etching-pcb

The radio amatuer in Australia is now left with some stark choices when it comes to the home production of copper printed circuit boards. There are superb personal computer based PCB design softwares that will print artwork onto overhead transparency film on a laser printer. Thats all well and good but have you tried to get photo sensitive resist materials in Australia lately ? It is just about impossible. RCS Radio in Sydney can sell you some pre sensitised material but it is very costly and has limited shelf life. The marvellous Riston coated board material is no longer being produced. This was a magic material, moderate exposure under a disco black light tube would result in high constrast rock hard traces. There were many other photoresists available in the eighties including some excellent positive resists that came in a spray can. I wrote to the marketing manager of Electrolube which make a superb positive photo resist but the reply was to the effect that it was not worth their time and money to market the product in Oz. Thats a very sad reflection on the state of manufacturing in this land. Jaycar sell small quantities of a presensitised board but it is extremely costly.

The Dalo etching pens are hard to use and I dont think I have ever been really pleased with the result. The resist is hard and very resistant to abuse but the pen has very uneven flow and dries and clags up very rapidly. It is useless for "coloring in" larger not to be etched areas. It is very good , however, for repairing resist areas that are too thin or badly exposed. I used a bog standard Texta or other spirit based marker pen for resist. The resist film is very thin but will last long enough to etch good quality lands , pads and lines. A long time ago I used bituminous paint to paint on the tracks. This was hard, messy, error prone and a complete pain. It is still a very effective resist and was in fact the very first photographic medium.

I am experimenting with something called collotype, which is a photosensitive material made with food grade gelatine which has been activated with ammonium dichromate to render it photosensitive. It is hard to use because gelatin is not one substance but a mixture of variable molecular weight proteins that are cross linked under the action of ultra violet light. It is , in fact, one of the oldest photosensitive resist materials and the collotype printing process is allmost as old as modern typography. Ammonuim dichromate will also photosensitise poly vinyl alcohol glue, but I have not done any experiments along that line yet. PVA is a more consistent material than gelatine and promises better results.

The method that I have been using mostly ,off late, for rapid prototyping is the Dremel method. Using a tungsten carbide cutter, quick and efficient removal of copper is achieved. As you are literally carving lands pads and tracks with the Dremel, this limits the scope of whats achieaveable. It is totally unsuited for digital work, but most of the type of analog circuits constructed by hams are ameanable to the Dremel. It is another variation on the dead bug style or the more pernickity "Manhattan Method" which uses precut PCB offcuts to create lands. Dont use a steel cutter, the glass in PCB material will instantly dull them. Insist on tungsten carbide!

There still remains the problem of etching PCBs. The many electronic enthusiast suppliers in Australia are still happy to supply either ferric chloride or ammonium persulphate etchants as either concentrates or ready made solutions. Ammonium persulphate should only be purchased as the powder, the solutions have no shelf live at all. Ferric Chloride still remains my favourite etchant despite its tendancy to leave persistant stains and corrode anything metallic within a ten meter radius. ( I am a firm believer in the telekinetic powers of ferric chloride. I am sure it has the power to rust steel tools a light year away! Probably can go back in time too!) Never do etching with ferric chloride indoors, your tools will rust into history!

Ammonium Persulphate is probably the "greener" etchant as this will harmlessly degrade to bog standard common fertilizer. Ferric Chloride is persistant (and nasty) and thats why I like it.

Ferric Chloride etchant can be re used allmost indefineately, indeed, I still use a solution thats over ten years old! (How green is that!)

The chemical reaction between copper metal and ferric chloride is a complex one and is still not completely understood by the chemical community. In it copper metal passes directly into solution without the evolution of hydrogen gas as would be engendered by dissolution in acid. You do not want to generate gas in any case because the gas will lift away the resist and ruin edge contrast. The copper metal is ultimately oxidised to cupric chloride in solution after being first oxidised to the partly soluble cuprous chloride. The oxidising potential of the ferric chloride is quickly consumed by the copper metal and very soon the initially vigorous reactions slows down. The etching action can be accelerated by agitation and heat, and this is the standard method.

Ferric Chloride can be regenerated. The initial reaction with copper metal reduces the ferric to the ferrous ion which is very reactive in the pressence of atmospheric oxygen. This results in the precipitation of a hydated ferric oxide and hydroxide which is highly insoluble and is the major component of the brown gunge that grows near and in your etching container. It also grows over the fresh copper surface inhibiting rapid etching.

Agitation by bubbling air through the etching tank is now an established method for accelerating the etching process. This has also the beneficial effect of reoxidising the ferrous ions back to ferric. There is however, a stochiometric deficiency of chloride and hydrogen ions. This can be made good be acidifying the etchant with biulders grade hydrochloric acid.

The resulting etchant takes on an evil looking deep green but has no turbitiy due to senspended ferric oxide. In this form the same etchant can be reused allmost forever, that is untill the high concentration of cupric chloride inhibits further action, due I guess, to the common ion effect. I havent experienced that in any of my etching baths yet.

With acidification with HCL and bath agitation with bubbled air, satisfactory etching action can be achieved without any heating and the etching solution does not need to be discarded but has a "near infinite" life.

Unwanted spent etchant should be mixed with calcium hydroxide/carbonate ( garden lime) before disposal and this will render this etchant safe as immobile iron and copper hydroxide/cabonates and will also nicely neutralize the remaining acid. Never pour unwanted etchant into a toilet bowl or stainless steel sink , it will result in an intractable stain. (and domestic woes )

The following images are my etching tank, constructed from glass sheets and silicon sealant. If you choose to make a tank from glass like this do use the so called acid cure external guttering grade silicone rubber. I made the mistake , in this model , of using a white rubber, which is filled with pigment, probably alumina. The pigment reacts with the acid. The air plumbing is standard garden drip feed tubing. The air nozzles are made by crimping the end of the tube shut by heating the tube and crimping with a heated pliers. Small holes are drilled in the pipe.

The pump, motor and air flow indicator (rotameter) were all found in skips at work. Aquarium pumps with some means of regulating air flow would be also satisfactory, cheap and readily available. You cant have too much air flow. If the air causes etchant to be expelled then you have too much! The air plumbing also includes a one way non return valve. This is to prevent any possibility of backflow by siphon action of etchant back into the pump. It also prevents corrosive vapours from finding their way into the pump. To be absolutely sure of preventing siphon action backflowing into the pump, allways mount the pump higher than the etchant tank.

This tank is constructed from recycled 3mm plain window glass. The inner ribbing is to provide support for the air hose and nozzles. The ribbing does not descend to the bottom, the gap is sufficient to permit the hose to bend underneath it. The far end prevents the hose from bending up when it is pressurised. A lid is provided to prevent ingress of dust and flying insects. The external ribbing is to provide structural support. You do not want the tank to dissasemble itself when charged with etchant. This tank hold just over a litre of etchant.

Choose the air hose carefully. My first nozzle array was constructed from garden drip rigid riser tube. It was blocked at one end and small 0.6mm holes drilled with a PCB drill. Very fine it looked and very well it worked when one fine sunny day (like 45 degrees C in february) I noticed that the sun had heated the tank and contents to some unbelieveable temperature. The hot ferric chloride had dissolved the plastic riser tube. I dont understand the chemistry of this, but the etchant under these conditions could have attacked and oxidised the plasticizer. Before you commit yourself to a nozzle material make sure it is compatible with hot concentrated etchant first.

The PCB should be suspended for that the resist does not bump into the walls during etching. Not shown here is a clamp which is made by sawing a length of PVC electrical conduit lengthways and using this to clamp the edge of the board. Just about anything else will dissolve. There are available all plastic paper clamps which I have also used for restraining the PCB.

Glass is the ideal material for tank construction as it can be guaranteed not to corrode. Use clear acid cure external guttering grade sillicone rubber to bind and seal the glass. see How to Cut Glass here. I used a white pigmented rubber for this tank and that was a mistake. ( it was what I had in stock) .The floor of the tank should be a double layer of glass bonded with sillicone rubber, this will render the tank "unbreakable", at least if the worst happens, you wont have to clean up a puddle of corrosive green goo.

The components of the air pump came from discarded mainframe peripherals. Sometimes you can find things like this at flea markets but a standard aquarium tank airator pumps will provide satisfactory air flow at low cost. I keep the tank outdoors, especially whilst etching because the spray will find and destroy your tool collection if you try this indoors. The air pump is mounted indoors. A three meter length of plastic irrigation drip feed hose has a sufficiently low head loss to provide sufficient air flow. My pump has a rotameter so I can monitor the tanks behaviour from indoors during Melbournes' many dark and stormy nights.