Bichromate Alternative

[motif]

Hello Everyone,

First of all I thank people of this forum for contributing wealth of knowledge here, I am really amazed.

I am exploring possibilities to use “Gum bichromate” method (Gum + potassium bichromate) to develop PCB (Printed circuit boards). Basically to transfer circuit from transparent sheet to the copper-cladded-board treated with gum-bichromate.

My sole concern is about safety. It is well informed all over Internet that chromate compounds are deadly dangerous (for both use and disposal).

1) I would like to know is there any alternative which is more safe, easily available and economical when compared to potassium/ammonium bichromate?

2) Secondly, do these bichromate remain dangerous/harmful after they are UV-exposed and developed with water? The harden material which remains after development, is that still hazardous?

Feel free to provide your views.

Regards,
Motif

 

[TheFlyingCamera]

According to "A Non-Silver Manual: Cyanotype, Vandyke Brown, Palladium & Gum Bichromate" by Sarah Van Keuren, the residual dichromate bound into the print surface upon development and processing is just that- bound into the gum/pigment mixture and is harmless. Potassium dichromate is less soluble in water, yielding slower exposure times. Ammonium dichromate is combustible and requires HazMat shipping, whereas Potassium Dichromate is not and therefore does not require HazMat handling processes. Avoid or otherwise minimize your handling of the sensitized but unprocessed emulsion to reduce your exposure to dichromates. If you do have contact with it, thoroughly wash the area.

 

[Gerald C Koch]

You can purchse photoresist already made for application to blank copper circuit boards. Any particuar reason you want to do it yourself?

 

[Steve Smith]

Or if you are making a lot of boards, you can screen print the tracking onto the copper then put it into the etch solution.

You can cure/dry the ink if you want but it's not always necessary to resist etching.


Steve.

 

[motif]

Thanks TheFlyingCamera for the alternatives, I was really looking for those. As you understand I am not much bothered about aesthetics and color, but have concern about quality. I don't want very fine PCB tracks/pads to merge or washed-down during development. Any Idea which one will be more suitable for me (among the alternatives)? Sorry If I am expecting too much .

Yes Gerald, I agree with you. Such pre-made dry film photo-resists are being used by number of people. Problem is that they are not readily available in every country and not that economical too.

Another reason is that I am really fascinated by these alternative methods for Image development. I believe this provides more scope for learning and experiments to satisfy bug within me

Yes Steve, Initially I had thought for screen printing. But due to elasticity of material used as screen, it may not serve dimensional accuracy required. Definitely for printing components names and annotation on PCB (final step after traces are done etc) it can be very useful.

Regards,
Motif

 

[paul_c5x4]

When I was involved in PCB manufacture, volume runs of single sided boards always had the etch resist screen printed. Dimensional stability was never a concern, but then again, we didn't do fancy stuff like multiple tracks between pins or high density surface mount.

 

[Steve Smith]

Yes Steve, Initially I had thought for screen printing. But due to elasticity of material used as screen, it may not serve dimensional accuracy required. Definitely for printing components names and annotation on PCB (final step after traces are done etc) it can be very useful.

The company I work for produces flexible printed circuits. Not copper, but screen printed conductive silver ink onto 0.125mm thick polyester.

We can print down to about 0.15mm track and gap with a 0.25mm tolerance of tracking to final part cut profile.

With a good stainless steel mesh, you can get very good dimensional accuracy.

Also remember that the first copper PCBs were produced by screen printing. Hence the printed in printed circuit board.

http://www.parlex.com/products/ptf.php


Steve.

 

[motif]

Thanks for the direction Steve, some how I underestimated screen-printing. Do you have any info what metal is used to create stencils (for different layers)? and can these stencils be done using commonly available materials/chemicals? I am curious to know what photo-transfer chemical/method is used for stencils.

Regards,
Motif

 

[Steve Smith]

We use a stainless steel mesh with a normal (machine) trough coated emulsion to both sides. I will get some details of the mesh and emulsion tomorrow when I go back to work. For most printing, we will use a nylon mesh. The stainless steel is reserved for tight tolerances or fine pitch tracking.


Steve.

 

[holmburgers]

Diazo sensitizers are a safer alternative. Join the exploration here -> (there was a url link here which no longer exists)

 

[motif]

Unfortunately Diazo is not available in my country

I am planning to use Cyanotype, as the chemicals are easily available, comparatively safe and economical.

I will update my findings after some time.

Thanks everyone for your views and suggestions. Really appreciated.

Regards,
Motif

 

[motif]

I tried cyanotype on paper and it worked perfectly fine. Now I wanted to try it on PCB but the solution is so thin that it doesn't stick to copper surface.

I am using 8% solution of potassium ferricyanide and 20% solution of ferric ammonium citrate (mixed in equal volume)

Any idea how can I make it work?

Regards,
Motif