Pinholes
A major part of pinhole problems when using
coated laminate is in the negative and work area. Even on short runs, dust
and muck can accumulate on the negative and create problems. Dirty glass
is another area for potential problems. The clear area in your negative
should be 100% clear to give a 100% result. Some customers have been using
negatives with 30% of the clear track area littered with tiny orange spots.
Check your negatives over a light table & scratch away spots etc with
hobby knife. Pin holing may be overcome to some extent by increasing exposure
time. Too much over exposure can cause bridging by increasing line width.
The longer the exposure the thicker the tracks.
Storage & Handling
Store laminate in as cool a place as possible.
A fridge can be used. Coating will expose itself in heat and with time.
Shelf life is approximately 2-12 months depending on conditions. Wherever
possible use yellow light in work area. Never leave board lying around
with coating up, even in subdued light. Develop the habit of face down
& covered. Board can be handled in fluoro lights and subdued light
for very brief periods of time. Prolonged exposure to fluoro lights, daylight,
sunlight, will develop surface and could render it useless. Establish the
time it takes to destroy a piece, by leaving a half covered piece lying
around for a minute or so in the area where you will be working. That will
give some indication of the handling properties in ordinary subdued light.
Always allow a generous amount around the border as coating may peel away
from where the laminate has been guillotined. Pinholes can be touched up
using alkaline based inks or paints when using acids like ferric chloride.
Test other touch up inks and paints with the acid you are using to ensure
suitability.
Exposure
Blacklight tubes 50mm below glass top gives
good results. Negative must be held tightly against sensitized laminate.
Do not remove plastic film until after exposure. Leave for 15 min before
developing. Guideline exposure time using 4 tubes as above, 15-30secs.
Always pre heat tubes before exposing. UV content deteriorates after much
use, but generally should not be a problem for small volume users. Emulsion
side of negative should be in contact with plastic coating of laminate
to reproduce tracks of same width as negative. The better the contact the
better the results. Ideally a vacuum should be used but a glass frame with
padded top should suffice in most cases. Too much over exposure can cause
bridging by increasing line width. The longer the exposure the thicker
the tracks.
Developing
Mix dry powder in water as per label. Do
not pour used developer back in with new. Store in separate container.
It can be re used until it takes too long to develop or causes tracks and
pads to separate from board. Common sense applies. Remove plastic sheet
before developing. Developer can be used cold, although heating when solution
is saturated will speed up developing. Heat to body temperature only. Overheating
may cause separation. Use sponge to develop. When sponge starts to drag
rather than slip across surface, board may be ready. To make sure use newer
developer to finish off. Like all things there is a learning process at
the beginning. If you are unsure of your ability, cut board into small
pieces, big paper guillotine may be suitable but blade will not last long,
and experiment. Tin snips can cut laminate, but make sure you do not lift
off the plastic top sheet. If you do, then exposure time must be increased
dramatically.
Developing DFP That's Past It's Used
By Date
Unlike the old Red Riston, (no longer available)
today's dry film has a limited shelf life. The longer it is unused the
harder it is to develop the board after exposure. It tends to leave an
almost invisible film on the copper surface that creates havoc in the etch.
The way to establish the age is to look at the colour. The darker the blue,
the old it is and the greater the attention to developing is required.
Use new developer only. It's advisable to scrub the board with a scourer
and a bit of abrasive powder like Ajax. Take care not to scrub too hard
and take the whole dang lot off! Exposure times should be the same as the
newer stock.
Drying
DFP is sensitive to UV. Drying in sunlight
may make removal of coating extremely difficult. Pad dry with newspaper
and place in subdued light until dry, or use heater or fan or hair drier.
Removal
Caustic will remove coating after etching.
Make sure you read safety instructions on can before using. Use gloves
and safety goggles when handling caustic or any other chemical. Establish
caustic strength by starting with weak solution. The weaker your solution
the less chance of accidents to eyes etc. Do not leave solution where children
may spill it on themselves or drink it. Do not use soft drink bottles to
store chemicals etc.
Cutting
Cutting coated board before exposure should
be done with sharp guillotine. Blunt cutting tools may cause lifting of
film near cut edge. This can be overcome by allowing sufficient clearance
from edge of your negative to edge of board.
New
Now no need for expensive positive acting
laminate or expensive negatives. Very good results can be obtained by using
a negative print straight from Protel. Use transparency film/overhead projector
film or laser printer plates or mylar.
If the density is not black enough you can
use double prints and lay them on top of one another or you can use a bit
of mylar drafting film (semi transparent) to diffuse the light. This inhibits
the light going through the non dark areas.
1. Place negative print on DFP, emulsion
side in contact with the photosensitive film on the board. Do not remove
plastic sheet on the surface of the DFP until after exposure.
2. Place light diffusing mylar or
any other semi opaque paper or drafting film over entire job.
3. Place in exposure unit and expose.
If you are using blacklight tubes 50-100mm away from glass then 25seconds
should be ample.
4. If you are having trouble with
boards not etching because you have not got the right exposure yet, then
use Ajax to clean board after it is developed. You can also use a scotch
bright scourer to develop board.
CLICK HERE to see our 'Do It Yourself' PCB supplies. E-mail: acetronics@acetronics.com.au for further information.