Sciencemadness Discussion Board

Stencil etching

Nixie - 26-12-2006 at 00:53

Is there any solvent that will dissolve transparencies (as used for overhead projectors) but will not etch areas covered by laser printer toner?

I make my own circuit boards, and I'd like to do reflow soldering, for which plastic stencils are needed to distribute the solder paste over in specific places on the PCB before arranging the components and baking.

Commercially stencils are made with laser cutters. How to DIY?

Tacho - 26-12-2006 at 02:30

Find a thin metal foil, do the artwork with water-insoluble ink, toner transfer or photoresist and etch your stencil either chemically or electrolitically.
This guy did something like it:

Nixie - 26-12-2006 at 02:45

I've tried his method. Unfortunately, thin foil crinkles and/or tears when you are spreading the solder paste, and for toner transfer, it's very hard to remove the paper without ripping the foil (I can't do photoresist as I am not equipped for that, I've always used toner transfer). Using thick foil has the problem that the etchant eats not only from the top, but from the sides of the holes, so that they are larger than the un-inked/un-tonered areas, and this is a problem for very small pitch areas (also uses a lot of etchant).

One thought I had is metallizing acrylic transparencies, then DCM would dissolve clear areas. If only there was a way to selectively metallize only inked (or non-inked) areas.

[Edited on 26-12-2006 by Nixie]

nicktoop - 26-12-2006 at 15:50

This is a really interesting topic for me too as I find it gets ever harder to solder fine pitch ICs to prototype boards even when I use a binocular microscope.

I believe one can buy aluminiumised acrylic sheets. Would sodium hydroxide remove this metallisation and would the toner resist sodium hydroxide? You could then use DCM.

Alternatively, could you stick the fragile thin metal foil to some supporting acrylic sheet and then use DCM to remove acrylic where there is no foil? Even if the acrylic under the foil was undercut the remaining acrylic sheet might still prove effective at supporting the foil in contact with the board.

Nixie - 26-12-2006 at 17:39

Maybe crazy glue the foil onto the acrylic...

I've been using hot air iron with a PCB preheater and it's great for limited board sizes, but at some point it just seems reflow becomes much more reasonable.

nicktoop - 28-12-2006 at 00:58

How about flexible PCB laminate... copper on polyester? This seems to be sold to hobbyists in the UK. Could you use your toner technique on this and then dissolve away the polyester?

Unfortunately, I have very little chemical knowlege. How do you arrange to dissolve the film? Do you stick it down on a surface first or do you cover the non-masked side with a resist? Do you just let it rest in the solvent or do you stir the solvent? Does it take hours?

Twospoons - 28-12-2006 at 13:13

I have made paste stencils with great sucess using toner transfer on 0.005" brass sheet. The brass can be found in most model shops. Toner transfer works much better if you use the proper paper - sure, it costs more but the result is worth the extra cost.

In prepping the artwork for the stencil I usually reduce the pad size by about 10-20%, otherwise too much paste gets applied - this can be done by making square pads into round ones.

Brass can be a little tricky to get toner to stick to. It must be scrubbed with abrasive (oil-free steel wool) and thoroughly de-greased (iso-propanol works). I usually do a double sided transfer (pattern both sides) so the etch process goes twice as fast, with less under-cutting. Ething can be done in persulfate, or ferric chloride - whatever your preference (I like persulphate as its much less messy).

Once the stencil is etched I attach (superglue) a bar of acrylic to one edge for support (could be anything really, I use acrylic because I have lots). For a paste squeegee I use a business card for small boards. The trick to pasting is to do it in ONE pass - the stencil must be firmly held to the board, and a big glob of paste squeegeed from one end to the other in one go.

Reflow I do in a cheap toaster oven, with a thermocouple placed next to the pcb. When the board hits 205C I turn off the oven, open the door slightly, and let everthing cool slowly. Giving the outside of the oven a bash while at 200C + helps settle the parts onto their pads.
These temps apply to standard tin-lead solder. I've not tried with lead free yet.

Nixie - 28-12-2006 at 16:59

Thanks, I'll have to look up that brass sheeting.

Boomer - 3-1-2007 at 07:42

You can buy the brass sheet WITH photoresist. Just print on transparent paper, radiate with UV, special (!) developer bath (negative btw, the black gives the holes), and FeCl3 bath. The resist does not like persulfates unfortunately.

Conrad used to have them in Europe, and I seem to remember RS or Farnell did as well. IIRC they are 0.25 mm, so not too fine a pitch is possible I fear.

Nixie - 3-1-2007 at 14:10

I don't have a UV setup. I've always used direct toner transfer. The best about that is that you can add a silkscreen the same way after etching.

Boomer - 5-1-2007 at 01:37

"I don't have a UV setup"

Yes you do, we all have one. Look into the sky! :P

Nixie - 5-1-2007 at 01:44

I'm in Vancouver. It's rain or at least overcast November through March.