unabe to develop PCB - positive photoresist

[kellogs]

Tried some old, from an open bag,  pure NaOH powder on an old Kinsten pre-sensitized PCB (protective foil was still firmly attached). Warm water, gobs of NaOH, nothing happened! Was it supposed to ?

The same setup went well on the blue stinky negative photoresist paint after leaving outside in the winter light for one hour. Same weather and time amount used for the presensitized Kinsten PCB.

Thank you!

[jpanhalt]

How many grams is a gob?  How many tablespoons of water was it dissolved in?

I am not familiar with that particular brand of positive photoresist.  The brands with which I am familiar show a difference in sensitivity to NaOH, not absolute resistance.  That is, a 0.4 molar solution can be used for development; four times that will remove even unexposed resist. 

[Benta]

Predestined to fail. You need to work more precisely than that.
First: exposure. Leaving it out in the sun for an hour is a joke. My old setup was a 300 W Osram Ultra-Vitalux UV-A lamp. Distance to PCB: 40 cm. Exposure time: 3.5...5 min.
Second: development. You need exactly 7 g of pure NaCl to 1 l of pure water. "Gobs" won't work.

[Siwastaja]

Remember that NaOH is poor man's replacement for the real developer and it's pretty finicky: concentration, temperature and time is critical with NaOH. I would not recommend using it except in a pinch. It's not the real developer!

NaCl as suggested by Benta does not work either. That 7g/l sounds eerily familiar though, maybe he meant NaOH.

You remembered to remove the protective film before developing, right?

Otherwise than that, chances are the exposure is not enough, or NaOH concentration is not high enough. Positive photoresist works so that light softens the photopolymer to be removed by the developer. Too little light, and nothing happens.

[Benta]

Remember that NaOH is poor man's replacement for the real developer and it's pretty finicky: concentration, temperature and time is critical with NaOH. I would not recommend using it except in a pinch. It's not the real developer!

NaCl as suggested by Benta does not work either. That 7g/l sounds eerily familiar though, maybe he meant NaOH.

You remembered to remove the protective film before developing, right?

Otherwise than that, chances are the exposure is not enough, or NaOH concentration is not high enough. Positive photoresist works so that light softens the photopolymer to be removed by the developer. Too little light, and nothing happens.

My bad! NaOH of course, sorry. Dunno where that NaCl brain fart came from.

But actually NaOH IS the developer for positive resist, I know of no other.
Here's the datasheet for Positiv 20, which is the most common resist for amateurs (if not using already lacquered PCBs).
https://www.tme.eu/Document/b7a6a16ed14d2aa6149ed9a14fb1b03d/TKC3%20POSITIV20.DTE.PDF
They mention 10 g/l, though.

[coppercone2]

are you reading one of those guides written by someone too cool to measure stuff ? (marketing technique)


You only ever not need to measure if you are slowly changing the viscosity of something, like thinning paint, where the idea is its a slow feedback loop where you observe the consistency proportional to addition. Using a similar technique for adjusting a pH is just not going to be conducive to anything other then a titration (that is, if you have a indicator). .


I ask what is your feeedback for adding gobs?

[jpanhalt]

I agree with Benta about needing some accuracy in making the developer for positive resists.  My presensitized boards were all from DigiKey from the mid-1990's to about 2010.

NaOH 7g/l is about 0.18 molar (MW=40).  I used 0.4 to 0.5 molar of KOH.  In the US, powder is available, but the more common form is solid half pellets.  Powder is probably less stable due to its reaction with atmosphere CO2.*  Over developing was not a problem, but under developing, over/under exposure, and over etching could be.  I removed mask after etching by adding a tablespoon of half-pellets of solid alkali to about 100 ml of developer.

*Both KOH and NaOH can also dehydrate to the oxides.  Thus any calculated molarity is an approximate, but still closer than gobs/squirt.  Exact procedure attached.  I placed  the boards on glass trivets during etching so the etchant could be easily stirred with a magnetic stir bar.  Essential for all positive resists is a photosensitizer.  Because of that, only near UV is needed and is probably preferred.

[kellogs]

So the NaOH does not spoil, right ?

>(8.4 g / 300 mL water)

I must have used something like 10-12g per 0.2L - maybe 10 times what I used on the blue paint negative photoresist in the past.

Also 1h on this weather & time of the year was the norm on the blue paint negative photoresist. Maybe i should leave it out for some 3-4 hours for these positive Kinstens ?

[jpanhalt]

According to a listing on eBay, the manufacturer recommends Na2CO3 (sodium carbonate) to develop (attached).  So, inactive NaOH is unlikely.  Are you sure it was NaOH?

[Benta]

Also 1h on this weather & time of the year was the norm on the blue paint negative photoresist. Maybe i should leave it out for some 3-4 hours for these positive Kinstens ?

How about a week or two? That's more realistic.
The Osram UV-A lamp I mentioned will give you a red sunburn if you hold your hand under it for 5 min. Try repeating that in the winter sun.

BTW, you'll see the PCB pattern in the undeveloped paint after exposure if you angle the PCB and squint along the surface.

[mikeselectricstuff]

But actually NaOH IS the developer for positive resist, I know of no other.

There is Sodium Metasilicate silicate developer which is much better than NaOH- it has a much wider margin between "develop" and "strip", which means it's much less temperature sensitive, and you can use it really strong so it will develop in a few seconds without overdeveloping.
Also the made-up solution has near- infinite shelf-life, whearas a tray of NaOH deteriorates in a few days in air

https://www.microchemicals.com/AZ-Developer-5.00-l/1000001

And don't dick around trying sunlight exposure - UV insect killer fluorescent tubes are pretty cheap and still  available - these will expose in around 5 minutes.

[mikeselectricstuff]

So the NaOH does not spoil, right ?

Made-up NaOh developer left in a tray will be useless in a few days - No idea of the chemistry, just from experience before I discovered silicate developer

[jpanhalt]

There is Sodium Metasilicate silicate developer which is much better than NaOH- it has a much wider margin between "develop" and "strip", which means it's much less temperature sensitive, and you can use it really strong so it will develop in a few seconds without overdeveloping.

What's best is what works.  I tried sodium metasilicate with the boards from DigiKey, and it didn't touch them.  The basic chemistry of positive resists is similar.  Most are Novolak based, but details of the chemistry and manufacturing process affect how they work.

[nctnico]

Oh this thread brings back memories. When I was an intern at an electronics workshop in a hospital, I had to etch my own boards. At some point I made the developer too warm and maybe added too much NaOH as well. I put the board in and within seconds I was left with a bare copper surface    For my home setup I always used pre-made developer kept at room temperature. Much easier to work with.

[jpanhalt]

Tried some old, from an open bag,  pure NaOH powder on an old Kinsten pre-sensitized PCB (protective foil was still firmly attached). Warm water, gobs of NaOH, nothing happened! Was it supposed to ?

That sentence has stuck in the back of my mind.  What did you expect to see?

Here's a test:
1) Take a piece of what you think is a positive-resist presensitized board.
2) Rinse in water.  Does the water bead up and run off?  If it doesn't bead up (photoresist is hydrophobic), it's probably not coated.
3) Place it in your favorite etchant (I use ferric chloride or cupric chloride), then rinse in water.,
4) Does it change color almost instantly?  The board will not show any changes in ferric chloride if it is covered with photoresist.  If it is bare, it will become duller and a salmon pink, in my experience.  (I have used that test to see whether the pattern is fully developed.  If not, you can always put it back in the developer.)
5) Now repeat with boards you have illuminated and processed in developer.  Some possibilities: a) The boards don't have photoresist on them; b) Your developer is so strong it is instantly removing the resist; c) Your NaOH is not NaOH; d) Other

[Siwastaja]

There is Sodium Metasilicate silicate developer

Yeah, that's it. But photoresists from different manufacturers are different from each other; if manufacturer recommends NaOH then obviously it should work and I stand corrected.

I get PTSD from Positiv 20 spray. Never again. Really, get negative dry film photoresist and a laminator and you have a real PCB process which works. Negative photoresists are physically so much easier to formulate (it is easier to make a polymer which hardens on UV light, than one that's hard enough to survive development/etch but softens on light). That's why professional processes were always negative.

The only reason to use positive process is that computers do not exist and artwork must be drawn by hand and hobbyist cannot afford a photographic extra step to invert the artwork. This reason disappeared around 1995 when computers (capable of running PCB EDA and inverting color) and printers (capable of printing the inverted artwork on a transparent sheet) became affordable even to hobbyists.

Pre-sentisized boards are much easier to work with than the spray, but pretty expensive. But negative dry film is cheap and easy to laminate!

[kellogs]

>BTW, you'll see the PCB pattern in the undeveloped paint after exposure if you angle the PCB and squint along the surface.

That's correct, and I can clearly see the imprint.

Also found the developer for them: https://www.wiltronics.com.au/wp-content/uploads/datasheets/kinsten-developer-safety-data-sheet.pdf
I understand that the Sodium metasilicate and NaOH developers are compatible. Correct?

Will resume after Christmas. Thanks everyone and have a merry one!

[jpanhalt]

Metasilicate is formed from NaOH/Na2O and silicon dioxide (glass).  Solutions of it were called water glass  It can me made by etching ordinary glass with sodium hydroxide.  Not sure what you mean by compatible.   When phosphate detergents and cleaners (e.g., Na3PO4) were outlawed, sodium silicate was usually used as a replacement.  It is a strong base but not as strong as NaOH and in my experience it tends to leave more residue than TSP or sodium hydroxide.  Any of the three can be used for denaturing and removing many paints.

For your exercise, if NaOH didn't work, I would not suspect sodium silicate to work.  So far as I know, there is nothing specific about the silicate that is needed.

@ Siwastaja, See later correction and retraction. I disagree about the use  of positive resists when you don't have a PC or draw by hand.  Sure, they are convenient in that situation, but they work well with computer graphics too. Most important, they  are convenient to use and don't require expensive and toxic organic chemicals for developer.  Anyone who wants to replicate a commercial process at home is wasting their time.

EDIT:  BTW, I didn't mention the brand of PCB boards I use.  The brand is/was Injectorall in Bohemia, NY. and sold by DigiKey.  Injectorall went out of business several years ago.  The photosensitive coating was orange-yellow.  The most effective lamp for it was a BL type. fluorescent bulb.  That is not germicidal, and its phosphor emits mostly around 390 nm.  A single, 18" bulb (15W) required about 12 minutes at 6 to 8" height.  When developing or stripping, the removed resist turned purple, and as mentioned, sodium silicate could not be used to develop it.  I have never used MG  branded boards, but from descriptions, they seem to behave quite differently.

[Siwastaja]

Most important, they  are convenient to use and don't require expensive and toxic organic chemicals for developer.

Wut? Dry film negative photoresist uses sodium carbonate (aka washing soda) as developer. Much more benign and much safer than NaOH. And not expensive at all. Like, playing around with NaOH you can accidentally make it boil and splash on your face while diluting, while sodium carbonate is... just "detergent".

For hobbyist, the benefit of dry film photoresist is that it hardens tougher than what Positiv 20 initially is, and process variation is wider. With Positiv 20, thickness of the coating, concentration of NaOH, development temperature and agitation were all finicky parameters. Whereas dry film photoresist is developed in sodium carbonate solution concentration of which can easily vary 1:3 without any issues, it doesn't matter if the development time is 3 or 6 minutes, and the film is tough enough to survive brushing with a toothbrush while in developer! (A tip: use hot air to bend the tip of a toothbrush 90 degrees to make a specialized PCB washing instrument.)

With wider process margin, hobbyist does not need to waste time in redoing and redoing PCBs.

Transitioning our club from Positiv 20 + NaOH, HCl+H2O2 (both finicky processes) into dry film photoresist + cupric chloride compressed air agitated etching was the game changer which massively increased popularity of DIY PCBs because of so much better yield and quality and less wasted time. Before that, some people supposedly had no problems with Positiv 20 but others did, I never was able to master the correct "magical touch" of spraying. Instead, I found dry film photoresist by coincident - had a 20-year or so old large roll lying around, still perfectly fine to use - and spent some time documenting the process for others to avoid the "magical hidden knowledge" phenomenon.

[jpanhalt]

@Siwastaja
Sorry about that.  My comment was based on faulty memory from years ago when looking at using a liquid resist (not dry film). Some of the solvent carriers used NMP (N-methylpyrrolidone) and aromatic organics. 

Please accept my apologies.

[Siwastaja]

No problem. In any case, liquid photoresists are PITA in home conditions, and putting them into a spray bottle seems easy but at least for me, spray coating correct and uniform thickness without the "orange skin" effect was nearly impossible, especially on a large PCB.

Pre-sentisized boards solve this coating problem for hobbyists who want a positive process and are willing to pay for it. For the rest of us, dry film easily produces perfectly uniform coating of exact correct thickness.

[coppercone2]

I believe the spray on coating to be a elaborate internet hoax

[Siwastaja]

I believe the spray on coating to be a elaborate internet hoax

Unfortunately it isn't. I can assure you Positiv 20 exists; I have personally used many bottles. Furthermore, it is expensive and has poor shelf life.

[coppercone2]

What does renew my interest is that I see people actually made spin coater using normal fans on youtube, assuming its not staged it makes me think liquid is possible


however, using the kitchen sink with a rubber plug to do the wet lay of the film is like the best method so far, if you have a big sink

[Gerhard_dk4xp]

This carbonate stuff is the developer for negative resist,
NaOH IS the right thing for Positiv-20 or Bungard pre-sensitized boards.

For example in Berlin  <     https://www.segor.de/#Q=FP2100x160B-35u&M=1    >
I normally use Bungard boards, and Positiv-20 for Rogers 4000 or TMM-6.

NaOH is also a good resist stripper after etching. Much better than any solvent
if you want to silver or tin plate your board.

My process is <  https://www.flickr.com/photos/137684711@N07/50834544133/in/album-72157720109860238/   >

It makes no sense to talk about prices. JLCPCB wins that hands-down.
Important is the difference from computer screen to solder: 1.5h vs 1.5 weeks.

Gerhard