TECHNICAL DATA / PRINTED CIRCUIT BOARD FILM

Customer Guide to Plotting
and Processing PCB Films
September 2005 • TI-2642
You have chosen to use Kodak film for your photoplotting
needs. We believe that you have selected the best quality
film available for your production. In this publication wewant to help you get the very best results from our product.
We want to show you how the right film, correct exposure,good processing, together with sensible maintenance willproduce consistent high quality results in terms of line widthand line edge.

How Film Works
Let us begin by briefly examining how film works. Whenyou expose your film in the plotter, the light energy from thelaser interacts with the silver halide crystals suspended in thecoated gelatin layers. Invisible changes occur to exposed
silver halide crystals with the formation of the latent (hidden)
image. Development, a chemical process, converts exposed
silver halide grains into metallic silver: the latent image
catalyses the reaction and the image becomes visible. The
next step in the chemical process is fixation. The fixer stops
development and removes the unexposed silver halide grains
in non-image areas. Left within the film, these would
eventually react with ambient light, turn into silver grains
and make the film brown.

Unexposedfilm

Latentimage

Following development and fixation are the important
steps of washing to remove residual chemicals, and drying.
In a subsequent section more information will be provided
on optimizing the processing of your films.

©Eastman Kodak Company, 2005

Precise Plotting
The important goal of this stage is to expose your film
correctly so that with optimal processing, you obtain highquality phototools that give the correct line widths whenused for imaging onto photoresist.
Before adjusting anything on the plotter, make sure thatthe processor is set up correctly for the film you are using.
Look at the film technical data sheet for the required
processing conditions. Dilute and mix the chemicals as
directed, and set the time, temperature, and replenishment
rates to recommended values.
Now you are ready to carry out an "exposure series."
Some plotters enable you to do this very quickly, as you canrun different exposure values on the one plot. Often you
have already been running a film, so you can choose a
relatively narrow range of exposure values, say +/-30% of
the setting that you have been using. If you are changing
plotter resolution or starting with a new film in a new plotter,a broader range of values will be needed for the firstexperiments.
Most plotter manufacturers also supply a test file withdifferent line widths and patterns. There should also be apatch of maximum density so that you can check its value.
Do the exposure with this file if possible, and process thefilm. Check the D-max with a densitometer. The value
should be between 4.5 and 5.0. If you do not have adensitometer carry out the following procedure. Examine
the pattern first with the naked eye and then using a 50x or
preferably 100x lens. With the unaided eye you should be
able to tell almost immediately if the initial selection ofexposure values is in the right range. If the film isoverexposed, it should be obvious on equal line and gap
patterns (and particularly on narrower line widths below
30microns), that there has been "fill-in"; the lines havegrown to fill the gaps. In the opposite case of underexposure,the smaller width line and gap patterns may not appear at all.

Evaluation and Process Control

Development

One note of warning on D-max measurements: do not thinkthat measuring maximum density alone is sufficient toestablish the correct exposure value. In an ascending seriesof exposure values, you will almost certainly attain a suitablemaximum density value before the correct line width (seegraph: density=red, line width=black).

Development occurs most efficiently at a high pH (alkaline,around 10.5 in KODAK ACCUMAX Rapid Access
Developer and Replenisher). During development, chemical
by-products are produced which tend to reduce the pH, so thedeveloper contains buffering chemicals that help maintainthis high value.
In today’s films there are other chemicals that enhance thedevelopment process to give very high contrast images withsmooth line edge quality. Some manufacturers incorporate
these chemicals in the film, while others divide thembetween the film and the developing solution. This is why
films may not develop optimally in another manufacturer’schemicals.
It may be helpful to think about development in thecontext of chemical reactions. These reactions proceed
most efficiently when the concentrations of the various
components are optimal and other conditions, such astemperature and pH, are correct. We explained earlier that
Kodak’s film design and chemicals have been optimized
with each other to give the best quality results; this is thebasis for our processing recommendations.
Within the context of seeing development as a chemicalprocess, replenishment is particularly important, as it servesto maintain developer concentration, remove unwanted
by-products, keep the pH stable, and a number of otherfunctions.
Some manufacturers distinguish between replenishment
for developer use and oxidation, and call replenishment for
the latter "anti-ox." But the same solution is being added, sowhen you are comparing different manufacturers’
replenishment rates, remember to add both together and thatwill give the total budget for developer solution.

Line width

Extra
exposureneeded to
get correctline width

Once you have established that you are in roughly theright range, look at the patterns with the lens and measure theline widths if you can. Choose the exposure value where theline widths are closest to their intended values. If necessary,
run another test with a narrower range of exposure values.
On some plotters, if you change the resolution you will
also need to determine a new exposure value. As a rule ofthumb, a higher resolution than the current one will require alower exposure intensity, and vice versa for a lower
resolution.
Once you are running production films, you will want tomonitor your film exposure process. Many customers do adaily measurement of line width and plot charts. These canbe used as a statistical process control, once control limits
have been established. Remember to react only to trends indata and not minor excursions.

Perfect Processing
As explained above in "How Film Works," processing is aseries of chemical reactions that reveal the latent image andstabilize it so that phototools can be safely archived. In
Kodak’s laboratories, film design is integrated with theprocessing chemicals to give optimal image quality.
Although reasonable quality images may be obtained inother manufacturers’ chemicals, the best results can only beguaranteed using Kodak film with Kodak chemicals.
Let’s look at processing in a little more detail. The latentimage produced during the plotter exposure needs to beconverted into visible silver grains. The chemistry ofdevelopment is complex, but the important features for ourunderstanding are that the developer contains a developing
agent (usually hydroquinone) together with other chemicals
that restrain development. These latter are necessary to
prevent "fog" formation, developed silver unrelated to lightexposure.

2Customer Guide to Plotting and Processing PCB Films • TI-2642

Fixation

Washing and Drying the Film

Fixation is another chemical process and is necessary to stop
the development reactions and obtain a stable image. Incontrast to the developer, the fixer is quite acidic (pHbetween 5.1 and 5.5), which stops further development.
Fixer also contains thiosulphate, which dissolves theunexposed silver halide out of the film. As the fixer is acid,it is important not to let it contaminate the developer.
Fixation also helps remove some dyes from the film.
Just like during development, fixer is consumed during
the process, and materials washed out of the film build up in
the fixer tank. Fixer replenishment helps to counteract theseprocesses.
One of the factors that reduce the efficiency of the fixer isthe build up of silver that forms complexes with thethiosulphate. At high levels of silver these complexes
become unstable, and silver drops out of solution, forming a
black sludge in the bottom of the fixer tank. If you areprocessing films with a low area of exposure, this problem
could be particularly severe. It is also possible that silver/thiosulphate complexes could be carried over to the wash
where they may break down, depositing silver in the washtank with the possibility that silver particles may stick to thefilm. Such problems may be largely avoided by the use of asilver recovery process. This removes the silver from thefixer, preventing the formation of the unstable complexes.
Over a few years such a unit will also save the customer
money.

Following adequate fixation, it is important to wash the film
to remove residual processing chemicals and by-products.
Most customers use ambient temperature water. Heatingthe water improves the efficiency of the wash, but also
encourages microbial growth. The latter is the major
problem associated with the wash phase. But careful
processor maintenance and judicious use of biocides will
reduce the problem.
The water replenishment rate should be at least 1 litre perminute.
The chemical and wash phases are complete and all thatremains is to dry the film. At this stage the most importantfactors are adequate drying of the film while maintaining
good dimensional stability. Kodak PCB films have different
gelatin content, and it is important to match the drying
temperature to the particular film. In principle, single-sided
films require a lower dryer temperature than double-sided
ones. Drying efficiency and thus the set temperature will
also be dependent on the ambient humidity. The
configuration of the dryer and size of film being used will
also influence the efficiency of drying. Ideally, drying
should be slow at a moderate temperature. One advantage ofthe 45 second development time is that it allows ample
drying time at moderate temperatures, lessening the risk ofchanging the film’s size.

3Customer Guide to Plotting and Processing PCB Films • TI-2642

Processors

Five Keys to Successful Processing

Most processors will give good results, but the features ofsome machines lead to more efficient processing.
Remember that processing comprises of chemical
reactions between the film and the developing and fixing
solutions. In a deep, narrow tank, the volume to surface areaof the tank is greater than in a shallow tank. This means that
there is relatively less oxidation of the developer at thesurface and the developer tends to be more stable.
Another feature which will have a small influence is thenumber of "roller strikes"; the more there are, the more likely
you are to obtain even development across large sheets.
Additional features that are desirable in a good processor
are filter units to keep the various solutions clean,
recirculation pumps to aid mixing of replenishment withexisting solutions, and level sensors.
There are other considerations when choosing a processor.
How easy is it to clean? Are the racks easy to remove andthe rollers accessible for wiping off? What are the control
features for turning the machine off at night or over theweekend?

1. Correct chemical dilution. You do not need to be
extremely accurate about dilution – use the containers
themselves and remember to keep count! If you areusing a mixer, make sure the values are input correctly.
Mix well to obtain an homogenous solution.
2. Set the replenishment rates correctly. For
ACCUMAX Rapid Access Developer and Replenisher
this is 350 mL/m2 and 540 mL/m2 for the fixer. Youmight think that you are saving money using lower
rates, but the recommended values will ensure goodand reproducible image quality over the lifetime of adeveloper batch. If you are processing less than 20 to30 films a day (24” x 24”), you will need to add extra
replenishment so that the total amount of developer
exchanged in a week is one tank turnover (25 L).
Adjusting the level of anti-ox does this.
3. Set the development conditions correctly. For mostof our films development time should be 45 seconds at
a temperature of 35°C (95°F). Slight deviations fromthese values will not affect the quality of processing,
but lowering the temperature to, say 30°C (86°F), will
seriously affect density and probably line width.
Severely shortening the development time will also
compromise image quality.
4. Set the fixing and drying conditions correctly. The
fixing temperature should be between 32 and 35°C (90and 95°F). Too low, and fixation may be inefficient;
too high and losses due to evaporation will beexcessive.
5. Maintain the processor properly. Racks should becleaned once a week. Do not use abrasive materials,
which might scratch the rollers. Filters should bechanged when the chemicals are changed, or sooner if
it is thought they are blocked. Some users may wish touse systems cleaner once a month or when thechemicals are changed.

4Customer Guide to Plotting and Processing PCB Films • TI-2642

Your Questions Answered
Q. Why does the developer go brown over time? Is it harming the process?
A. The most important component in the developer is the hydroquinone. This compound interacts with the oxygen in the air toproduce a brown compound. Although it is unsightly, this does not affect the development process or stain the film. Sulphite in thedeveloper helps prevent this oxidation.
Q. How important are time and temperature settings in development?
A. If you follow the manufacturer’s recommendations, your processing quality should be consistent. If you suspect some fault
with the processor, check the temperature of the different tanks and the time of film passage. Slight variations (5% or less) will notaffect the quality of processing.
Q. What should I do if the film is not developing properly?
A. Check the dilution of the developer. Check that you are using the correct time and temperature settings. Make sure that thedeveloper has not somehow been contaminated with fixer. Check that the replenishment and recirculation pumps are working.
Q. How often should I change the processing chemicals?
A. About every six weeks, depending on the use of the recommended replenishment rates. A low replenishment rate will either
cause exhaustion or over-activity of the developer depending on the amount of film being used, but in both cases will lead to earlier
exchange of the chemicals.
Q. Do I need to check the pH of the developer and fixer concentrates?
A. No, this is not necessary. This is rigorously checked at the factory using instruments that are maintained and calibrated
according to international standards. Local measurement of pH in the concentrated solutions is very difficult and prone to serious
error.
Q. Sometimes the film comes out with a blue or pink tint. What does this mean?
A. The films contain various dyes: sensitizing, anti-halation and safelight protection dyes. Normally these are decolorized and/or washed out in the developer and fixer. If the processed film has a colored tint it is most likely that there is something wrong withdevelopment. There are many possible reasons: pH too low, wrong dilution, exhausted, temperature too low, poor agitation, one orother of the replenishment and recirculation pumps faulty.
Q. My films have a milky appearance.
A. Most likely, fixation is not working properly. Check the dilution of the fixer; make sure that replenishment is at recommendedlevels; check function of the pumps; when was fixer last changed?
Q. How can I prevent drying marks on the back of my films?
A. Check the alignment and condition of the squeegee rollers just prior to the dryer. If water is being recycled you might consider
adding some detergent to improve film wetting. If the water is "hard," consider installing a water-softening unit.
Q. How can I stop bio-contamination of the water tank?
A. Every user occasionally encounters this problem. Good maintenance can help limit the occurrence and inconvenience of these
episodes. One of the most effective procedures is to thoroughly clean the wash tank using a small amount of bleach, then rinse
thoroughly and allow complete drying before refilling. If possible, fit filters to the water supply. Overnight and/or the weekend addsome biocide to prevent growth in the standing water.
Q. How do I know that my process is in control?
A. If you have chosen the correct exposure value on the plotter to write your chosen line width, and the processor is operating at
the recommended settings for time, temperature and replenishment, then processing quality should be maintained over the life of thedeveloper (about five to six weeks).

5Customer Guide to Plotting and Processing PCB Films • TI-2642

Customer Guide to Plotting and Processing PCB Films

NOTICE: The sensitometric curves and data in this publication represent product tested under the conditionsof exposure and processing specified. They are representative of production coatings, and therefore do not apply
directly to a particular box or roll of photographic material. They do not represent standards or
specifications that must be met by Eastman Kodak Company. The company reserves the right to change
and improve product characteristics at any time.
Aerial and Industrial Markets

EASTMAN KODAK COMPANY • ROCHESTER, NY 14650

Customer Guide to Plotting andProcessing PCB Films
KODAK Publication No. TI-2642

Kodak and Accumax are trademarks.

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