Think & Tinker, Ltd.
P.O. Box 1606, Palmer Lake, CO 80133
Tel: (719) 488-9640, Fax: (866) 453-8473
Sales: Sales@thinktink.com, Support: Support@thinktink.com
|Low-cost UV Exposure Source
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)
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.
- 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
- 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.
- Wire the ballasts, starters, tube sockets, fuse holder,
power switch, and utility cord according to the schematic provided by your vendor.
- 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!!!!
- Centered along the bottom of each semi-cylinder, 3" (7.6 cm) from each end,
drill mounting holes for the #10 machine screws.
- 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.
- 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).
- 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.
- Apply the 1/2" wide double-backed adhesive to both edges of the inner surface of
- Cut the aluminized Mylar reflector into strips 17" (43.2 cm) long by 6.3"
(16 cm) wide.
- 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.
- Conform the Mylar to the surface of the trough and press the second edge into the
opposing adhesive strip.
- Repeat this operation for each cylindrical reflector.
- 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°.
- 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
- The source is now ready for calibration.
On the web since 1994
Sales: 1-(719) 488-9640 Tech Support: 1-(719) 488-9640 Fax: 1-(866) 453-8473
Copyright © 1994 - 2014 Think & Tinker, Ltd.