what is minimum trace width i can get on presensitized boards

hi

can anyone tell me what is the minimum trace width i can get on presensitized boards?
i needed 4 mil trace width for my pcb

thanks

depends on your PCB manufacturer capability. in olden days 8/8 mil (width/ seperation) was common. But Now most of the manufacturers are capable for doing 4/4 mils but at a higher cost.

NIKHILRAJ said:

depends on your PCB manufacturer capability. in olden days 8/8 mil (width/ seperation) was common. But Now most of the manufacturers are capable for doing 4/4 mils but at a higher cost.

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thanks for your reply... actually i have already bought presensitized pcb board
can i get 4mil trace width on these boards: **broken link removed**

thanks

I use presensitized positive resist boards made by Injectorall (Source: DigiKey). You can get very fine traces with experience. I generally stick to wider traces than 4 mil, but I use a "zero width" line as a cutting guide, It generally comes out pretty good and is certainly in the single-digit mil range.

First off, what experience do you have? Are you using positive resist? How are you making your transparencies? How are you etching the developed board?

The etching process is probably one of the most important variables when you go to very fine lines. Some etchants cause significant undercutting compared to others. Considering only the acidic etching baths, I believe ferric chloride has about the least undercutting, cupric chloride is a little worse and is sensitive to the acid concentration, and the peroxide-based etchants (e.g., HCl + hydrogen peroxide) give the most undercutting.

Here are two references related to etch quality:

Printed Circuit Board Fabrication - Etching Outer Layer (Look about halfway down under quality. NB: The equation for etch factor is inverted relative to the reference below.)

**broken link removed** (Shows some typical etch factors.)

In sum, I think 4 mil may be doable by a hobbyist, but it will require some experimentation and very careful control of the conditions.

John

Edit: Here is an image of a portion of a board I made. The smaller traces are 24 mil. The "J " is considerably smaller. As calculated from the font used, the line width is approximately 6 mil. Unfortunately, I had no pictures of the outline I use as a cutting guide, but you can see from the letters, that 6 mill and possibly smaller would be OK.

thanks for the information john it is very helpful.
i have experience of using toner transfer method and with that i can get 10mil width. i am using ferric chloride for etching

Since you have used toner transfer, I assume you are planning to use your laser printer to make the transparency for exposure of your photoresist. It will work, but it may not work as well as an inkjet for that purpose. Laser toner is opaque, but it is still particles, and light can get through the spaces to cause roughness and uneven edges. Inkjet tends to flow, so there are no holes. Inkjet does not need to be completely black. The colored inks block enough of the light to be just as effective. In fact, I ran out of black (actually, the cartridge plugged up) and I could not tell the difference between those boards made with a transparency with black ink and those made without it.

I did a brief study comparing inkjet to laserjet and to a modified laser jet transparency. The modification was simple. I took a Dry Erase pen and swabbed it across the laserjet transparency. After the ink dried, I wiped it off with soft tissue. The Dry Erase fills the tiny holes in the laserjet toner. If you try this, just apply the ink with a full, smooth stroke. Since you are removing excess later, cover the entire circuit image. DO NOT SCRUB the transparency with the marker, or you will remove some of the laser toner.

Shown below is a microscopic image of the laserjet transparency of a pad, a similar pad after treatment with a Dry Erase pen, and a pad done with inkjet. I hope the pictures are self explanatory. If not, I would be happy to give more details.



John

thanks john for the explanation and pictures... very informative.. it seems using inkjet will give better results

I did a brief study comparing inkjet to laserjet and to a modified laser jet transparency. The modification was simple. I took a Dry Erase pen and swabbed it across the laserjet transparency. After the ink dried, I wiped it off with soft tissue. The Dry Erase fills the tiny holes in the laserjet toner. If you try this, just apply the ink with a full, smooth stroke. Since you are removing excess later, cover the entire circuit image. DO NOT SCRUB the transparency with the marker, or you will remove some of the laser toner.

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great advice in case we want to use laser jet...

some time back i saw a video on youtube (Making PCB: Using Pre-sensitized Board w/o UV Lamp- YouTube), the guy used normal paper with oil to make it translucent for exposing, since paper will absorb the laser jet ink so i think there are chances that on absorption ink will spread on paper and eventually will block all the spaces making it completely black.

Laser jet ink is really solid particles, like a pigment. I would not expect it to spread. Although, hydrocarbon/mineral oil coated paper is certainly an option.

As for ink jet verses modified laser, the results are extremely similar in my experience. One of the variables we haven't discussed is the transparency used for the inkjet printer. Many years ago, I stumbled upon a transparency made for artists by Mitsubishi called Pictorico. It has a very fine, I believe ceramic, coating and is superb in holding the ink and getting great resolution. You cannot actually see the coating. With oblique light, the coated side is just a little duller than the uncoated side. Here is a link: **broken link removed** I have not tried the Diamond Jet product. Interestingly, an older link, pictorico.com goes to the Diamond Jet site. You can see some of the artistic results, which are impressive.

Unfortunately, UV light tends to reduce the perception of sharpness, so a few years ago Pictorico added a UV blocker and sold that as an enhanced version. That would obviously be bad for our purposes. I bought quite a supply of the non-UV blocked material when I learned of the enhanced version. I do not know the current status of Pictorico and UV blocker. That is, whether Pictorico ("Pictorico Premium OHP Transparency film") has begun putting the UV blocker in all its OHP films or only in the enhanced film. Maybe Diamond Jet is the enhanced film?

Bottom line is that I wouldn't give up on using a laserjet transparency. Just try the Dry Erase, if your fine lines look a little moth eaten. If you can get a very high quality inkjet transparency, then I think that would be better. In contrast, I have heard of people using regular, cheap inkjet transparencies and getting pour results because the ink does not stick well.

John

Lots of good info, jpanhalt. I'd like to add a few tips.

1) It might be obvious, but it's always worth repeating. When doing photo-exposed boards, always make sure that the printed layer of your artwork is on the BOTTOM, touching the copper. Otherwise your art is raised off of the board the thickness of the transparency, making for a fuzzier exposed image, since light will bleed under the traces.

2) Heating/agitating your etchant will speed the etch time and can help to minimize undercutting the thin traces. This is something that just takes experimenting with to dial it in.

3a) The heat of the laser printing process can soften/stretch a transparency. With intricate work, it's not a bad idea to print out the artwork on a sheet of paper as well as on the transparency, and then lay the transparency copy over the paper one. That will show you any distortion in the transparency-printed version.

3b) I've never used it, but others have gotten good results by printing the artwork on vellum paper (tracing paper) instead of transparencies, since the paper doesn't distort due to the heat of laser printing.

4) It's been many years since I used the photo method to make a board, but a decade or so ago when I got some new boards I had the same question about how small my traces could be. I created an image in Photoshop simply consisting of a series of lines, starting with one pixel wide, then two, then three, etc, etc. I then printed/exposed/etched that to see what I could get away with. I do remember that I was surprised with how small my traces could be. I'd suggest you do something similar as an experiment so see what can be done with your materials/process.

Good luck!