Etch Resist Pen

[Newz2000]

This weekend I will be making a circuit board using some soic and a tqfp PIC cpu. The TQFP has the finest pitch of the devices I'm working on at 1/32nd inch (0.031 inches). The pins are .015 inches wide.<p>My typical way of doing this is to use an exacto knife to cut out a stencil for the traces on a rigid piece of paper (cardstock). Then I color in the stencil with my etch resist pen (a black sharpy). It just so happens that my etch resist pen says it can make lines as narrow as 1/32 of an inch.<p>Usually when I've soaked the board in acid long enough to remove all of the bridges, the acid has eaten away at the traces a little so that they're not real smooth. This concerns me since this experiment will be the smallest I've ever worked on. I'm afraid that the acid will completely eat through my thin traces before it removes all of the copper between them.<p>Since I'm making a stencil, is there something else I can use that will be a little more resistent to the etchent then the ink from the sharpy?<p>I have no laser printer, so I can't use that cool looking blue paper for making circuits. It seems like all of the coolest tricks require a laser printer

[Engineer1138]

I haven't tried it, but oil-based block printing ink *should* work. What I would do is use a mylar or other plastic stencil film and apply the ink with a soft rubber brayer (roller). You can get better resolution with plastic film than with thin cardboard. Screen printing ink is normally used for art prints in this manner, but it's thicker, so you may not be able to get very fine lines with it. Block printing (relief) ink can make very thin lines.<p>An art supply store will have the plastic film, the ink and the brayer. Total cost is probably about $15 or so. I know ferric chloride washes away with water, so I wouldn't use the water-based ink.

[philba]

I've had variable luck with resist pens. They work ok, sometimes. I think you are reaching the limit of your current technology. Are there transfer decals for tqfps? maybe that's an approach.<p>However, all is not lost. If you have access to an inkjet printer and a copier, you can still do toner transfer (you don't need the pulsar paper at all).<p>I use a copier for toner transfer on regular inkjet paper and it works pretty well. I print my art on the inkjet and then make a copy. There is an advantage to this in that you can crank up the toner density on the copy to get more toner than you would with a laser printer.

[Edd]

Sir Matt:
wow….wow……WOW !
Since I see that you are utilizing the “ stencil method” that I initially used waaaay back in the ‘50’s .How’s about pulling out a little scrap of PCB stock to test etch and try my old method with a small sample with the closest trace width and spacing parameters that you might need to use. It worked in the “contact vinyl” separation process due to two things, the fact that there was a minimal or no coagulation of the flat or ultra flat Krylon formulation when sprayed into the PCB’s bare foil areas as a film .That was along with the proper timing in stripping the vinyl Contact stencil away from the board. That being just after the initial drying point. The resultant resist layer is quite hardy, and otherwise, a quantum leap from any of the resist pens that I have ever tried. In reality I had the best luck with a Japanese Uchida brand opaque black paint pen., which you shake up and initially press on paper to get the ink flowing from the end hollow metal tube nib and then it will flow for you on application, it works better than the “Sharpie” style resists pens and the paint resist coating is very tenacious. Try sampling the stencil mode first , however. And now, old information contained within my last post in …..A Blast from the Past is:
http://206.131.241.58/ubbcgi/ultimatebb ... t=000386&p <p>73's de Edd
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[Newz2000]

Sir Edd:
ow...ow...OW!<p>The instructions you gave were excellent. However, it proved to be too difficult to cut out the traces (1/64th inch thick) without destroying the gaps between them. My brother and I both tried and we estimated our chance of success to be doubtful, so we gave up.<p>I think I'm going to get some of that blue press 'n' peel paper. I'll just have to find someone with a laser printer or photo copier.

[Edd]

Sir Matt:
Sorry that it couldn't work out for you….with your apt description of the structuring of the template that you were making from cardstock ,with acid fibril cleaning to boot, I thought that you were having some degree of success with that technique.
Possibly you are using some older ATmel type of PIC in that TQFP case profile, with possibly 32 pins ?... or Lord forbid no more than that. Oh well, remember that technique in case you are working up into a larger scaling in the future .<p>73's de Edd
[email protected] .........(Interstellar~~~~Warp~~~Speed)
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<p>Think about it…..If you mixed vodka with orange juice and milk of magnesia, would you get a Philip's Screwdriver?

[EPA III]

Art supply stores sell rolls of adhesive tape in various widths. You can buy several and apply them directly to the board after doing the pads. It's a good idea to carefully run over them with a seam roller (as in wallpapering - Home Depot, etc.) to be sure they are no voids under them where the etchant can travel. <p>Some art stores even sell dry transfer (rub on) circles that can be used for the pads. <p>Paul A.

[Newz2000]

Edd - 44pin TQFP. I will use that technique though; spray paint sounds much easier than sharpie and the contact paper is no harder to cut than paper but should give sharper edges.<p>Here's my messy schematic:

<p>The traces are .015 inches wide. There's a 44 pin TQFP pic 16f877a cpu, a crystal, two capacitors, an SMT eeprom and an SMT temp sensor. Two rows of headers for plugging into a breadboard and header along top for programming.<p>Funny thing about the temp sensor... when I got it, I thought it was the smallest chip I'd ever seen. Then I realized I wasn't looking at the chip... I was looking at the package the chip was in!<p>[ September 27, 2005: Message edited by: Matt Nuzum ]</p>

[philba]

matt, if i were in your situation, I'd take a look at having a board house fab the board.<p>spark fun will do them for $5 a sq inch
http://www.sparkfun.com/shop/index.php? ... 232&cat=86&
this is for a double sided, solder mask, silk screened board.<p>It gets better, if your order submission is perfect (no errors in the files you send), they will cut the rate to 2.50 a sq inch for subsequent jobs..<p>your board looks pretty small so it should be fairly cheap.<p>[ September 28, 2005: Message edited by: philba ]</p>

[Engineer1138]

I don't know why this didn't occur to me earlier. The problem is that you have CAD software, but not a laser printer, right? You should be able to print to file (either PS or hp PCL) and then take that file to Kinko's or the like and have them print it on overhead transparency, or something better. Then you can use that film to expose coated PC board (Circuit Specialists sells the stuff cheap) then develop & etch.<p>Much easier than mucking about with an X-Acto knife and getting 12-mil traces is trivial.

[Newz2000]

Thanks for the suggestions Phil and Engineer... Both good ones. My ink jet will print transparencies real nice, so I could do the photo sensitive version my self. I'm first going to draw my circuit in Eagle, which is going to be a challenge because Eagle is about as intuitive a program as Genghis Khan was a friendly neighbor. Once I learn it, I'm sure I'll be glad I went through the effort. (I hope)<p>The x-acto knife trick does work well for through-hole boards using DIPs and etc. The bigger traces are actually very easy to cut out. I'm gonna try the paint/contact paper soon. I've got a 555 circuit that I'm using as my fake engine coil so that I can simulate engine RPMs, and it's just dieing to be on a circuit board (and off my breadboard!).<p>[ September 28, 2005: Message edited by: Matt Nuzum ]</p>

[ezpcb]

Matt,<p>I have some 44pin TQFP PIC boards,

I don't know whether you can make use of them. they are free for you if you want.<p>Mike

[philba]

Eagle is amazingly powerful, especially when you consider the price ($0). what you have to do is forget anything about windows. The copy/paste metaphor is different (and clumsier, imo). the terminology is wierd as well. The Al Williams book Build Your Own Printed Circuit Board is a very good eagle tutorial. I've seen it in B&N.