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Old 8th May 2014, 02:48 AM   #1
Few is offline Few  United States
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Default Viability test: DIY screen printing for planar magnetics

I thought I'd report on my recent test of the viability of diy screen printing as a technique for creating the pattern of conductors on a planar magnetic diaphragm. I wasn't happy with the results (or work) associated with cutting strips of aluminum so I decided to try etching the pattern instead. High body acrylic paint works great as a resist, and I've found the dried paint can be removed with denatured alcohol and a bit of rubbing. The adhesive I used to attach the aluminum conductors to the mylar substrate seems not to be bothered by denatured alcohol so the next task was to come up with a way to apply the resist (paint) to the aluminum in the desired pattern. For my planar magnetic driver I wanted to make 2 mm wide conductors separated by 0.5 mm so I made a small test pattern to test the resolution of the approach I had in mind.

To do the screen printing I used StencilPro High Resolution pre-coated screen printing sheets and inkjet transparencies, both purchased from Circuit Bridge. The screen printing screens are photosensitive so a conductor pattern can be transferred to them if it is first printed on a transparent substrate.

Here's a quick run-down of the steps I used:
1) Create a conductor pattern as opaque black lines. I drew them in Adobe Illustrator.
2) Print the pattern on a transparency (I purchased transparencies from Circuit Bridge).
3) Lay the transparency on a photosensitive StencilPro sheet, cover it with glass, and expose the sandwich to UV-A light (long wavelength ultraviolet light) for 45 seconds. I used a compact fluorescent black light bulb positioned one foot from the transparency.
4) Develop the screen in water for ten minutes to soften the portions of the photosensitive emulsion that were not illuminated by the UV light.
5) Use water and a soft brush to remove the portions of the emulsion that were not exposed to UV.
6) Fully develop the remaining emulsion by exposing it to UV for at least 10 minutes.
7) The resulting sheet permits resist (paint) to pass through regions that received no UV illumination. The UV-illuminated regions resist the passage of the paint. The screen therefore acts as a stencil to transfer the pattern to a substrate (aluminum foil, in my case).

Results:
The original pattern is shown in the upper left panel of the image below. The long lines are 2 mm wide, the short lines are 1.5 mm wide. There are gaps of 0.5 mm and 1.0 mm to determine the resolution of the technique.

The bottom left panel shows the pink emulsion on the screen printing sheet after it was exposed through the transparency and then rinsed out. The areas that received no UV illumination appear white. The 200 thread per inch screen fabric not coated with pink emulsion allows paint to be squeezed through during the screen printing process.

I used the test sheet to screen print acrylic paint onto 12 micron thick aluminum foil stuck to adhesive tape (used just for testing purposes). The paint I used was white Liquitex Heavy Body acrylic artist's paint. It is thick enough to retain the pattern without smearing or running, and acts as a very effective resist when etching the aluminum foil. The screen printing result is shown in the bottom right panel. The aluminum appears black in the photo because it is reflecting a dark region of the room.

The upper right panel shows the transparent adhesive tape with the resist and aluminum foil that remained after the unprotected aluminum was etched away by a ferric chloride solution. The photo shows the white paint still coating the aluminum traces because I haven't yet removed the paint with denatured alcohol. I've done that step with other 2 mm wide conductors so I know it works reliably.

I want to make a 220 mm tall by 60 mm wide planar magnetic driver prototype. This test suggests this technique will do the trick. It also looks like the screen printing sheets, once exposed and developed, can be reused enough times so that I can make the many diaphragms I'll need for two planar magnetic line arrays.

Few
Attached Images
File Type: jpg ScreenPrint Test Pattern quad.jpg (192.9 KB, 438 views)

Last edited by Few; 8th May 2014 at 02:50 AM.
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Old 8th May 2014, 03:20 AM   #2
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cool P.S.B (printed speaker board) ha.
good luck way cool.
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Old 8th May 2014, 01:20 PM   #3
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Hi Few,

Very intersting, I was thinking about etching PYRALUX® FLEXIBLE CIRCUIT MATERIALS for doing that.
Did you tried with such a product?

How do you attach the aluminium to the mylar substrate?
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Old 8th May 2014, 01:39 PM   #4
Few is offline Few  United States
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Thanks guys. The only substrates I've tried are thin mylar left over from an electrostatic speaker project and heavy transparent 3M packaging tape. I used the latter just to simplify gluing the aluminum in place when testing the screen printing. I want to maximize the efficiency and bandwidth of the drivers I'll be prototyping so I want to use the thinnest substrate (diaphragm) that I can make work. When using mylar I glued the 12 micron thick aluminum foil using 3M Super 77 spray adhesive.

I'd consider Kapton as a diaphragm material, which is used by many of the commercial planar magnetic driver manufacturers because it handles high temperatures, but I can't find it readily available in a very thin form. It also tends to be crinkly. I'd really prefer something that is both limp and able to handle the mechanical and thermal stresses. I've never looked into Pyralux. How thin can you get it?
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Old 8th May 2014, 01:44 PM   #5
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Hi Few,

Have a look at this :DIY Flexible Printed Circuits

Nicolas.

Last edited by nicotin78; 8th May 2014 at 01:58 PM.
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Old 8th May 2014, 03:20 PM   #6
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Great Stuff, Few!!!!

A long while back I had got some samples of Kapton but it I could only get it in 1" and 3" widths.

I thought I had got it from Chemplex but I now don't see it on there list, they have precut pieces but the stuff I got is on a roll.
The stuff I have is 1 mil.

I was also able to get a small roll of some .3mil as well in 3" widths from chemplex.

You can find it by searching up Electron microscopy supplies.

Thin Film Selection Guide - Thin Film Selection Guide - Thin-Film | Chemplex Industries, Inc.

I just found this one for some 6"x100ft tape but it doesn't say what gauge it is, I assume that it is probably 1 mil.
Maybe it is thicker as it does not say.

Order Kapton Tape for 3D Printer Platforms - 6" x 100' at MediaSupply.com

Uline also has some but again they only stock up to 3" widths and 1mil and 2mil thicknesses,

Kapton Tape, Kapton in Stock - ULINE

I once had a line on some .5 mil stuff I think, but I forget where it was at this time and it was not cheap as you know.

For that I was looking to build something bigger like my Apogee Duette's or along those lines, You know what I mean.

This was about the best I could find after many deep search's without having to buy a huge roll from Dupont for $5,000 or more.
I have had my samples since 1994 when I first got in to this hobby.

It is out there, But it is not easy to find someone that will supply a small quanity's.

I got lucky with some samples after I had intrigued the sales person with my explanation of use for the product in the design of ESL's and Planer ribbon speakers.

I had got the Kapton for an attempt at some ESL's suitable in the high temp environment of automobiles, But I haven't got that far yet.

Good luck!!!

jer

P.S here is some 1mil x 4"wide,

http://www.kaptontape.com/1_Mil_Kapton_Tapes.php

One more 8"x 8" square sheet,

http://printrbot.com/shop/kapton-tape-squares/
Attached Images
File Type: jpg Kapton 1.jpg (48.9 KB, 385 views)
File Type: jpg kapton2.jpg (60.8 KB, 375 views)
File Type: jpg Kapton3.jpg (109.0 KB, 372 views)

Last edited by geraldfryjr; 8th May 2014 at 03:32 PM.
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Old 8th May 2014, 03:40 PM   #7
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Mcmaster Carr has 1mil x 6" wide with a 1.5mil sticky backing,

McMaster-Carr

Cheers !!

jer

P.S. this may be where I got the .3mil stuff,

http://www.spexsampleprep.com/equipm...artnumber=3511

Here is some 24"wide stuff but it is only in 2 mil,

http://www.professionalplastics.com/KAPTONFILMPOLYIMIDE

Last edited by geraldfryjr; 8th May 2014 at 03:53 PM.
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Old 8th May 2014, 04:13 PM   #8
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Here is some 25"wide by .5mil, Not cheap !!!!

https://shop.eis-inc.com/sap(bD1lbiZ...nbr=KAPHN.0005

The whole list of stuff,

http://www.eis-inc.com/suppliers/pro...wcase_no=19681

jer

Last edited by geraldfryjr; 8th May 2014 at 04:15 PM.
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Old 9th May 2014, 03:36 PM   #9
Few is offline Few  United States
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Nicolas: I had to smile when following the link you provided because we had a Tektronix Phaser printer where I work but that was 20 years ago. I don't think it's sitting in a closet anywhere, unfortunately. Interesting idea, though. I guess I'll be continuing with the screen printing for now.

Thanks very much, Jer, for all the kapton information. You dug up more than I was able to. It certainly is hard to find exactly what you'd like in the kapton world. The 8 micron thick stuff looks very interesting; too bad it's not a bit wider than 2 7/8".

I found a patent awhile ago that suggests urethane as a better diaphragm material if the goal is a lower resonance frequency: " Examples of suitable film materials include Urethane, Tefzel® and Teflon® polymer thin films." (Here's a link to Bruce Thigpen's patent.) I think the less crinkly nature of the materials would be advantageous as well. Unfortunately, I've not had great luck looking for readily available thin films (0.5 mil/12 micron or less) of polyurethane.
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Old 9th May 2014, 03:45 PM   #10
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Hmmmmm.........

I was not aware that polyurethane's were available as a thin film now, what really surprised me was the Polycarbonate can be had in a very very thin film!!!

Ya, the .3 mil Kapton I have is the 2 7/8" stuff and I tried looking but it seems that this is the only width that is available short of buying one of those 2 ton full width rolls from Dupont.

jer
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