Epoxy Resin PCBs

[George Sephton]

Hi,
Ive been making the move from etching to milling and I was wondering about silkscreening. I understand silkscreening from my art classes and I have found places that sell heat resistant white ink for the legend but where can I get the green epoxy that covers all the tracks except for the pad.
And how would I use it? Just create a template, Im thinking CAM printer for pads, place ontop of silkscreen, paste over epoxy lift up leaving a paste over all tracks except for pads then add the fixer? or put in oven depending on how it works?
Any advice appreciated,
George S.

 

[Andrew Cowan]

In commercial PCBs, I think the green epoxy is a layer on the PCB originally. I don't know how possible it would be to add it afterwords.

A

 

[George Sephton]

How would that leave the pads open and the tracks to be milled, it'd have to be added afterwards or the tracks would be exposed.

 

[Dippy]

If by "green epoxy" you mean the solder-resist thrn that is screen printed on after etch, pth & clean pocesses (sorry Andrew, didn't mean to contradict).
But, sorry, haven't got a clue where you would buy it.

maybe go to your local PCB manufacturer and swap a pot for a few notes?

If you design PCBs and check the output Gerber files, for double sided you will see;
1. Vias
2. Top Copper & pads (which can be separate if you want)
3. Bottom Copper & pads (which can be separate if you want)
4. Top ident. (i.e. the component outlines and numbers and any other text, usually white).
5. Optionally bottom ident
6. Top Solder mask (& optionally bottom solder mask).
7. Paste layer top (& optionally for bottom) - this is the screen print for solder paste SMD.

The Solder mask layer is the screen print for the solder resist - the green epoxy. I prefer blue. Green is so common
It is usally the 'negative' of the pads layer, but the 'pad' diameters are a tad larger. (Some people call it padswell).
Some people include vias in the Solder Mask, some don't.
All good CADs will give options for just about everything.

 

[Andrew Cowan]

Confusion of terms - on the PCB I am looking at, there are two green layers - one on top of the tracks, and one darker green layer below the tracks. I was previously talking about the lower one.

A

 

[Dippy]

You're probably confusing the solder resist with the board material Andrew - if you ever saw boards being manufactured then you would see what I mean.
Depending on manufacturers the laminations are sometimes very visible and sometimes not too visible.

George said about paste over the tracks. I assumed he really meant the solder resist but didn't know terminology.

 

[George Sephton]

Solder resist! That's it. Is that like a thermo-setting thing or a resin, ie a paste with another thing to set it. And then where would I get that or is there a special name I could tell the DT guys at school who know where to buy stuff like that.

EDIT: For example would this work: http://www.rapidonline.com/Tools-Fasteners-Production-Equipment/Soldering-Equipment/Accessories/Peelable-solder-resist/30466 or will that no be strong enough?

 

[Dippy]

No, that stuff is just a temporary rubbery layer. Its a bit like Copydex - the stuff that if you put it on your hands and rub it rolls up into bogies.
Some people call that stuff a peelable mask.
e.g. if you were wave soldering you'd plop that onto areas that you don't want soldered and then wash/brush it off after.

The proper stuff is like this:
http://www.p-m-services.co.uk/solder_resist.htm

I haven't time to Google, that's your job.
I haven't got a glue where to buy it , and I'm sure your teacher will love you using that stuff at school; making smelly messes and trashing screens.
It would be nice if you could make a negative stencil and spray it.
(I hope it doesn't come out of my taxes )

PS. Why don't you dig out Yellow Pages, find a local PCB manufacturer, phone them (yes, speech, not txt pls thnx tb) and ask them what it is, tell them its a school project for science (and that the teacher is a 21year old blonde) and you may get a free sample.