Photoresist for PCB etching process

[Cifrocco]

Years ago, the local electronics store sold small aerosol cans of MG Chemicals Positive Photo Resist, along with the complete line of PCB etching chemicals and equipment. They offered both positive and negative fabrication of PCBs.

Since several years however, I can't find the Resist in aerosol can or any other form. When I emailed MG Chemicals about it, they said they don't sell it anymore because it causes cancer. Honestly, that's what they said. I guess it's ok to smoke cigarettes from the age of 18 everyday of your life, but it's too dangerous to etch the odd PCB every now and then, a process which can be carried out with sufficient safety precautions to eliminate exposure to any harmful chemicals. I think the only place you can get Resist nowadays is from DuPont and it costs about $1,000 per liter or something like that. You can, however, purchase pre-sensitized boards from MG Chemicals, which come in assorted sizes.

Anyways, I was curious to know what folks here use to etch their boards. What would you say is the best technique, short of sending them out to a PCB factory to have them custom made?

 

[jrz126]

Alot of people (myself included) use the toner transfer method.

it involves printing your circuit onto photo paper using a laser printer. Then you iron it onto a regular copper clad board. soak it in water to remove the paper over the copper that you want to remove. (not covered up by the toner) The toner will stick to the copper. Then use an etchant to remove the copper (I use sodium persulfate). Then alittle acetone to remove the toner, drill, solder and your're done.

Thats a quick rundown of how I do it. google for it, there are quite a few guides out there.

 

[Oznog]

Photosensitive Riston films are made for commercial production, but with some technique they can be applied at home and with better results than the spray resist. It's got a protective layer on either side, you peel off one side and roll them onto a warm board while applying some more heat with a hair dryer or heat gun. The trick is to avoid air bubbles, although you can pop them with a needle, get the air out, and reheat it without causing too much of a problem. Then you can expose & develop it and get really fine, reliable lines without hairline breaks or anything. Uses common spa chemicals, really cheap & easy to mix.

 

[Cifrocco]

Hey fellas, thanks. Much appreciated.

 

[Cifrocco]

For those who are interested, I found this on Jon Elson's Home Page:

Resists
I currently use DuPont Riston 4713 dry film resist. I was lucky enough to get a dry film laminator at a good price. For lower volume, I would suggest you buy the board pre sensitized with the dry film resist. I used to use pre-sensitized boards from Kepro, in St. Louis, MO, USA. There might be someone closer to you who supplies similar material. I tried using the liquid resists, like Kodak KPR, and had very erratic results. It was also hard to tell when the resist gave a good print, because it was nearly clear. The Riston has a dark color, so you can easily see the traces and pads, and any defects can be touched up. The main benefit of the dry film is it is remarkably tough, as you will find out if you have to touch it up with an exacto knife. I won't even try the peel-off techniques, I know they won't work for the 2-sided work I'm doing.

Exposing Light Source
The resist hardens (cross polymerizes) under mercury-band UV light (357 and 406 nM). I used to use a 400 W mercury-vapor lamp with the outer, shield bulb removed, at about 30 cM, for 16 minutes per side. I have switched to using UV fluorescent lamps that have a peak output around 350 nM wavelength. Apparently the lamps used in backyard electric 'bug-zappers' are quite similar to this, and those can be picked up in your local hardware store.

Setting correct exposure time is somewhat trial and error, but using a Stauffer ST-21 step tablet (available at photo supply stores) will make the process much easier. You should try to get an exposure that causes the resist at step #7 to remain solid after development. This ensures that a clear spot on the film will result in an even harder resist than what was under your #7 step, because it got even more light. (On the ST-21, step 1 is nearly clear, while step 21 is totally black.)

Exposure Frame
For best results, the emulsion side of the master film should be held in close contact with the resist surface. I use a home-made vacuum frame. It consists of 2 sheets of 1/8" Plexiglas. One sheet has a 1/8" NPT angle pipe fitting threaded into it, for the vacuum hose. The other sheet has a ring of .060" plexiglas strips glued around the edge. Just outside this ring is a piece of very thin (about 1/8" OD) Tygon plastic tube, used as an O-ring. The ends are glued together, and it is tacked down in a few spots with contact cement. The vacuum pump is a small diaphragm-type pump. it really doesn't need to provide much suction, due to the large area of the sheets. Mine is about 12 x 18", large enough to do just about anything I'd be likely to do. Right now, I expose one side at a time, but the films are aligned, so I can flip the vacuum frame over without risking that the board will slip out of alignment with the side already exposed.

Note that the dry film resist should be exposed with the cover sheets on, and then held in a dark place for 15 minutes before removing the cover sheets and developing.

Developing the Resist
I develop in Sodium Carbonate for about 3 minutes, at 35-40 C, with very light wiping with the bare fingertips on the resist. Some people are sensitive to the developer, and say a sponge can be used safely. Wash the board very thoroughly, or any softened resist can dry back onto the bare copper. I wipe again with the fingertips during the rinse step. Etch in any acidic etchant, but not alkaline, that will dissolve the resist. Ferric Chloride works, but is messy. I'm looking at Cuprous Chloride. A spray etcher is best, spraying 35 C FeCl2 on both sides of a double sided board at the same time for about 5 minutes. This resist is so good, you can use a dip tank, but it takes about 17 minutes, and the edge definition is poorer. You need constant agitation if you dip or float the board on the etchant. The resist is stripped with NaOH (Lye).

A Trick for Aligning Artwork
I align the two sides of a double-sided board's negatives by gluing them together to a strip of scrap board. I have a plastic sheet which is .060" thick, same as a PC board +/- a few thousandths of an inch. I put it between the two negatives and align them on a light table until the holes line up. I glue the PC board scrap along one edge with rubber cement, with the clear plastic still between the films. I put a heavy weight (perhaps a kilogram or so) over the glue, and then make a final alignment. I usually get registration of .015" or better, depending on what equipment was used to make the films. Note that laser printers are not very accurate, and so any board bigger than, say, 100 mm (4") on a side will not line up at all the holes. You need to take your master artwork, however you make it, and make a negative on high contrast film, such as Kodalith (Lithograph-type) film. The resist is low contrast, and so the negative must be very high contrast, or you will have light leaking through the areas that are supposed to be black, and causing the resist to not all wash off there. That will ruin the board.

I have modified a Calcomp plotter to hold a light-pen (optical fiber), and write on litho type film at 1:1 size. It's a haywire contraption, but it does work. This looks like a positive, black lines on a clear background (like a laser print would look) and so I then have to reverse it by contact printing onto the same type film. This improves contrast a bit, too. Then the films are contact printed with the UV light to the circuit board resist.

I have recently built a 1000 x 1000 DPI photoplotter, using a diode laser to write the artwork on the film. Software can make it positive or negative, so i eliminated the step of contact printing. The accuracy is MUCH higher, so all the holes of 2-sided artwork line up across even a large board.

 

[justDIY]

i havent' seen postive that you can spray on, but I did run across some negative spray on

**broken link removed**

there is the big ugly url or just visit https://www.minute-man.com and look under the pcb etch resist section

 

[fever]

Alot of people (myself included) use the toner transfer method.

it involves printing your circuit onto photo paper using a laser printer. Then you iron it onto a regular copper clad board.

photo paper? is it the same paper that used to take printouts of digital photos.and u said use laser printer for printing.i doubt on the thinkness of the lines being printed.can they resist etchant?

 

[Phasor]

i doubt on the thinkness of the lines being printed.can they resist etchant

How small do you think the tracks are going to be? Most hobbyists wouldn't go below 5 mil, which should be no problem for toner resist.

 

[sand]

how can i use a negative photoresist & whether any particular quantity for that

 

[jpanhalt]

@Cifrocco

I would like to try the Riston films. Was your post simply a quote or do you have experience with them? I have not found any place to buy Riston in hobby quantities. I have been looking on and off for at least 3 years.

As for alternatives, I use Injectorall pre-sensitized boards from DigiKey. The company will also sell direct. The resist is orange-yellow and appears from various communications with other users to be a little less sensitive to UV than MG. I simply use dilute KOH for developer. So, except for the initial cost, the other chemicals needed are very cheap.

John

 

[picbits]

Wow - really old thread bumpage .......

 

[jpanhalt]

I think we have all made that mistake once or twice. Sorry. John

 

[Hero999]

I know this is an old thread and I don't care - there's some pretty interesting stuff here.

I use the toner transfer method with magazine paper for most PCBs and the photo method for more complecated PCBs. I've never used photoresist spray anduse presensitised boards

 

[3v0]

If you try to make the photo resist process as cheap as possible you can get some very poor results.

You are going dirt cheap on toner transfer, I suspect you are getting what you pay for in this case.

EDIT: I could be wrong and perhaps your toner boards are as good as it gets, but then why would you need photo resist.

The question to ask is how good is good enough?

 

[philba]

toner transfer can make some pretty good boards. I have good luck with 12 mil traces and separation. At 10 mil it gets a bit dicey for me. Photo can go lower than 10, I've heard of people doing 8 mil traces.

 

[3v0]

toner transfer can make some pretty good boards. I have good luck with 12 mil traces and separation. At 10 mil it gets a bit dicey for me. Photo can go lower than 10, I've heard of people doing 8 mil traces.

Toner transfer. I like to use 12 mil too. I made a few PCB that were 10 mil but had one or two traces on each board that were broken.

The pulsar site shows traces down to 6 mils.
They use 1/2 oz copper to reduce undercutting and cover the toner with foil prior to etching, this reduces the chance of a bad trace due to pinholes in the toner. I use surplus "mystery" surplus PCB stock and often skip the foil so I am sure there is room for improvement.

I am not sure that my inexpensive laser printer can produce such fine lines. I have started working with SMD so I have a chance to go smaller in the next few months.

 

[buddog08]

MG Chemicals now has introduced a negative photoresist film, catalog no. 416DFR-5. A 12inch x 5ft roll costs about $20. It is applied to the PCB using a hot laminator, and requires the Negative developer (MG No. 4170-500ml). Info is on the MG Chemicals web page.

Which leads me to another question? How can I produce negative artwork? I'm using Gerbtool, and it only appears to produce positive print to my laser. I can export to a bitmap and invert with a photo/paint program, but then the image scale is all screwed up. Is there a Gerber Viewer/Toll that can directly output negative images? MG Chemicals recommends IvexView, but that tool is no longer produced.

 

[sum]

i was searching internet for dry films and found few other companies manufacturing other than Dupont , have u tried these other companies

Product Lines for Electronics and Electronic Devices: Dry film photoresist
**broken link removed**
**broken link removed**

Please let me know about these alternatives.

Thanks