PCB making

The coating can be a variety of chemicals that resists the etching chemical.

For low volume a ultra violet sensitive coating can be used, but in the vast majority of cases the image is screen printed on to the bare copper.

John.

Wow! Screen printing! Didn't that go the way of the steam jitney?

Tolerances on boards today are so tight (not to mention multi-layer alignment) that photo lithography is mostly used.

On the really low-tech side, you can buy a pen to draw your resist traces on the board. Looks pretty funky, but if it's a simple circuit it works OK.

Okay, so most of my pcbs are low volume and I get them made on the cheap by screen printers...

Usually because I don't trust these software pcb layout packages, they take too long to get it to put the parts / tracks / pinouts etc... in the 'right' places...

So I still use a light box, transparent film and sticky UV blocking tape / pads etc...

Trouble is doing it to a scale of one to one means you need good eyesight etc...

For one or two offs I make them myself using UV coated boards, a UV exposure unit and a bubble etch tank with ferric chloride...

Funny how that ferric chloride gets everywhere isn't it?

John.

Yes, I've noticed that myself. It's tricky stuff to contain.

Ferric Chloride is a fairly dangerous poison, more so when it is loaded up with a few grams of copper (come up and see my etchings!!), and as such is banned in Germany, like for the last 20 years or so.

Although I have forgotten the name of the stuff we use, it is relatively safe and only etches when near boiling point AND can be given up in small quantities to the local government when it is not usable further. It will also develope boards that have been exposed....

That's interesting about ferric chloride! While we never drank the stuff, we did almost bathe in it. Seemed fairly harmless other than the fact that it stained like hell and you never knew what it might start corroding.

I know they use other chemicals in industry these days, but I believe you can still get a bottle of ferric chloride from your local electronics store. And they still don't mark the bottle with the "skull & cross bones."

Also, I don't think your alternative is much good... I've been involved in lots of fires where a particular chemical has to be kept at some exact temperature range to work. One little glitch in the heating controls, and everybody is running for the doors while the firemen are coming in, wearing HASMAT suits.

That is not the chemical we used, it would only etch when hot, it was considered to be completely safe in all respects. I will investigate as to what it was....

Also, try to get some info on what happens if the temperature became too high. Did it break down into nastier stuff?

Thanks, Andy

I use ExpressPCB, they do good work in a few days.

The simple method using a pen (Edding are very good) is to intimately clean the pcb with a scouring agent, then wash using a good washing up liquid and warm water. Finally clean with methylated spirits and DO NOT touch the surface with your fingers or hands at this point!!

Draw your circuit with the Edding pen, shielding the PCB from your hand with a clean sheet of paper when needed.

Another method is with special plastic transfer foils, you print on them with a laser printer, and then iron the print onto the clean PCB. I have never had much sucess with this method though personall.

Let dry and then etch......etching can be done with many different liquids, some (the good ones) need to be hot to work!!!

I prefer the foil and a UV light source and a photo sensitive PCB....develope the PCB and then etch. Its one chemical more (sometimes), but works best I find.....

Only use glass or plastic to hold etching liquids.....

So you've done work with this stuff, too. In a perfect world, the best way to make a home circuit board is to get a bottle of that clear photo-resist. Make sure (as you said) that the board is really clean. Then, coat the board with the resist (it's clear), and leave to dry out of sun light! Next, lay out your circuit on a piece of clear acetate using tape or ink pens or whatever. The nice part of doing it this way is you can mess with it until you get all the traces nice and clean - just make sure your ink (or whatever, is really opaque). Put the acetate on top of the copper with the resist, then expose to UV. I find that direct sunlight is fine, but it takes a little trial-and-error to get the exposure time right. Once exposed, clean and etch. Ferric Chloride still seems to be the old standby for home stuff - there's many kinds of etchants used in industry these days; but some are not so friendly.

For that you have to first laminate the copper panel with dry film and after exposing thro" the exposing machine you have pass it thro the developer which creates the permanent image on copper panel thereby you can use it for next chemical process.

Naveen

dear freind some photo resist sprays are available in electronic parts market the process are written on spray tin 2nd uv ink with screen printing are commonly used in manufacturing pcb 's some proffesionls are use a chemical rockyphotbut itis expensive

if i want some pcb's i normally use hp 3p laserjet output it is best .out put may be on normal paper .after cleaning with no.400 cilicon paper, diagram placed on pcb and press with iron, iron set on silk setting, time atleast 30 second .then after few seconds remove paper from pcb. then dip it in a tray poured with ferric chloride sulution.and stir the sulution in the tray.after eatching drill the hole for components.

thank you every one who had spend their time and express their idea. But still I could not get what is the name of the chemical used for pre-sensitizing the raw copper board? that is what acually I asked for .

I personally cannot recommend coating your own boards, why you may ask?

Due to surface tension, the edges of the PCB get a thicker coating, the thicker coating needs a longer exposure than the rest of the board where it is thinner.

Also, the thickness of the coating needs a test PCB to find out the exposure time before making the final board.

Therefore the board needs to be much larger than needed, so that you can cut the final board out from an area where the thickness is the same!

You will end up with a lot more chemical than you need for one board, what will you do with the rest???

A much better idea is to buy pre-coated board, it is here about twice as expensive, but the coating is always the same thickness and I find it is much, much better and far more reliable in quality. Keep the black plastic covering on right up to just before exposure, develope and etch immediately afterwards....

I agree with you there Andy, but for one thing, the precoated boards do have a very short storage life, I think its only 6 months at room temperature.

Quite often I've been frustrated by etching a board only to find the UV coating has softened and it all etches away....

I made a dip coating tank back in 78 when it was difficult to buy the precoated boards, and it worked fine, I made it so that you had to wind it at a set speed out of the tank to give a uniform coating, the last edge of the board you cleaned off the drops and left it to dry.

I can't remember what the name of the chemical was, as I 'borrowed' 5 litres from work at Marconi Instruments, it was just called photoresist positive working on the bottle.

I have tried using the spray cans of photo resist you can buy from RS etc... but the results are far more unpredictable....

John.

I did not realise that the coating had such a limited life, I only bought as and when required.....thanks for the tip.

The chemical we used (if my translation is correct!) is called Sodium Persulfate I believe (or that is what my German to English Dictionary gives me!!

At one strength it can develope the exposed PCB and at another strength (and hot) it will etch the PCB.

We were informed that this was much safer than Ferric Chloride (which is banned for home use due to people putting it down the toilet and causing problems in the sewage water system!). It would be interesting to hear from someone who understands chemicals better than I do (did?) on that score.

Heck we just drank what was left over!!!

No we didn't. However, it can hold so much copper that we could use a liter bottle of it over and over, and never get to the point where it wouldn't etch a circuit board.

See http://www.bidservice.com/ , http://memscyclopedia.org/su8.html , and http://en.wikipedia.org/wiki/Photoresist

Photoresist used in printed circuit board manufacturing is a short-chain polymer which changes to a long-chain polymer when exposed to ultraviolet light (unless things have changed dramatically in the last 20 years). The long-chain polymer is less susceptible to dissolution with solvents leaving it behind while dissolving the short-chain polymer. The board is placed in an etching solution after curing the remaining polymer and copper is etched away leaving the circuit traces.

why not try this site, download their CAD for layout and then submit to find the cost, they do really good work.

http://www.expresspcb.com/

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A related but not identical method is something I want to try to mount some Cree XR-E LED's onto 0.2mm copper clad FR4 epoxied onto aluminium as heatsink. The copper will be broad to spread the heat to overcome the poor thermal conductivity of FR4 board. My intent is to spray a etch-resist layer directly to the board, then scribe narrow gaps, then etch, leaving the remaining copper as wide tracks to handle heat and current. I tried a spraycan of metalworking layout marking fluid, but that stuff didn't scribe cleanly, or resist even the most careful handling long enough to finish the task. There are many kinds of paint in spraycans, but I don't know what to use because analysing their content is rarely simple even when there is any info at all on the can. Does anyone know of a paint that can adhere well to clean degreased copper, be scribed through very cleanly, and remain intact during etching, then be removed easily with an acetone soaked rag? It is important that the paint must not cure beyond initial solvent evaporation. Please don't guess or suggest ideas, there are too many things to try already. I'd like to know of something specifically known to be useful for this, something that someone already uses and knows to be good.