Think & Tinker, Ltd.
P.O. Box 1606, Palmer Lake, CO 80133
Tel: (719) 488-9640, Fax: (866) 453-8473
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Low-cost UV Exposure Source

Introduction

A low-cost, source of steady UV light with the right spectral power distribution can be found in the "high-power" black-light fluorescent tubes from Sylvania. The 32 Watt, 18" tube (P/N F18"T12/350BL/700/PH with 202-BTCP ballast) offers the highest Watt-density (Watts/inch) available in a fluorescent tube lamp (the same lights are used in the ever popular "bug zapper"). One of the perceived disadvantages of using fluorescent tubes to expose high-density PCB artwork is that every point on the cylindrical surface acts as a Lambertian emitter (i.e. emitts a cosine distribution of radiant energy) rendering any thought of collimation completely moot. However, since collimation is purely a matter of geometry (diffraction effects notwithstanding), a very simple collimator fabricated from an "egg crate" commercial lighting grid allows the use of fluorescent tubes even when artwork features are smaller than 0.005" (0.127mm). The downside is that, to achieve adequate collimation, a significant amount of the output power ( almost 50%) has to be discarded. These fluorescent lights can be effectively used in a low-cost cylindrical-reflector-based illuminator that is easily built at home.

Don't have time to find the components and build your own exposure source? Check out the DKS UV1218.

Components you will need:

  • 18" high power black lights (P/N F18"T12/350BL/700/PH
  • ballasts (202-BTCP)
  • starters (FS-12) and sockets
  • bi-pin end sockets
  • hook-up wire
  • on/off switch
  • slow-blow fuse and holder (rating depends on the number of lamps used)
  • utility cord with 3 pronged plug
  • mounting plate (fire-proof)
  • 1/2" square, 1/2" thick black commercial lighting grid
  • 4", thin-wall PVC water pipe
  • aluminized Mylar® reflective film
  • 1/2" wide double-backed tape (3M 467)
  • #10-24 (or metric equivalent) flat head machine screws (2 for each reflector, 1" longer than needed to pass through the mounting plate)
  • #10-24 (or metric equivalent) hex nuts (2 for each reflector)
  • 1/4" (or metric equivalent) flat washers (10 for each reflector)

Construction:

  1. On one side of the mounting board, mount the bi-pin tube sockets such that the central axis of adjacent tubes will be about 4.1" (10.4 cm) apart. The inner faces of each pair of sockets should be 17 - 1/4" apart. Drill holes near each socket to pass the connecting wire through. Be sure to deburr the through holes on both sides of the mounting plate.
  2. On the flip side of the mounting plate, along lines midway between the axes of the bulbs, mount the ballasts and starter sockets. Since the reflectors will be mounted aligned with the bulbs, you must make certain not to locate the starter sockets or ballasts where they will interfere with the reflector mounting screws.
  3. Wire the ballasts, starters, tube sockets, fuse holder, power switch, and utility cord according to the schematic provided by your vendor.
  4. Cut the 4" PVC pipe into 17" (43 cm) lengths, and split each section in half, lengthwise, to form a set of semi-cylindrical troughs.
    Building a cutting jig will make this a much safer operation.
    DO NOT TRY TO MAKE THE LENGTHWISE CUTS FREEHAND!!!!
  5. Centered along the bottom of each semi-cylinder, 3" (7.6 cm) from each end, drill mounting holes for the #10 machine screws.
  6. Along the centerlines of the bi-pin sockets, drill matching holes in the mounting plate. To insure accuracy, use the semi-cylinders as templates to locate each hole.
  7. Using the #10 machine screws and 1/4" flat washers (as spacers between the pipes and the mounting plate), mount the pipe sections such that the inner surface of each semi-cylinder is 1" (2.5 cm) from the central axis of each fluorescent tube. This geometry should provide a strip of fairly uniform illumination 4" wide by 19" long at a distance of about 16" (40.6 cm) from the lamp (the location of our copy board).
  8. Tighten the screws until their heads are pulled flush with the inner surface of the troughs. This will cause the plastic wall of the pipe to slightly extrude into the topmost flat washer, forming a secure, self-centering attachment.
  9. Apply the 1/2" wide double-backed adhesive to both edges of the inner surface of each semi-cylinder.
  10. Cut the aluminized Mylar reflector into strips 17" (43.2 cm) long by 6.3" (16 cm) wide.
  11. To mount the reflective Mylar to the semi-cylinders (mylar side facing the tube lamp), carefully align one edge of the mylar to an edge of a trough and press firmly into the adhesive strip.
  12. Conform the Mylar to the surface of the trough and press the second edge into the opposing adhesive strip.
  13. Repeat this operation for each cylindrical reflector.
  14. Sandwich two sections of lighting grid together and carefully align them so that each opening forms a clear cell 1/2" x 1/2" x 1" deep. In the absence of sophisticated optics, source collimation is most easily accomplished at the cost of some of the source energy. Using this simple 1/2" x 1/2" fluorescent fixture grid, stacked to a total thickness of 1" and spray painted flat black, an effective geometric collimator is made which "extinguishes" all of the light with an incident angle larger than 35°.
  15. If you will be mounting this collimating grid with the square cells parallel to the axes of the tube lamps, position the top of the sandwich 2.63" (6.7 cm) from the edges of the reflectors. This location will insure that Moiré fringes do not impair the uniformity of illumination at the copy board, 13.5" (34.3 cm) from the bottom of the grid.

    Rotating the grid so that the rows of cells form an angle of 30° to 45° with the tube axes will eliminate Moiré fringes altogether and allow to collimating grid and copy board to be positioned wherever is most convenient. However, trying to cut the fragile grid material on an angle will significantly increase the difficulty of fabrication.
  16. The source is now ready for calibration.
A major benefit of this approach is that it can be scaled indefinitely. To make a wider area of illumination, simply add more tube/reflector assemblies. Since the output of adjacent lamps will slightly overlap in the exposure plane, it is a good idea to make the total source area a bit larger than the object being irradiated to avoid the reduced intensity at the edges. In the case of a 12" x 18" (305 mm x 457 mm) substrate, a suitable lamp consists of four such subassemblies mounted side-by-side with no gap between adjacent reflectors. Mount the lamp assembly such that it is about 13.5" above the surface where you will expose your sensitized substrates. It is imperative that you calibrate this source prior to any attempt to make an actual circuit. This is most easily accomplished using a 21 step Stouffer exposure gage and following the directions that come with most dry-film products.


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On the web since 1994

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Copyright © 1994 - 2014 Think & Tinker, Ltd. Updated 2/13/2014 8:36:56 AM