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Printed Circuit Imaging |
Overview
As treated in this manual, any method that can be used to deposit a protective (etchant
resistant) pattern onto copperclad prior to final circuit formation, is considered to be
an "imaging technique". Although historically reserved for image formation using
photosensitive polymers, the list has grown to include:
- photoresist imaging:
- exposing circuit patterns into photoresist using an ultra-violet source and photographic artwork
- soldermask imaging:
- exposing pad masters into soldermask using an ultra-violet source and photographic artwork
- liquid-ink direct imaging using an X-Y plotter:
- creating a resist pattern directly onto copperclad using special plotter pens filled with a low-viscosity, chemically resistant ink
- UV light direct imaging of photoresist using an X-Y plotter:
- creating an exposure pattern in photoresist using a fiber optic "light pen" and a high-intensity UV spot source.
- ink-jet direct imaging:
- creating a resist pattern directly onto copperclad using a scanning ink-jet head filled with a low-viscosity, chemically resistant ink
- thermal toner transfer imaging:
- creating a chemically resistant "decal" using a common laser printer and thermally transferring it onto copperclad
- mechanical etching:
- although it does not involve the deposition of any kind of protective layer, mechanical etching "images" a circuit pattern by cutting away unwanted copper to isolate each circuit element
Dry-film photopolymers used in the printed circuit industry (e.g. photoresist and
soldermask) are exposed using ultraviolet (UV) radiation in the range of 300 nm
(nanometers) to 440 nm. In a production environment, exposure equipment routinely provides
as much as 6000 Watts of irradiation from banks of high-pressure mercury arc-lamps. The
primary goal of these systems is to provide intense, collimated UV light at levels
conducive to high board throughput and reliable, repeatable image transfer.
For the home shop, a much smaller system is usually adequate. Since throughput is a
secondary concern, the radiant output can be much lower and still provide acceptable
performance. Two things that cannot be traded off, however, are source collimation and
uniformity of illumination.
Source collimation, or the degree of parallelism with which the
UV light illuminates the artwork/sensitized substrate, ultimately determines the fidelity
of image transfer and the minimum size feature that can be reliably resolved. With most
practical light sources (lasers not withstanding), there is always going to be a tradeoff
between collimation and the total radiant flux reaching the photoresist. At one extreme,
you could place your source at infinity (or pretty far away if infinity is out of the
question) and achieve a very high degree of collimation. The inverse square law , of
course, would reduce the total amount to light actually striking your laminated copperclad
to next to nothing, so exposure times on the order of millennia might be expected. At the
other extreme, you could put the source right up against the board being exposed to
achieve very short exposure times, but, the divergence of the incident light would be so
extreme that resolving fine details would be impossible.
Uniformity of illumination determines the consistency of exposure
from one point to another and affects the trace-width uniformity in the developed image.
From a uniformity point of view, the ideal source would be an isotropic emitter whose
emission area was as large as, or larger than the substrate you are imaging.
Any light source intended for use in PCB production must strike an acceptable balance
between these two factors and the cost of implementation. Needless to say, you can
significantly increase the efficiency of your illuminator by using reflective and/or
refractive optics with intense point sources but the uniformity of illumination in the
exposure plane usually suffers. Large area emitters, while technically feasible, sacrifice
collimation in favor of uniformity (not to mention extreme cost).
There are a number of very good commercial exposure units available priced from US$500
to US$2,500 (not including shipping). If exposure times on the order of 4 to 7 minutes per
side are not objectionable, you can make an
exposure source capable of resolving 0.005" (0.13mm) traces on 0.010"
(0.254mm) centers over a 12" by 18" (305mm x 457mm) exposure plane for under
$200.
Regardless of the source used, the basic process of setting up yo
If you are uncertain of how much time is required to reach full exposure with your
light source, obtain a 21 step Stouffer Sensitivity Guide or "step
tablet" from your local camera shop and follow the instructions included with the
soldermask or photoresist. A Stouffer step tablet consists of a series of 21 steps which
vary in density from totally clear (step 1) to totally opaque (step 21). For the UV
wavelengths used in printed circuit applications, the ratio of effective exposure between
any two consecutive steps is a constant factor of about 1.414 (i.e. square root of 2, or ½"f" stop).
To calibrate your light source:
- Laminate a piece of substrate with the soldermask (or
photoresist) that you intend to use.
- Position the 21 step Stouffer gauge between the soldermask (or photoresist)
and a clear area on your artwork. Be certain that nothing on the artwork
overlaps the step tablet.
- Put the ensemble into a vacuum frame (if available) and draw it down.
- Calculate an initial exposure time by dividing the power output of your UV lamp (in
Watts) by the total area being illuminated (in cm2). Divide the result into the total
energy requirements of the dry-film (in miliJoules/cm2). Multiply the result by 2. The
final number that you come up with will be a pretty good first estimate of the needed
exposure time in seconds.
- Expose your "board".
- Let the board sit for 15 - 30 min., strip off the Mylar cover sheet and develop as
described below.
- When the pattern is completely developed, examine the image of the Stouffer gage. The
highest number step that still shows some material present after developing is your "exposure number". The highest number that shows no removal of film is referred to as the "step held". In the case of DF-8030 Soldermask, the "exposure number" should fall between 9 and 12. For
DF 4615 Photoresist, the
"exposure number" should fall between 7 and 10.
EXAMPLE:
If you get an initial exposure number of 5 (as shown, step held = 4), you should increase
the exposure time five full steps to reach the center of the recommended range (10 for
Vacrel 8020); i.e. multiply by a factor of; 1.414 * 1.414 * 1.414 *1.414 *1.414 =5.66.
In other words, multiply your initial exposure time by 5.66, and run the test again.
- If the initial exposure number is too high, your board is over exposed. Count back to
the center of the desired range and divide your exposure time by the appropriate divisor.
EXAMPLE
If you get an initial number of 13, calculate the proper exposure by dividing your initial
time by:
Exposure = (initial time)/(1.414 *1.414 * 1.414) or Exposure = (initial time) / (2.83)
It may require as many as three test exposures to "zero" the process in but,
once you have determined a working exposure time, it will always be a good starting point,
even when your bulbs age or need replacement. This calibration should be conducted on
every photoimageable material you will be using and the results recorded in your dated
process log (Don't keep one? Now is a good time to start.)
Once you are satisfied with the calibration of your source, you are ready to burn some
serious images. Although it is not absolutely necessary to use a vacuum frame to insure
intimate contact between your artwork and the photopolymer, working without one can limit
the ultimate resolution attainable during the imaging process. In other words, if you
don't have a vacuum exposure frame, get one (or at least build a good approximation). The
following assumes the use of such an appliance.
- Laminate the drilled. plated, and cleaned copperclad with the
photopolymer of your choice.
- Set the copperclad into the vacuum frame.
- Carefully align your artwork, emulsion side down,
with the proper side of the copperclad and tape into place. Before applying the tape, fold
over one end of each piece so that you will have a "tab" to make removal much
easier. The tabbed end should point toward the center of
the film.
NOTE: It is easier to position the artwork and fasten it in place if the
copperclad is a bit bigger than the artwork film.
- Close the cover of the frame and draw down to maximum vacuum. Allow at least one minute
for all of the air to be evacuated from between the artwork and the photopolymer
Mylar cover sheet.
- Expose the dry-film for the amount of time determined during exposure calibration.
- Release the vacuum and allow the frame to bleed up to ambient pressure.
- Open the frame and remove the film/copperclad.
- If you are exposing a two-sided board, leave the first piece of artwork in place, flip
the board over, and repeat steps 2 through 6.
- Carefully remove the artwork (DO NOT REMOVE THE MYLAR COVER SHEETS!) and let the board sit for at least 5 minute in a cool, UV
safe area.
NOTE: A UV safe or UV proof area can be made by
coating all windows and overhead fluorescent lights with amber UV blocking filters. Generally speaking, incandescent
lights have virtually no effect on modern photopolymers made for PCB applications so short
exposures (less than 5 minutes) are allowed. Sure beats working in a darkroom.
- Peel off the Mylar cover sheet(s) and develop the board (see 4615 Processing Specifications for more details).
- Carefully rinse the board and dry the remaining photopolymer in a 100°C oven for 5 minutes. Do not leave the board in the oven too long or you will never get the photopolymer off if you need to.
- Your imaged board is now ready for further processing.
As above, once you are satisfied with the calibration of your source, you are ready to
burn some serious images. Although it is not absolutely necessary to use a vacuum frame to
insure intimate contact between your artwork and the photopolymer, working without one can
limit the ultimate resolution attainable during the imaging process. In other words, if
you don't have a vacuum exposure frame, get one (or at least build a good approximation).
The following assumes the use of such an appliance.
- Laminate the drilled. plated, etched, and cleaned circuit with soldermask.
- Set the board into the vacuum frame.
- Carefully align your artwork, emulsion side down,
with the proper side of the etched circuit and tape into place. Before applying the tape,
fold over one end of each piece so that you will have a "tab" to make removal
much easier. The tabbed end should point toward the
center of the film.
NOTE: It is easier to position the artwork and fasten it in place if the
copperclad is a bit bigger than the artwork film.
- Close the cover of the frame and draw down to maximum vacuum. Allow at least one minute
for all of the air to be evacuated from between the artwork and the photopolymer
Mylar cover sheet. If you illuminate the film with a monochromatic source (use a yellow bug
light with no UV content), you should see "Newton's Rings" start to form as the
artwork is drawn into intimate contact with the dry-film.
- Expose the dry-film for the amount of time determined during exposure calibration.
- Release the vacuum and allow the frame to bleed up to ambient pressure.
- Open the frame and remove the film/copperclad.
- If you are exposing a two-sided board, leave the first piece of artwork in place, flip
the board over, and repeat steps 2 through 6.
- Carefully remove the artwork (DO NOT REMOVE THE MYLAR
COVER SHEETS!) and let the board sit for at least 5 minute in a cool, UV safe area.
NOTE: A UV safe or UV proof area can be made by
coating all windows and overhead fluorescent lights with amber UV blocking filters. Generally speaking, incandescent
lights have virtually no effect on modern photopolymers made for PCB applications so short
exposures (less than 5 minutes) are allowed. Sure beats working in a darkroom.
- Peel off the Mylar cover sheet(s) and develop the board (see 8030 Processing Specifications for more details).
- Carefully rinse the board and dry the remaining photopolymer in a 100°C oven for 5
minutes. Do not leave the board in the oven too long or you will never get the
photopolymer off if you need to.
- Your imaged board is now ready for further processing.
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