Spraying photoresist

[HerbertMunch]

Does anyone have any experience with spraying + photo resist on to pcbs?

When i use commercially coated boards, i get really good results, however when i spray my own, the caustic soda just rips off all the coating during developing.

Also I cant seem to achieve a consistent coating. speckles appear all over the surface, like if you were spraying water onto a water proof surface.

Id really like to use the spray, as its much cheaper than buying the ready sprayed ones.

Many thanks,
chris.

 

[3v0]

Circuit specialists used to carry spray photo resist. Although it was in theory possible to get good results few people did. They no longer carry it.

We used the stuff years ago so the product may have changed since then. The problem I recall was that it tended to form ridges.

 

[SPDCHK]

Chris, I had the very same problem. Repeatability. I could never repeat a good batch the next time round.

I had to go to the toner-printer method, and since then I have a near 95% success rate. The trick is to have the PCB about 1cm on the borders bigger than what the print requires. Then you will have 100% success, but with some PCB space waisted. I can now even have traces between IC pins where with the Positiv 20 method it always came out broken.

 

[Boncuk]

Hi Chris,

I wouldn't even consider spraying photo resist.

This is the experience I gained using spray: The PCB material has to be posted absolutely plain and horizontal in each direction (100% levelling is necessary).
Spraying the coat of resist it must be applied also 100% equally all over the board so the wet and glossy film is thick enough.

Drying the resist can be done in ambient air in absolute darkness. That process lasts 48 hours or more at temperatures below 25deg/C. Drying in an oven at a max. of 70deg/C (with no hot spots) the process is accelerated considerably, but here again there is the problem of absolute darkness.

If the photoresist isn't dried properly you will achieve an absolutely worthless PCB. You can observe how the traces develop and a second later you can observe how they diminish.

There are better ways to spend money. Take your wife out for a good dinner - that's a better idea.

Boncuk

 

[HerbertMunch]

Hi thanks for the replies.

3v0,
for me, not ridges, more like 100% speckled

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Its a shame because as i metioned before, its so much cheaper than buying the precoated ones. If i were to buy a 100x160 FR4 ss from rapid, it would cost £0.90. The same board coated in + resist costs almost £3!

I have tried the toner transfer method endlessly. For me it doesnt even produce close to comparable results as the photographic method. For instance with the photographic method i can achieve traces that are a fraction of a mm, and extremely close together (with no need to worry about toner smudging).

I have tried many different papers, (magazine, glossy, transparencies, etc).
The only paper i havent tried is the commericial stuff. I just cant warrant buying this stuff as i cant bear to cut out bits of paper and then stick it to an A4 backing sheet.
This is what I had to do with the magazine paper, as it would jam my laser otherwise.
+ not to mention that having to purchase the p&p type stuff, would bring the cost up to much, and i might as well just buy the pre coated boards.

It all just seems like an awful lot of effort, and I find that repeatability is terrible.

SPDCHK, what type of paper do you use?

Anyway, has anyone used the pre coated ones before?
The results that can be achieved are stunning and no problem with the non exposed stuff coming off in the developer.

The resist seems to have been sprayed in a pattern. Almost like they sprayed it through a very fine mesh of some sort.
Do you think that is how they achieve the consistant covering?

Thanks all
Chris

 

[jpanhalt]

Eastman Kodak Company will tell you the one thing it knew how to do was to spread thin, uniform films on a substrate. It's not as easy as it seems to be. Spraying is one option, but requires skill. Dipping is another; silk screening and spinning are other options. The liquid resist is probably in a volatile organic solvent, so you may want to consider building a box in which you can create an atmosphere that is saturated with the solvent vapor, which will slow evaporation and allow leveling. It is a lot of bother and probably not worth it on a small scale.

As an alternative, DuPont makes a thin-film version of photoresist that can be laminated to bare PCB boards. It goes by the name of Riston 9000. I have found it, but only in very large quantities or small, very expensive (more expensive than pre-sensitized boards) quantities. Has anyone here tried it or used anything similar?
John

 

[Boncuk]

Hi Chris,

why don't you use a safe way with excellent photo quality film?

Print a pure black copy of your layout. Then get a film (available at print stores) which is UV-sensitive. Use your laser print and copy to the film as a contact copy. There are two types, positive and negative. (The negatives can be used for electrochemical through-holes and vias for later differential etching)

Using a laser print on a PCB the toner will more or less rub off leaving areas out of focus and blind spots which you will have to touch up before etching.

The films I mentioned are ultra thin and there will be little refraction if at all. They use an extra developer and a fixer. (The fixer takes care of a absolutely black copy.) As a contact copy they are as accurate as the original print. After the process the film is reusable many times. If you handle it with care (no sharp edges on the PCB) you can stow it away in a dust cover (or envelope) and use it at any time you need it.

I made lots of double sided PCBs applying that method and except for the missing solder stop they looked professional.

Regards

Hans

 

[SPDCHK]

SPDCHK, what type of paper do you use?

Plain high gloss magazine paper. The photographic paper people use for photo's is too thick for my laser printer (HP Laserjet 4V)

Anyway, has anyone used the pre coated ones before?

Honestly, no. I've always made my own PCB's. As mentioned before, I used the Positiv 20 method, but I did not have muck luck with that. Maybe one day, if I spend the time and money and try development with a pre-coated board I would sing another tune, but for me as a hobbyist, I'm going for cheap and nasty

 

[3v0]

If I ever got to the point where I could not do the job with toner transfer I would go to pre coated sensitized boards.

For toner transfer I use the Pulsar (now PulsarProFX) paper. I use it with a good laminator and would never go back. It is poosible tomake do with an iron but from my viewpoint it is just one more thing to go wrong.

Touchup is rare and resolution is as good as I need for through hole or surface mount. On through hole boards traces between pads are ho hum, and you can get two with just a bit a care using .010 traces.

I switched to 3M's blue painters tape for all the taping. Other brands may work too. the current roll I have is labeled Intertape. It is easier to use the the mailing labels which are far too sticky. With a bit of care the same A4 sheet can be used to print 3 or so images.

I have posted images of boards and would be happy to do so again if you would like to see them.

 

[SPDCHK]

This is a project I finished over December. The circuit and PCB layout was designed with Eagle. I usually export the track image as a BMP file, and manually just do some tough-ups, e.g. make the through hole solder surface a bit bigger etc. I then print on a normal high gloss magazine (usually one of the wife's female magazines.) I have noticed that if there is RED on the magazine page, that tend to come off on the PCB as well. I therefore always try to find a page that is as white as possible, else a page that does not have many colour images on it.

I etch my board with a third each solution of H2O2 (40%), HCL and H20. (Laymen’s terms; 40% Hydrogen Peroxide, Swimming pool (hydrochloric) acid and water)

Once the board is etched and after I've drilled the holes, I treat the board with a Tinning solution. Here in South Africa I found a product called "Tin-Posit LT-34". I bought 1 liter for about R100 or about $12.00. For the amount of this liquid I use for each board, it should last me forever.

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[Hero999]

A nice board.

The only problem is that there isn't enough clearance between the mains and the DC side. For safety reasons you should allow at least 5mm (I can't remember the exact figure, I'll have to check the IEE regulations). This is because there could be 2kV spikes on the mains and any moisture/impurities on the board could cause it to track over to the DC side.

I strongly recommend you cover the mains traces with a protective lacquer or conformal coating which will provide an additional layer of insulation.

If the entire project is encased in a plastic insulated box and no external circuits are going to be connected to the the DC then there's no need to bother but otherwise I wouldn't risk it.

Another way of providing similar protection would be to bond the negative rail of the DC supply to the protective earth conductor.

 

[picbits]

Nice to see I'm not the only one who uses SMT decoupling capacitor directly under the PIC power pins

 

[Hero999]

I tend to use SMT parts whenever I can nowadays as it saves drilling holes.

 

[SPDCHK]

A nice board.

The only problem is that there isn't enough clearance between the mains and the DC side. For safety reasons you should allow at least 5mm (I can't remember the exact figure, I'll have to check the IEE regulations). This is because there could be 2kV spikes on the mains and any moisture/impurities on the board could cause it to track over to the DC side.

I strongly recommend you cover the mains traces with a protective lacquer or conformal coating which will provide an additional layer of insulation.

If the entire project is encased in a plastic insulated box and no external circuits are going to be connected to the the DC then there's no need to bother but otherwise I wouldn't risk it.

Another way of providing similar protection would be to bond the negative rail of the DC supply to the protective earth conductor.

Thanks for the advice. I followed some standard where I read something about that 1mm is enough protection for about a 1000V. To be quite honest I usually don't fill the unused parts of the PCB. I tried it here and came out quite nice, BUT I seem to have violated some safety rules in the process.

 

[Nigel Goodwin]

Thanks for the advice. I followed some standard where I read something about that 1mm is enough protection for about a 1000V. To be quite honest I usually don't fill the unused parts of the PCB. I tried it here and came out quite nice, BUT I seem to have violated some safety rules in the process.

Your mains connections are really badly (and dangerously) designed, why on earth have you got copper filling in between the mains pins?.

But the actual PCB looks really great!.

 

[Hero999]

A 1mm air gap is fine for 1kV

A 2mm air gap is fine for 2kV.

But neither of these measurements account for tracking which can occur at much greater distances.

Tracking occurs when water condenses on the surface of a material placed between the electrodes, forming microscopic droplets which effectively reduce the electrical distance between the two conductors.

If you don't have enough room leave a 5mm gap between the mains and DC side and don't want to bother with a conformal coating you can cut a slot in the PCB between the traces. This stops the tracking as there is no surface where condensation can build up on.

There are different classes of insulation, single insulation. and double or reinforced insulation.

On a 200VAC to 250VAC circuit:

Single insulation only needs to hold off the nominal mains voltage and must have a minimum breakdown voltage of at least 500V.

Double or reinforced insulation must not ever fail under any circumstances and must have a minimum breakdown voltage of at least 2kV.

Single insulation between an exposed metal part and the mains is fine, providing the part is earthed. If the part isn't earthed, double or reinforced insulation must be used.

An example of single insulation is a 1mm gap between PCB traces or some thinly insulated wrapping wire.

An example of double insulation is a mains appliance's cord - there are literally two layers of insulation between the user and the mains (the insulation on the cores and the outer sheath).

An example of reinforced insulation is a multimeter test lead - the insulation is very thick and won't break down at 2kV.

Another example of reinforced insulation is a 5mm gap between PCB traces because there's no chance of tracking even at 2kV.

The same rules apply to separating a SELV (Safety/Separated Extra Low Voltage) circuit (a circuit with a nominal voltage not exceeding 25VAC or 60VDC), (which is what the DC side of your circuit is) and the mains. You can earth the SELV circuit (in which case it becomes a protected ELV circuit) and use single insulation or no earth it and use double insulation.

In short electrical safety can be a nightmare which is why home building of mains powered projects is discouraged.

EDIT:
Is the copper filling between the mains connectors a ground plane?

Just earth it, it'll prevent the ELV side from floating at mains potential.

 

[pitronix]

I have been doing small quantities PCB manifacturing for some time and again I have to advice people to try using silk screening with PVC colors. It is easy enough and pays out in a long run much more then all other methods. You just print as many pices as you need and etch it with H202 and HCl solution. For using Positiv 20 and another sprays you need lot of practice but it can be done if you realy take some time to finde right way to do it

 

[HerbertMunch]

I have been doing small quantities PCB manifacturing for some time and again I have to advice people to try using silk screening with PVC colors. It is easy enough and pays out in a long run much more then all other methods. You just print as many pices as you need and etch it with H202 and HCl solution. For using Positiv 20 and another sprays you need lot of practice but it can be done if you realy take some time to finde right way to do it

would you recommend silkscreening for single boards?

 

[Hero999]

Here's a couple of links regarding clearances.
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It's quite complicated, so just sticking with earthing, conformal coating or ensuring that the secondary side has more than 5mm of clearance from the primary side should keep you safe enough.

 

[3v0]

I have been doing small quantities PCB manifacturing for some time and again I have to advice people to try using silk screening with PVC colors. It is easy enough and pays out in a long run much more then all other methods. You just print as many pices as you need and etch it with H202 and HCl solution. For using Positiv 20 and another sprays you need lot of practice but it can be done if you realy take some time to finde right way to do it

It is my GUESS that anyone who can not manage a toner transfer will have little chance of making and using a silk screen. Could be that the process has changed since I used it.

What chemicals does it take?
How do you create the mask?