Tom, thanks for your input.
I have been playing around with the dry film lately. It is definitely
the a lot better than spraying liquid resists and cheaper than pre
coated boards. I can explain some of my experiences. The dry film is
typically sold in 500 ft rolls form the distributors. Price in Australia
is AU$600 for a box of two 150 meter x 300mm (500 ft x 12") rolls. This
is standard package size since laminating is almost always done double
sided. Ok, I realize this is way beyond any hobbyist budget. The way to
obtain small quantities of dry film is to contact your nearest PCB
fabricator and ask them to save 10 or 20 meters or so left on the roll
when its time to replace a new roll on the laminating machine. If they
are not too busy they should be willing to do this for a small charge. I
found the largish PCB shops did not want to be bothered as much as the
smaller shops. If you can obtain a used roll then your in business.
∗ How to apply dry film without using a $500+ dry film laminator
filling half your home workshop.
The dry film is normally applied with hot roll lamination. The hot roll
melts the dry film and applies it to the copper surface by pressure from
the roll. This melts the dry film into any irregularities on the copper
and forces air out of the way by the rolling action. The result should
be a smooth and strong adherent bond without trapped air. This is tricky
to do without the proper equipment. I've heard people using office pouch
hot roll laminators which tend to be expensive items also. The ordinary
house cloths iron was almost impossible to get on neatly and wrinkle
free. The best method I have found to date is to use cold wet
lamination. All that is required is an electric oven capable of 80-90°C
and a rubber ink roller available from art supplies. This is basically a
smooth rubber roller molded to a steel shaft with two bushes at each
ends. The roller is held by a wooded handle perpendicular to the roller.
Ink rollers are perfect because they are very smooth and accurate.
The PCB is first cleaned as normal so it wets with acidic water. You
should etch the copper in etchant for about 2~5% of time it normally
takes to etch copper completely away. This leaves the surface slightly
rough for good adhesion of the resist (at least that's what the experts
say). Rinse the board with tap water well, but do not re-apply soap or
any alkaline solutions since the copper surface cannot be allowed to
have any alkaline residues (at least that's what the experts say).
Ideally you should finish off with a quick rinse of deionized water.
With the board wet lay it down on the bench with the side to laminate
facing up. Carefully remove the polyethylene protective cover off the
dry film and hold the dry film at one edge with your finger tips. Lay
down the bottom end of the film at one edge of the PCB. Using your other
hand grab the rubber roller and carefully roll down the film over the
PCB using much pressure as possible. You only get one shot at it ! The
film should be smooth, flat and have no visible signs of bubbles or
foreign particles. Ok, the film is on.
Next you must heat the board to about 80°C - 90°C for 5 minutes or
longer. This fuses the film into the copper. An electric fan forced
cooking oven is perfect for this.
Expose and develop the dry film as normal.
After developing you should bake the board once again to make sure all
water is driven off from within the film. I found this necessary when
electroplating the PCB as some spots in the dry film would not
completely block plating and would deposit some tin under the resist. I
think for normal etching this will be less critical. The post bake
guarantees the film is adhered and acting as a good resists.
Using this method to apply film seems to give results good as hot roll
laminating but without the expensive equipment.
twb8899 wrote:
>Brian and the group,
>
>I spent over twenty years in the circuit board business and probably
>tried every method that came along. The most difficult part for the
>hobby board producer seems to be good artwork generation and etching.
>With this in mind you might want to spend some of your money on a
>used process camera for making negatives.
>
>Most circuit board films are now made with photoplotters and this has
>put many of these cameras on the used equipment market not to mention
>the ones that were thrown in the dump. If you print your designs with
>a laser or inkjet printer at 2:1 scale and photograph it at 50% on
>the camera a very accurate film negative can be produced. A film made
>this way will not have the print density problems that arise when
>trying to expose through paper artwork from printers and plotters.
>
>Dry film photoresist is one of the best ways to sensitize the copper
>panel. This film is put on the panel with a roll laminator but I have
>had excellent results by gently laying it on the clean copper surface
>and using a clothes iron to heat the resist for proper adhesion to
>the copper. A small glass contact printing frame can be purchased at
>most camera or photographic supply stores. This contact frame will
>give the proper contact between the negative film and the photo
>resist. A sunlamp works good as a UV exposure source.
>
>Developing of the exposed resist can be performed in a small tray of
>1% sodium carbonate (soda ash). I have also used Tide laundry soap as
>a developer but it seems to take a bit longer.
>
>Ferric chloride is a good etchant for hobby boards. It will last for
>a long time and can be used in a tray, bubble, or spray etcher.
>Other etchants are available but many of them have a limited tank
>life. Ferric chloride does not have this problem and is stable for a
>long time, just heat it up and start etching.
>
>A small high speed drill press with carbide bits will do a good job
>for making the holes. KBC Toools sells a small drill press like this
>for less than $150. I have one of these and I like it much better
>than any of the Dremel drills. The Dremel Tool drills have way too
>much spindle runout and their drill press stand is very flimsy.
>
>I only make circuit boards for the instruments that I produce and no
>longer make boards as a merchant shop. When I got out of the
>commercial business I kept just enough equipment to perform the
>processes discussed in this message.
>
>The following equipment is what I'm using for my board production:
>
>CAD system
>Photoplotter
>Vertical camera
>Template (stylus) drills
>Pin router
>Small spray etcher
>Various silk screening equipment
>
>Some of this equipment can be built in the home shop or picked up at
>auctions or on eBay.
>
>Concerning photoplotters, I saw a table top photoplotter just like
>the one I use on eBay. This was about a year ago and it never got one
>bid! This unit could have been picked up for less than $500 and it
>would have been a bargain. This type of plotter makes the whole
>process very easy so it would be a wise move to latch onto one of
>these units.
>
>If anyone is interested, I would be willing to make photo plots for a
>nominal price. The plot data would have to be supplied as a Gerber
>plot file with aperture list or file. The films would be made on
>Kodak 7 Mil film with a negative or positive image. Email me off list
>if there is any interest in this.
>
>Tom
>
>
>