubbles a small air pump is used. This pump was donated by a friend and is a very small capacity aquarium pump .During etching the printed circuit will needs to be suspended in the ferric chloride this could be done with plastic clips however I decided a more robust solution was required. I found a sink mat, which is simply a plastic grid of squares. I trimmed this to size, and gently heated it while bending it into shape. I then used a number of cable ties to complete the holder. The holder is open on the right-hand side allowing the printed circuit board to be slid into it.
You will note the slightly longer Perspex on the base of the tank, this allows the tank to be secured to a baseboard, thus preventing spillage of the ferric chloride. The baseboard is an offcut of 18 mm plywood, covered polypropylene skin. This happens to be a cheap domestic chopping board. During the construction of the tank, some use of the local pound shop was made :-) The switches for the heater and air pump were mounted on a small plastic box, and to save some space, the air pump was also housed in the same box. Hence the large hole in the lid to enable it to fit!
