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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

Photoresist Imaging

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

Exposure Calibration

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:
  1. Laminate a piece of substrate with the soldermask (or photoresist) that you intend to use.
  2. 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.
  3. Put the ensemble into a vacuum frame (if available) and draw it down.
  4. 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.
  5. Expose your "board".
  6. Let the board sit for 15 - 30 min., strip off the Mylar cover sheet and develop as described below.
  7. 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.
  8. 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.)

Photoresist Imaging

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.
  1. Laminate the drilled. plated, and cleaned copperclad with the photopolymer of your choice.
  2. Set the copperclad into the vacuum frame.
  3. 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.
  4. 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.
  5. Expose the dry-film for the amount of time determined during exposure calibration.
  6. Release the vacuum and allow the frame to bleed up to ambient pressure.
  7. Open the frame and remove the film/copperclad.
  8. 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.
  9. 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.
  10. Peel off the Mylar cover sheet(s) and develop the board (see 4615 Processing Specifications for more details).
  11. 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.
  12. Your imaged board is now ready for further processing.

Soldermask Imaging

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.
  1. Laminate the drilled. plated, etched, and cleaned circuit with soldermask.
  2. Set the board into the vacuum frame.
  3. 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.
  4. 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.
  5. Expose the dry-film for the amount of time determined during exposure calibration.
  6. Release the vacuum and allow the frame to bleed up to ambient pressure.
  7. Open the frame and remove the film/copperclad.
  8. 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.
  9. 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.
  10. Peel off the Mylar cover sheet(s) and develop the board (see 8030 Processing Specifications for more details).
  11. 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.
  12. Your imaged board is now ready for further processing.


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