DIY Plated Through Holes

Here's a method I have used to produce high quality plated through hole PCB's in Rogers RO4003C material, although it should work on any double sided board. Unlike some of the commercial and other methods, this requires just 1 tank of fluid and the materials can be bought for around £20.

First, why? A little introductory background.

I packed away my home circuit board etching equipment (or more likely threw it away!) many years ago. For the last decade or more, the easiest and cheapest way of getting good quality double sided (or even multi-layer) printed circuit boards in FR4 has been to simply produce the Gerber files on your favourite PCB design package and send them off to China. You can get 5 boards made for less than £5, making them yourself is simply not worth it. I know, I know, giving money to China etc.

The trouble is, FR4 is not really great for microwave. It has a high dielectric constant which can be quite variable depending on what particular board the supplier happens to have. Worse, it has different properties when tracks are run in the X or Y directions due to the coarse woven nature of the product. Printed filters for example become very variable on standard FR4. The loss is also quite high at the sort of frequncies we like to play with.

The solution is to use Rogers teflon (or similar) based laminates that have lower loss, a lower and gauranteed dielectric constant and predictable performance. They are also available in very thin sections, which is useful at the higher frequencies. They are not cheap (£10 for an A4 sized sheet) but they are well within the amateur budget.

[ Image: rogers laminate ]

The essential problem is of course, getting small numbers of boards made in Rogers material is expensive, even though the base material is not that costly. The cheapest quote I could get for 5 or 10 small pre-amp boards was in excess of £400. This make prototyping an eye-wateringly expensive passtime if you have it done commercially.

Why not just make them in the kitchen then, like we all did back in the 90's? Well, with modern microwave techniques plated-through-holes are often an essential part of the design, forming joins between tracks, earth-plane bonds or earths under the pads of SMD components where minimal inductance to the groundplane is essential. There are methods but they are complex.

[Image: cross section of PTH]

How is it normally done?

I'm going to take it as read that we are all familiar with the process of drilling holes in a board, getting an etch resist pattern onto it in some way (usually photogrpahically) and etching it in something that stains kitchens. If your kitchen floor has multiple yellow stains, you'll know the process. There are plenty of guides to this, so I won't cover it here.

The way it is done in commercial production, and indeed has been done in research labs and universities (and even one or two keen YouTubers ...) is to use "holewall activation." Essentially, the board is chemically cleaned, then soaked in a tin solution (normally Stannous Chloride in acid), washed, then soaked in a Palladium or Silver solution (Palladium Chloride and an acid for example) ... finally it is placed in an electroless copper plating solution, before being electroplated. So why not just do that then? Well, the chemistry is not out of reach, but when you hear the word "Palladium" you would probably also feel a slight twitch in the wallet. It is crazy expensive (£200/gramme for Palladium Chloride) Silver can be a cheaper alternative, but then there is the electroless copper.

By "electroless" I mean copper applied without needing a source of electric current. The solutions are well known and not complex (Copper Sulphate, EDTA, NaOH and Formaldehyde) but they are very unstable. They are trying to deposit the disolved copper onto anything and everything. If you dip in a nice bit of circuit board, freshly prepped with lots of nice Tin and Palladium ions, the copper will jump at the chance to stick to it, and within a few minutes you'll have a nice brown coating, just enough to deposit a nice thick layer of electrolytic copper onto with a plating process.

Variations of these methods exist, I'll provide links to some excellent YouTube videos below, but they vary between needing 8 and 30 separate stages and as mentioned, the expensive and unstable mixtures. These make them an adventure, but not exactly practical.

So whats the alternative?

One of the videos mentioned that the researcher had heard about conductive inks being used, but not had time to follow it up. I started digging. I found a mention of using a simple water-based graphene ink, basically squeegeed through the holes in the board, blown clear with a little air, dried in an oven and then plated. This sounded promising, so I started looking for the named ink, and found it, at £45 for 15ml ... not exactly cheap. I began researching alternatives and found whole sites that specialised in graphene based inks, many used for printing carbon nanotubes and other exotics. They also had similarly exotic price tags, clearly aimed at relieving universities of their budgets.

I found other suppliers such as "Think and Tinker", a USA company that offers a PTH ink for $224 per hundred grammes ... cheaper, but not cheap enough for me.

So I had a look on eBay (where else!) and found some conductive ink by a Manchester based nano-tech company called "LiquiWire" at the bargain price of £4.95 for 50ml ... I had no clue if it would work, but it was worth finding out.

Testing

I cut some 10mm x 50mm strips of Rogers RO4003C 0.508mm material and drilled some test holes. 0.4mm seemed a good place to start. Using 800 grit wet and dry I de-burred the holes. I did not wet the board as I did not want to upset the chemistry of the hole walls, nor get them overly damp.

I squeegeed the ink through the holes, blew it clear with a straw (gentle compressed air would work, as would gentle vacuum). The idea is to make sure the holes are not plugged with ink, but also to leave a good even coating on the walls. If you are too agressive with the air, you can clear all the ink off the walls.

I wiped the front and back of the boards with a slightly damp cloth and popped them in the oven at about 80C for a few minutes, and then 150C for about 15 minutes, just to make sure they were good and well dried.

A quick wipe with the wet and dry and a final inspection to make sure none of the holes were plugged with ink or copper dust and then into the electrolytic copper plate. Don't be scared by this step, it is simple, quick and easy and comes in a bottle. 1 litre of "standard acid" copper plate costs about £13 and will do many tens of boards.

You just need a plastic container, a couple of copper anodes (ideally "PDO" copper in anode bags if you want to be posh, but my tests were initially just with bits of copper strip and it worked fine). A copper anode on each side of the board connected to positive, the PCB connected to negative and apply a plating current. The "rule of thumb" is 10A per square foot, or 10mA per square centimetre ... don't forget it is double sided, so double the area. No, you don't need to try and work out the area of the holes. Somewhere between 5 and 15A per square foot will be fine. If you want to be clever, start off at the lower setting and turn it up after a few minutes. To give you an idea, a 100mm x 50mm board needs around 1 Amp.

Leave it in for around 20 minutes to an hour, take it out, wash it. Thats it! Really, simple quick and reliable. I struggled a little keeping 0.3mm holes clear of ink, 0.4 was better and 0.5mm a doddle. I tried holes up to around 2mm and they all plated well, I used a small paintbrush to ensure the holewall was well coated in ink.

Is it good enough?

I tested several 0.4mm holes that had 30 minutes of plating and found they took at least 10A without issue. I didn't test them to destruction, but 10A through a single 0.4mm via seemed "acceptable" to me. I drilled rows of 0.4mm holes down some more test strips and plated them through, then cut them off and tested each strip with a multimeter to see if they had properly connected through and they had, I got bored after I had tested 20 or so holes, they all worked.

Exposure and Etching

The next step is of course the exposure and etching. I used a "dry photosensitive film" again from eBay, 30cm x 5m is less than a tenner. It is important to use this method to sensitize the board to UV light as it will lay on top of the via holes and protect them from the etchant. it is really important that the vias are protected and the dry-film method is widely used in industry. It is "reasonably" light sensitive, so do it in a dimly lit hovel. The film consists of the sensitive film sandwiched between two sheets of plastic. This must be removed before use. Cut a piece of film a little bigger than the board, then apply a bit of sellotape to each side. If you are lucky, you'll be able to pull the plastic sheet away on one side leaving the film and plastic on the other.

There are now two methods you can use, I prefer the "dry" method. If you have a laminator (typically used in offices for laminating notices etc) you can simply place the film on top of the board and run it through the laminator. A "not too hot" clothes iron with a piece of paper over the top can also work well. Some people have used a wet method, there the film and board are sprayed, the excess water squeegeed out and then left to dry for 24 hours. I'm impatient, so I used a laminator.

The last layer of plastic film must then be removed. After this the board is ready for exposure. Depending on the detail needed, you can use "overhead projector film" in a laser printer (typically 600 dpi), or send off for professional film masks (about £30, but 4000 dpi!) Expose in your UV light box, develop and etch as normal.

Conclusion

Plated though hole PCBs in Rogers laminates are widely used in amatuer microwave designs, and I believe a lot more designs would make it from the drawing board to reallity if a simple, cheap and accessible method of protoype production of PTH boards was available. This method will allow the amateur experimenter to execute reliable through hole plating in a simple and reliable process, with 1 process tank and an expenditure of a few tens of pounds.

I've not seen this low-cost method documented elsewhere, although I suspect people may have done it.

Suppliers:

Liquiwire:

https://www.ebay.co.uk/itm/223903744472

https://liquiwire.tech/products/liquiwire-conductive-ink

Plating supplies:

http://www.youplate.co.uk/copper.html "High Build" copper, not the bright one.

https://gsplating.co.uk/component/j2store/products/pdo-flat-copper-tank-anode?Itemid=1314

Photo tooling for masks:

https://www.jd-photodata.co.uk/photo-tooling/low-resolution-film/9-x-12-photo-tool.html

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