PCB fabrication using spray photo-resist

[Stuart Wallace]

Hi all,


It's been a while since I've posted; I hope that someone can help with a
problem that I'm having. I apologise for the length of this posting;
I've tried to be as complete as possible in my descriptions.

I'm trying to make PCBs at the moment. I'm having a problem using
"POSITIV 20" spray photo-resist.

Here's my process:

1. Brush a piece of single-sided copper-clad board with a Scotchbrite
pad to get a decent shiny surface, then wipe off the accumulated copper
dust.

2. Clean the board with a degreasing substance (Autoglym engine
degreaser) three times.

3. Clean the board with an alcohol-based cleaner three times.

4. Shake the can of photo-resist well, then spray side-to-side across
the board as evenly as possible from a distance of about 20cm. I do this
under reduced light.

5. Place the board in an opaque container, sit the lid on the container
such that light is blocked but fumes can escape, and leave at room temp
(ranges from 15-25 deg C in the UK at the moment) in a pretty dark room
for 24 hours.

6. UV-expose the board. My exposure unit is a gutted flat-bed scanner
containing two 8W UV tubes mounted about 5cm below the glass. The inside
of the scanner case is lined with reflective foil. The artwork is a load
of test patterns that I drew on a transparent OHP sheet (acetate) using
a solvent-based black permanent marker.

7. Develop the board in a solution of sodium metasilicate made up
according to the manufacturer's recommendations.

My problem is that after developing, I either end up with "tracks" so
faint that they can be rubbed off by hand, or the board barely develops
at all. I have varied the UV exposure time from 1 minute to 15 minutes
in 1-minute increments, and I am unable to get anything approaching a
satisfactory result.

Does anyone have any suggestions? Some people seem to swear by spray
photo-resist; others think it's worthless. Also, how much resist should
I apply? So far I've been using the minimum necessary to coat the board.
Should I load more on there?

I'd be grateful for any help with this problem -- it's been driving me mad!


Thanks,

Stuart Wallace.

 

[Anthony C Smith]

-Snip_

The pen you are using may be black to visible light but not to UV

I get good results with HP laser toner on acetates or better using Terston
drafting film
this is definately UV proof
Otherwise try using a uv drafting pen for the job.
makesure the print you make (traces etc) are facing the pcb, otherwise thin
tracks will under etch
because of the thickness of the acetate causing paralax errors.

you could also try reducing the exposure time- testing the exposure time
with something know to be uv proof- such as as piece of card will be
benificial.
regards
Anthony

 

[Stuart Wallace]

The pen you are using may be black to visible light but not to UV

I hadn't actually considered the pen -- thanks for pointing it out,
Anthony. I'm going to try printing some test sheets using an inkjet
printer. I can't believe that I may have made such a stupid mistake...


Regards,

Stuart Wallace

 

[Scott Stephens]

I'm trying to make PCBs at the moment. I'm having a problem using
"POSITIV 20" spray photo-resist.

Here's my process:

1. Brush a piece of single-sided copper-clad board with a Scotchbrite
pad to get a decent shiny surface, then wipe off the accumulated copper
dust.

2. Clean the board with a degreasing substance (Autoglym engine
degreaser) three times.

3. Clean the board with an alcohol-based cleaner three times.

After the scotch-brite, I pre-etch the board with dilute etchant.

My problem is that after developing, I either end up with "tracks" so
faint that they can be rubbed off by hand, or the board barely develops
at all. I have varied the UV exposure time from 1 minute to 15 minutes
in 1-minute increments, and I am unable to get anything approaching a
satisfactory result.

Does anyone have any suggestions? Some people seem to swear by spray
photo-resist; others think it's worthless.

I didn't like the stuff. Couldn't apply a consistant coating thickness.
I had to make sure the developer temperature, etchant temperature and
rinse temps were all pretty much constant.

Then I baked the board in a toaster to harden the resist after
development. Nothing like seeing the traces rinse off your board to give
you a bad day.

I'd be grateful for any help with this problem -- it's been driving me mad!

Try help here: http://groups.yahoo.com/group/Homebrew_PCBs

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

 

[Mike Harrison]

Hi all,

Does anyone have any suggestions? Some people seem to swear by spray
photo-resist; others think it's worthless. Also, how much resist should
I apply? So far I've been using the minimum necessary to coat the board.
Should I load more on there?

I'd be grateful for any help with this problem -- it's been driving me mad!

Don't waste your time with spray photoresist. Just buy the pre-coated stuff.
With the spray it is next to impossible to get a consistent coating thickness and no dust. May be OK
for really low-res boards but useless for serious stuff.
Decent pre-coated stuff can do 40-50 tracks per inch with no defects - spray would be too hit & miss
to do that.

 

[Frithiof Andreas Jensen]

I'm trying to make PCBs at the moment. I'm having a problem using
"POSITIV 20" spray photo-resist.

Two items of advice:

1) By the precoated boards - the spray stuff is rubbish IMO (It's probably
toxic too).

2) Don't bother at all, Ship the CAD files to one of the many prototype
manufacturers f.ex. http://www.pcbpool.com and have them do it. Get the
results in the mail & let them worry about resist coating.

Considering the time & money one wastes faffing around with obtaining
chemicals, scrapping boards, drilling holes and so on, I think it is worth
the money. I haven't bothered with a DIY board for years!

 

[Michael]

I hadn't actually considered the pen -- thanks for pointing it out,
Anthony. I'm going to try printing some test sheets using an inkjet
printer. I can't believe that I may have made such a stupid mistake...

Regards,

Stuart Wallace

Whatever you use to "draw" on your acetate, hold up the result to strong
light and look for light leaking through on "copper" areas. If you can
see light, you're definitely drawing with the wrong stuff.

If all else fails, you might want to inkjet your artwork, contact print
that on high contrast (lithographic) sheet film, develop, contact print
that on your sensitive board. Back in the '70s I used the
litho-film-and-contact-print method exclusively, with *terrific*
results. Yeah ... develop/dry was indeed a royal pain but the PCBs were
dy-no-mite.

 

[Stuart Wallace]

Hi all,


I just wanted to thank you all for your helpful and informative replies
to my original message. I think I'll be able to get my process running
smoothly now!

Top quality consultation free of charge from s.e.d. once again!


Regards,

Stuart Wallace