Optimizing process controls for the next generation diazo phototool films.
A few years ago, the market offered a variety of diazo emulsion
designs. The diazo films then were commonly referred to as standard and
double-sided emulsions. Diazo film was designed for silver film
duplications, a matte emulsion version and clear emulsion, and the
first generation of scratch and chemical resistant emulsions.
Today,
the needs and requirements of end users have changed as the U.S. and
European PCB production models have moved from a blend of prototype,
short production runs and "long runners" to primarily prototype jobs.
China's demand for diazo films for long runners is growing and it's
used in both primary imaging and LPISM applications.
This
transition has pushed diazo manufacturers toward refining their product
offerings to a single design that features an improved blend of a
chemical and scratch resistant emulsion. Diazo is the increasingly
popular choice as the working photo mask (phototool) for liquid photo
imageable solder inks allowing for easy registration to the panel.
Also,
diazo has a home in both the outer and inner layer imaging areas
because of its outstanding ability to reduce both artwork and board
touchup/rework/scrap associated with handling scratches during the
production process. The new and improved chemical resistance features
of the diazo film allow for easy cleaning and removal of soldermask
inks from the film.
This same ability to resist
chemical degradation also prevents damage to the actual image on the
film. Silver film gelatin emulsions are easily attacked and permanently
damaged by many of the liquid soldermask inks on the market. Finally,
diazo films' scratch resistance provides superior protection when
compared head to head with silver phototool film on the production
floor.
Superior resolution properties are the result of
reducing the matte particulates while still allowing for air evacuation
during the contact exposure stage, both from the silver master to the
working diazo phototool and during the imaging step for on the
production floor. The medium-orange tinted emulsion was specifically
designed to fall between the darker style emulsion diazo films and the
lighter style diazo emulsion films for ease of registration to the
panel.
The actual color of the film has no
relationship to its potency at blocking UV light. Manufacturer D-Max
release specs are in excess of 4.5 D-Max - more than adequate for any
and all imaging applications - from dry film photo resists to liquid
photo imagable soldermask inks. Actually, the mid-range color is a
significant benefit on the production floor and assures easy visual
registration verification on the panel.
On the
processing side, it is pretty much business as usual. You expose,
develop and touch up in a similar manner as with past diazo films.
However, you should take these steps with care. Only then can you
achieve optimum results.
Let's review the entire
process. In the artwork generation/photo lab, inspect the diazo or
silver master for any damage and touch up any pin holes or scratches.
Verify image acuity/sharpness and check for visible damage. Re-plot
your silver master if you encounter any fuzzy/blurred lines, creases or
dents on the film.
Next, clean the master with the
appropriate film cleaner. Clean the exposure unit to reduce any
imaged-on defects caused by dust, dirt, lint or other influences.
Center the films in the contact vacuum frame emulsion to emulsion and
start the vacuum drawdown. I recommend increasing the drawdown time to
at least 60 seconds, preferably 120 seconds.
With
less matte in the emulsion, air evacuation during this drawdown stage
requires a little additional time. Check for Newton rings over the
entire film surface - this ensures a good contact and produces a
resulting image that has crisp sharp line edges on the diazo duplicate.
If you are using a Stouffer 21 step wedge to verify the correct
exposure time you want to see a clear step 1 and 2 with a visual step
3.
Once in the development stage, make sure the
developer has had the proper warm up time of approximately 30 minutes
and then verify the developer "film plane" temperature (what the film
actually sees going through the developer). You can verify this using
temperature tapes; disregard the thermometer on the side. Often the
thermometer on the developer only indicates the tank temperature and
not the film plane. You want a minimum of 155°F and a maximum of 165°F.
Verify that the metering pump is delivering 15-17
cc/ml in 10 minutes. I recommend using 28-30 "baume A" ammonium
hydroxide, not technical or diazo grade. Now pass the film through the
diazo developer. With the improvement to the diazo chemical resistance
feature, I recommend a minimum of four passes through the developer to
ensure complete development. Remember that unlike a silver film, diazo
cannot be overdeveloped. So if you have any doubt whether the film is
completely developed, just pass it through the developer a few more
times. If you are using a densitometer to verify D-Min and D-Max,
always first perform a "burn back" on the diazo by placing the film
base toward the exposure lamp and exposing at two times the original
exposure time setting before taking the density readings.
Next,
visually inspect the film for any image defects and touch up
accordingly. Clean the film and place it in a protective sleeve. You
are now ready to send the film to the production department. PCD&M
Jeff Jarvis is global business manager for InteliCoat Technologies. He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..
