Etching Brass and Photoresist

I’m trying to etch text onto a brass plate using the same technique as described by Neil in Model Engineer magazine back last year. I’m using spray on Electrolube positive photoresist, a U.V. box, and ferric chloride. I find after exposure the developer can’t completely remove the exposed region (without also removing the unexposed region). The image of my text is very distinct after development, but none the less I’m left with a green film that stops the ferric chloride. I don’t think I’m accidently exposing it to U.V. during preparation. However, my photoresist can is about 8 years old – before I rush out and buy a new can does anyone know if this is a symptom of shelf life ?

David

Edited By David Cambridge on 02/06/2015 08:13:11

Hi David,

It sounds more like underexposure of the resist. Try exposing it for longer? Does the developer come ready mixed or do you dilute it yourself? Might not be strong enough. When I last used the photo resist method the developer I used was ordinary sodium hydroxide solution.

John

Hi David

It could be one (or more) of many things. I would say that 8 year old photoresist is probably pushing your luck a bit. But it could also be that the developer is a bit too cool - have you tried it warmed up? Try it about 20 deg. but not 'hot' or it will strip it before you can blink! It could also be too weak a developer or underexposure, as John says. I would try increasing the temp a bit and also gently rubbing the surface of the plate while it is in the developer (usual H&S common sense applies).
What are you using as a mask? If it is allowing light through at all you will, of course, get this result.

It's a black art to be sure !!

Diane

It does sound like an exposure time problem. You could try a test piece with several different exposure times.

Personally, for etching text on a flat plate I prefer a dry film negative resist. The un-exposed parts are removed so the master is simply a positive transparency - so just ordinary text printed reversed on the rough side of transparency film with an ink jet printer. I have been using this one and had the same problem initially due to overexposure. I found by trial and error that 3 seconds exposure to full sunlight here in November was about right.

Russell.

My experience is that this can be also be caused if the developer is too concentrated and therefore too aggressive, but this happens very quickly. I use plain caustic soda, and had this problem until I halved the concentration from what the net suggested.

The other times I have has this are when the resist has been under-exposed.

I calibrated my set-up by putting a resist coated strip about 5cm long of PCB material on the UV-box, with just 1cm exposed and a long bit of card covering the rest. Tape the PCB (or brass) so it can't move. Expose, and after 30 seconds pull the card along by 1cm. After 2 1/2 minutes the whole pieces is being exposed, so leave exposed for another 2 1/2 minutes. You now have five adjacent sections exposed for 2 1/2 to 5 minutes in half-minute steps. Drop it into the developer (agitating it well, and it can help to use a VERY soft small brush to move the dark strands off the piece) and you should rapidly be able to see the strips clear of resist in turn. Make a note of how long the work is in the developer at the point where you get a clear demarcation between two strips. You know know a good, reliable balance of exposure time and time in the developer.

Also, if you put the developed work in the etch all exposed copper or brass should turn pink almost instantly. if they don't, RINSE WELL and back to the developer. My last board was underexposed along one edge (probably greasy fingers - I am sure finger marks stop UV) so I ended up leaving that part of the board in the developer for much longer.

My exposure time is about 4 minutes and developer 2-3 minutes, information which will be of little use to your, as your set up will inevitably be different.

I used to use inkjet film, but now I use 3M laser film in a HP deskjet (because a £5 laser cartridge that does 3,000-6,000 pages is cheaper than three injet carts at four times the price...)

Neil

Thanks all. I’m using some laser printer transparency film from Mega electronics that’s sold for the purpose, and the developer is ‘Universal PCB Developer’ I bought from Maplins some time ago (it’s a dry powder you mix up, so hopefully hasn’t gone off). There’s a few suggestions to try above and before I buy some new photoresist. The exposure time seems like the sensible thing to start with. I’ll give it a go and see what happens …..

David

I find that my ink jet printer on the "photo" setting gives better density than the Laserjet. The transparancy film I use is Xerox brand and has one side roughened for printing on.

The sun here is cheaper than a lightbox and gives a very even exposure

Russell.

You can use tracing paper instead of expensive transparency film since the tracing paper is pretty transparent to UV. Over the years I have tried a few printers for making masks for printed circuit boards with varying success. The best one was an old Epson inkjet which gave very good solid black images. All subsequent printers have not been as good and I have had to resort to printing two images ( one a mirror image ) and aligning them exactly one on top of the other with the printed surfaces touching. With luck the defects in one image are masked by the second layer.

I now use a laser printer and tracing paper, but still have to resort to making two layers to get a decent mask. I think some laser printers are better than others, but you don't know how good a particular machine is until you try it out.

Doug

For PCB's I always print mirror image so the toner is on the side in contact with the photo resist. I print on drafting film, the plastic semi-opaque stuff. This gives sharper edges. Only expose for just as long as you need and no more or you get problems with the mask density and the you end up with etched pit's where you dont' want them. With good quality pre-sensitised board I can happily do 6 to 8 thou tracks. Spray on is a different question. You have to be carefull to get a thin even consistant layer.

Regards Martin

This really is essential. If the image is not in contact with the resist, light will spread between the top surface and the resist, giving unsatisfactory results. You need 'right reading ... emulsion down'. I never use a vacuum.

I have never found any combination of desk top printer (be it inkjet or laser) and commercially available media to give satisfactory results. There is invariably a degree of opacity in papers, even OHP film, that has to be compensated for in the exposure time which then leads to ragged edges to the image. To make nameplates I only ever use lith film produed by an immagesetter (availalbe from an ever decreasing number of traditional printers) high enough resolution and opaque enough to give acceptable results. Nothing else comes close. This is probably not necessary for PCBs but if you want to etch very fine images it's worthwhile trying to find a firm that can supply lith film. They will work from a PDF supplied.

Diane

Bowing to Diane's huge experience in this field, I might add acoupel of thoughts on printers:

I used to have a an inkjet that would, with a following wind, print two successive layers in register about half of the time. If you have one that is good enough to do this, it's worth trying.

Look at the media settings on your printer and try a few different ones - 'transparency' is not always the one that gives the densest black. Also, make absolutely sure that you aren't in ink-save or economy mode!

Finally remember "no one ever got fired for buying HP"

Neil

Hi

I could not get a dense enough print from my laser printer, I ended up using 2 prints carefully superimposed on one another, this gave much better results.

It took me several tries to get an acceptable etch, I found the photoresist thickness, UV light exposure time and image blackness (+ a bit of luck) are all variables affecting a good result. when developing I found it helpful to carefully clean the surface with a soft brush.

Nigel

I'm surprised that some people seem to struggle with laser-printed masters.

This is one of my early inkjet-boards:

And this is a double side board made with the help of a laser printer (the Samsung, I think, which I don't think is as good as the HP). For a double sided board like this (about 100mm square) you don't want dodgy masters. The minimum track width is about 12 thou, so on the generous side, I go smaller if I need to go between pins.

Neil

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Is this progress, or slogan-jacking ?

When I were a lad, it was IBM

MichaelG.

I would have though it was Chamber HB ?

.

Sorry, John ... You've competely lost me there

MichaelG.

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O.K. ... I think I've got it ... The missing s fooled me.

Edited By Michael Gilligan on 04/06/2015 22:17:00

Sounds like under exposure to me. I use ready-resist printed circuit board. Exposed with mylar print (laser/mirrored) face down for 2.5 minutes in a 36watt 4 tube uv nail-gel lamp box. Tubes are about 100mm from board. Board/print sandwiched in 2.5mm glass. Developed in sodium metasilicate (30g in 1 litre water). Developer needs to be slightly warm or it is very slow, too warm and it strips the resist. I leave bottle on radiator for an hour before use. I tried caustic soda but found it too critical in strength/temperature. This works for me on fairly fine tracks. Experiment is my advice, maybe your resist is a bit thicker than the commercially applied stuff.

> slogan-jacking ?

I think it's been 'jacked' by several other companies.

> I would have though it was Chamber HB ?

No, Fullers HSB

.

I'm sure no-one ever got fired for buying that [unless at lunchtime]

MichaelG.

Regarding suitability of laser printers for masters. PCB's typically have 35 micron copper foil which is no great depth to etch. If you are doing nameplates (which I have never done) I suspect you want a deeper etch. If there are thin spots on the master the resist 'gives up' in places after a long etch time. When the etchant get too old it needs a much longer etch time I get pits in the tracks where the etchant has punched through the resist. If I stick to fresher etch and a faster etch time there are no problems with pitting. The other issue with laser print is the the larger areas are less dense in the centers of the blocks than the edges (due to charge distribution on the drum so you get more toner at the edges and a higher density). I doubt if inkjets suffer in the same way as the don't work like that so they may be better suited to large blocks. I use 1200 dpi and print draft quality on a HP510.

regards Martin