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-   -   Etching onto positive photo-resist board (https://www.diyaudio.com/forums/parts/20779-etching-onto-positive-photo-resist-board.html)

PaulHilgeman 26th September 2003 03:17 AM

Etching onto positive photo-resist board
 
I am new to this photo-type. Here is a link to it:

http://www.partsexpress.com/pe/showd..._ID=1170&DID=7

I am curious, do I simply make a transparency sheet on my printer, set it on top of the board, and expose it to sunling to etch off the resist?

Then proceed as normal, by soaking it in the disolver, drill it, then tin it?

Thank you,
Paul Hilgeman

jackinnj 26th September 2003 01:14 PM

You can make your transparency with overhead transparency material from MMM or Avery. I like to use Avery full sheet clear labels -- these you can stick right to the board. They are available at Staples.

You can do your work with the board using a red safelight bulb -- but don't get too close even though the PCB material is "orthochromatic".

Instead of the vagaries of sunlight, you might want to get a couple of the fluourescent bulbs which are sold as replacements for incandescents (Home Depot has them as do most hardware stores.) I use a lightbox with fluorescents -- roughly the same thing -- expose for 8 minutes.

The board is developed (you can again do this with the safelight on) in a dilute solution of sodium hydroxide (lye or drano) or sodium carbonate -- if you use sodium hydroxide in a non chemical or pharma grade form you may have to experiment with the concentration -- its just a few grams per liter. Use latex gloves and have access to plenty of water.

DRILL BEFORE YOU ETCH !!! Don't get carried away with your beautiful board and try to etch right away. The action of drilling will heat the pads and they may lift of the board.

I use both carbide and high speed steel drills -- both from Digikey -- with the HSS drills just throw them away when you are done -- the carbide drills are unforgiving if they are subjected to lateral torque (i.e. bending).

Peter Daniel 26th September 2003 01:44 PM

I prefer to drill after etching. The small, etched dot in a center of a pad, centers the drill on a pad. Also any uder-etching of the holes is avoided. Here's good info of the whole PCB making process: http://www.diyaudio.com/forums/showt...5&pagenumber=2

peranders 26th September 2003 01:44 PM

Quote:

Originally posted by jackinnj
DRILL BEFORE YOU ETCH !!! Don't get carried away with your beautiful board and try to etch right away. The action of drilling will heat the pads and they may lift of the board.
I myself drill after the etching but I'll guess this is a matter of taste? If you do it afterwards you won't risk damaging the photo resist.

jackinnj 26th September 2003 01:50 PM

i'll correct myself then. in my personal experience, if you use the HSS bits drill before etching, if you use carbide bits it doesn't matter.

to get the "holes" which peter mentioned to center the bit you MAY have to specify "prototype" in your PCB software -- elsewise the software just chooses the "tool" and the index for the hole, rather than printing it out on your mylar.

stokessd 26th September 2003 06:17 PM

DO NOT DRILL BEFORE YOU ETCH! Unless you are amazingly careful. If you scratch the fragile polymer resist coating you'll have to touch it up before you etch. It's a total nightmare to drill this way. Etch first and use sharp carbide bits. I etch all my pads with a hole in the middle so the drill bit falls right into the small hole and centers itself (no bit walk). Here's my instructions that I include with all my circuit board designs:


Sheldon


Board Etching Tips
The artwork is printed onto transparency film from a laser printer, print it three times. Cut out two of the prints
with about a quarter inch of clear space around the circuit board image. Then carefully tape these two copies to
the uncut one after carefully aligning the traces of the overlay to the uncut sheetís traces. When finished, there
should be three perfectly stacked copies. This increases the contrast of the final image. When a transparency is
printed with a laser printer, there are usually holes in the black printed parts. And the blacks arenít all that black
when it is held up to the light. Overlaying makes the blacks much more black, and gets rid of the holes. Now
the artwork is ready to use. For double sided boards, the two sheets of artwork can be taped securely together on
three sides after carefully aligning the traces on each side. this forms an envelope which the circuit board gets slid
into. Itís helpful to tape the board in place inside the envelope with a single piece of tape. This will prevent the
board from shifting when it is flipped over to expose the second side.
This method uses GC positive sensitized boards and developer. The FR-4 fiberglass 1 Oz. grade board works
very well (they can be gotten local electronics stores). The board emulsion is sensitive to UV light, A good source
of UV to expose the board is a GE sunlamp. The sunlamp is hung so the bottom of the bulb is about 12Ē above the
board. The exposure time is 9 minutes. With a yellow incandescent bug light-bulb on, pull the protective coating
off the board and carefully align the artwork on top of the board. Then cover the artwork with a piece of glass to
hold the artwork against the board (just like making a contact print in photography). Then turn the sun lamp on
for 9 min. If a sunlamp is unavailable, the sun at noontime (on a clear day) can be used exposing the board for
about 20 minutes.
The exposed board gets dumped into the developer which has been mixed up beforehand. The developer says
to use a 1:9 concentration of developer to water, but a 1:5 mix can be used, which works faster and can yield
slightly better results. However the timing is more tricky, so it is not recommended for the first time. Submerge
the board into the developer (A photography developer tray works very well), and rock the solution back and forth
over the board. The exposed parts with start to dissolve. The emulsion is green and it will wash away exposing the
copper underneath. This is the tricky part. The board must be removed when all the emulsion is off the exposed
areas. If the board is removed too soon, the emulsion wonít be completely dissolved off the exposed areas and it
wonít etch, if the board is in the developer too long all the emulsion dissolves and all that is left is a bare board.
With the 1:9 solution this time window is about a minute, with a 1:5 solution itís about 20 seconds. The board is
removed from the developer and washed off with room temperature water, then scrape at a an exposed area and
see of there is any emulsion left there. if there is, place the board back in the developer for a few seconds. Repeat
this as necessary until the exposed areas clear. With a little practice, itís pretty obvious when itís time to pull the
board out. Do all the developing using the yellow bug light. When the board is done, wash it off and let it dry. Be
careful of the emulsion, itís easily scratched, especially when fresh from the developer.
Next, drop the board into an etching solution. Ferric Chloride is available from the same electronic outlets
where the GC boards and developer are purchased or from Radio Shack. Ferric Chloride is a nasty smelling,
iodine looking, serious staining stuff. Pour out the developer from the tray, wash it out and add the etchant. Then
put the board into the etchant and rock gently back and forth for about a half hour or so, until all the exposed areas
are clear. Then remove the board and wash it clean. The emulsion can then be removed with acetone or alcohol.
Then all the holes need to be drilled in the board. A Dremel moto tool works well for drilling the small holes,
a small drill press would also work.


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