Homebrew_PCBs

I'm new here but find this list very interesting. A good UV lighting
system is the plain old mercury vapor lamps. These are commonly used
for dusk-to-dawn lighting. The bulbs are available in several
wattages such as 175, 400 and 1000 watts. One of these bulbs and a
reflector makes a great UV exposure system. Visit any large warehouse
and check out the lighting system since these are very commonly used
in larger buildings.

I have a commercial exposure system that uses this type of bulb. It
is a Colight Model DMVL-930 with 1000 watt bulbs on the top and
bottom. The exposure time when using 1.5 mil dry film photoresist is
17 seconds. If there is any interest in this I can photograph the
"guts" of this machine next time I clean the reflectors and post the
photographs in the photo section of this list.

It really would not be hard to make this exposure unit. Polished
aluminum sheeting or aluminmum foil could be used to make the
reflector system. I have helped several of my ham buddys set this
type of unit up and they have all been amazed at how fast this can
expose the photo resist. You can also make silk screens with this
setup.

I made my first hobby circuit board in 1966 and owned a commercial
printed circuit business for 23 years. We closed that plant last year
due to foreign competition and rising costs. I did keep the best
"small" equipment for my own home shop. This still allows me to make
boards for the instruments I make as well as my ham radio projects.

We gave away the plating operation because it was really a drag
putting up with the evironmental issues etc. I kept one smaller
photoplotter, several semi-automatic drilling machines, silk
screening equipment and all of the tooling and setup stuff. The big
Excellon CNC drilling and routing equipment was all sold off.

I started my company on the kitchen table in the late 1970's and it
looks like I've gone full circle but this time ended up in the
basement! It's much more fun this way I might add. We made boards up
to ten layers back then but it was just way to much work and the
headaches just kept getting worse. Too much work, too many hours, too
little pay and no more fun !!

I do enjoy the hobby side of this technology and would be willing to
help out with ideas and maybe some equipment projects that could be
posted on this list. If there is any interest let me know. Sorry
about the long post... sometimes I get going and don't know when to
stop!!! Hope I can help out.

Tom

Don't stop! It is good to have you here.

So can you tell us, is my memory faulty or does it only require
near UV to expose PCB photoresist?

I used to use a plant grow light incandescent floodlight bulb,
150W. I don't recall the exposure times but they weren't terribly
long or terribly short. IE, somewhere between 30 seconds and 5
minutes is the closest I can remember, with the bulb about 1 foot
away and the film/PCB held under glass. The film was mylar with a
variety of things on it, drafting tape, those rub-on pads from
Radio Shack, paint, copier toner, and anything else opaque.

What about those inexpensive 150W quartz halogen shoplights? Or are
they built to block too much of the UV?

An idea I had long ago but never tried out- use a bare xenon
flashtube, calibrate by number of flashes. Use one of the larger
tubes rather than the tiny ones in modern cameras. Easy to power it
with AC and to set up a programmable counter circuit to flash it. A
PIC or even just a 555 timer that enables flashing.

It is my understanding (could be wrong) that a fair amount of UV
(low UV, anyway) gets through the tube and that the plastic cover
on the flash serves two purposes, color balance and UV block.

Someone said standard window glass blocks some UV. What types of
glass block less UV? What percentage is blocked? Is it just a
matter of using a thinner sheet of glass to hold the artwork to the
PCB?

Steve Greenfield

--- twb8899 <twb8899@...> wrote:

> I'm new here but find this list very interesting. A good UV
> lighting
> system is the plain old mercury vapor lamps.

-snip lots of good stuff-

> I do enjoy the hobby side of this technology and would be willing
> to
> help out with ideas and maybe some equipment projects that could
> be
> posted on this list. If there is any interest let me know. Sorry
> about the long post... sometimes I get going and don't know when
> to
> stop!!! Hope I can help out.
>
> Tom

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I have agree with Tom on the plain mercury vapor street lamps. they are
cheap, readily available and work great. I use 400W, no reflectors, 400 mm
distance and get 90 seconds exposure on dry film resist. I have heard from
several people about using quartz halogen lamps. Quartz halogen run hot and
put out significant UVA and even some UVC (if not shielded by normal glass),
the UV level depends on the operating temperature. Bulbs that run extra hot
and short lived are better, such as a photographic lamp. You could try
running one of those 12V 80W quartz halogen ceiling lights at say 15V.

Here is some info on dry-film photoresist spectral sensitivity,

http://www.circuitree.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2133,3647,00.html



Standard 'soda' glass (windows) stops pretty much everything shorter than 350
nm. The sensitivity of dry film resists starts dropping off past this anyhow.
You won't loose much from the glass, Even a thick sheet, say 8 mm.
I tested the effect of glass by doing a 10 step exposure on a sample of dry
film. The effect of 4 mm glass sheet was virtually unmeasurable (passes
greater than 80% of light). probably somewhere around 90% - 95%.


Good work Steve on setting up this forum.

Adam.


Steve Greenfield wrote:

> Don't stop! It is good to have you here.
>
> So can you tell us, is my memory faulty or does it only require
> near UV to expose PCB photoresist?
>
> I used to use a plant grow light incandescent floodlight bulb,
> 150W. I don't recall the exposure times but they weren't terribly
> long or terribly short. IE, somewhere between 30 seconds and 5
> minutes is the closest I can remember, with the bulb about 1 foot
> away and the film/PCB held under glass. The film was mylar with a
> variety of things on it, drafting tape, those rub-on pads from
> Radio Shack, paint, copier toner, and anything else opaque.
>
> What about those inexpensive 150W quartz halogen shoplights? Or are
> they built to block too much of the UV?
>
> An idea I had long ago but never tried out- use a bare xenon
> flashtube, calibrate by number of flashes. Use one of the larger
> tubes rather than the tiny ones in modern cameras. Easy to power it
> with AC and to set up a programmable counter circuit to flash it. A
> PIC or even just a 555 timer that enables flashing.
>
> It is my understanding (could be wrong) that a fair amount of UV
> (low UV, anyway) gets through the tube and that the plastic cover
> on the flash serves two purposes, color balance and UV block.
>
> Someone said standard window glass blocks some UV. What types of
> glass block less UV? What percentage is blocked? Is it just a
> matter of using a thinner sheet of glass to hold the artwork to the
> PCB?
>
> Steve Greenfield
>
> --- twb8899 <twb8899@...> wrote:
> > I'm new here but find this list very interesting. A good UV
> > lighting
> > system is the plain old mercury vapor lamps.
> -snip lots of good stuff-
> > I do enjoy the hobby side of this technology and would be willing
> > to
> > help out with ideas and maybe some equipment projects that could
> > be
> > posted on this list. If there is any interest let me know. Sorry
> > about the long post... sometimes I get going and don't know when
> > to
> > stop!!! Hope I can help out.
> >
> > Tom
>
> __________________________________________________
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> Yahoo! Tax Center - online filing with TurboTax
> http://taxes.yahoo.com/
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Hi Tom -

About 25 years ago I made a small number of silk screened PCB's at
home. I would like to start making a few PCB's again with this
process. I have forgotten the little bit I knew then, so I have many
questions.

Someone recently let me cart off some 50 silk screens with PCB
patterns (all unidentified); they are up to 2 decades old. How can I
clean these for reuse? Household bleach and water rinse?

What photosensitizer would you recommend? The simpler for home use,
the better.

Will PCB patterns laser printed on transparency work for making the
screens?

I am interested in a step-by-step "cookbook recipe". I did buy a
book by Kosloff on "Screen Printing Electronic Circuits," but it is
fairly general. I look forward to hearing from you.

Grant

Hi Grant and the group,

I use two types of silkscreening emulsions. They are Ulano CDF-4 and
Autotype Five Star. Both of these are film types. Here are the web
sites:

Ulano www.ulano.com

Autotype www.autotype.com

I like the Five Star film emulsion the best. It exposes in 30 seconds
on my 1500 watt mercury vapor lamp unit. After exposure it requires
development in a weak hydrogen peroxide solution for one minute. A
low pressure warm water wash will remove the excess and leave the
desired image. At this point you lay the wet film on a flat surface
and put the clean pre-wetted blank screen onto the film. The film
will adhere to the screen and the excess water can be blotted with
newsprint paper. A fan can be used to speed up the drying time. When
dry the plastic carrier sheet easly peels off and the screen is ready
for printing. This type of film is called INDIRECT because it is
processed first and then applied to a clean screen.

Another Autotype indirect emusion I have used is called NOVASTAR.
This emulsion is developed using only warm water. The only negative I
have found with this film is the lack of resolution on very fine
detail. For hobby circuits this will probably never be a problem.
On the plus side, you don't have to use any special developers.

The Ulano CDF-4 film is similar but is applied to a wetted screen,
dried, exposed, washed out and dried again. This film is called a
DIRECT emulsion because it is applied first and then processed. Your
exposure system must be able to hand the thickness and size of the
screen frame. The CDF-4 exposure times are much longer than the
indirect types and can take up to five minutes even on high power
units.

Cleaning these emulsions from your screen requires bleach or the
special cleaners from Autotype or Ulano. Be careful when using bleach
with aluminum screen frames. Bleach will attack and discolor the
aluminum. I have also noticed that the bleach method works best if
you apply it to the screen and let it sit for up to an hour on
stubborn emulsions. Scrubbing with a nylon bristle brush will help to
remove every trace of emuslsion. Don't worry about stains left behind
in the fabric. One way to check for complete emulsion removal is to
hold the screen up to a light source and inspect the fabric with a
magnifying glass. The slightest bit of emulsion will show up as
clogged holes in the fabric mesh.

I use a UV cured etch resist that is stripped using a weak, 1%
caustic solution. Common store bought lye will work fine. Screenable
resists that will air dry are also available. I have a UV curing
system and prefer that method because you never have to worry about
the ink drying in the screen and clogging the pattern. There are
spray cleaners available to solve that problem so its not really a
big deal. The only time you will have a problem with drying is when
you stopped printing for more than five minutes or so and don't clean
the screen.

I purchase my inks from TAIYO. They have all types of screening inks
for circuit board production including solder mask and legend inks.

Taiyo America www.taiyo-america.com

Another good source is Lawson Screen Products. They have almost
everything needed to screen print and even offer training seminars.

Lawson Screen Products www.lawsonsp.com

Except for very small quantities, I process all of my boards by
screen printing. I only have a manual press and it is not difficult
to print up to 250 impressions per hour using these methods. You can
also print control panels, boxes and enclosures etc. Screen printing
is a valuable skill.

Tom

Tom - many thanks for the great info! (I already sent a post to
say thanks but it seems to have disappeared into Yahoo cyber la-la
land).

> I use a UV cured etch resist that is stripped using a weak, 1%
> caustic solution.

Is this ink from TAIYO? What is it called? (I couldn't find anything
but solder mask etc inks on the web site).

I found a local supplier for Five Star - they even have it on sale if
I buy six rolls. I told them I was a hobbyist and that would likely be
more than I would ever need!

Grant

Grant,

The UV etch resist ink I use was made by LeaRonal in Freeport, NY but
they are no longer around. They were bought out by another firm but I
can't remember who that was. There are several suppliers who offer
these inks, check out Enthone and MacDermid. If a supplier offers
soldermask, they will usually have the etch resist. I had a gallon of
this ink left over from my commercial shop and even though its date
code had expired it still works good. We thin the ink a bit with a
special solvent for UV inks.

If you are just getting started with screen printing I would suggest
getting the air dry etch resist. This type of resist is inexpensive
and works good. Buy the smallest quantity they offer. These inks will
cover about 3000 square feet per gallon when using a 305 mesh
polyester screen.

If you want to try out the soldermasking process get a single or two
part epoxy soldermask. You can use an old electric kitchen oven to
cure the soldermask. It takes about 30 to 60 minute to cure depending
on the temperature. This really makes your boards look good.

BTW, the screen printing emulsions are negative acting so you will
need positive films emulsion side up.

Tom

Tom, thanks for the excellent information again.

I already have a litre of some black Inktech satin poster ink (and a
gal. of 'wash up'), which someone advised could be used for PCB's. The
wash up smells up the house quite strongly, and for a day or two, and
since I work out of the house with clients, I hesitate to use it
often.

I think I will look for something for PCB's which can be removed with
a lye solution instead.

-- In Homebrew_PCBs@y..., "twb8899" <twb8899@y...> wrote:

> Grant,

[snip]

> If you are just getting started with screen printing I would suggest
> getting the air dry etch resist. This type of resist is inexpensive
> and works good. Buy the smallest quantity they offer. These inks

will

> cover about 3000 square feet per gallon when using a 305 mesh
> polyester screen.

[snip]

> BTW, the screen printing emulsions are negative acting so you will
> need positive films emulsion side up.

My PCB software (Ivex Winboard)will not print my artwork as a
positive, so I will be looking for a way of turning the art into
positives. Kodak used to have a high contrast film which was good for
this, but it doesn't seem to be available anymore. I noticed a product
for this purpose mentioned recently on this list. Any other ideas?

Grant

Grant,

Use a solvent to clean the screen but do it outside! Acetone
solvents work good and seem to make less odor. The lye or caustic
solution is only used to strip ink from the etched board. Remember
that the screen emulsion is water based so you will not be able to
use any water based chemistry to clean the screen. Always wear gloves
as the solvents will really dry out your hands.

BTW, I have a photoplotter that makes negative or positive films if
you need help with your layout. It requires Gerber photoplot data and
aperture list or RS-274X Gerber data with the apertures imbedded in
the plot data.

Tom