Document Title

Laboratory Exercises in Photochemistry

1. Printed circuit boards

It is possible to buy uncoated printed circuit board stock and to coat it yourself.
- The uncoated boards are available from most electronics suppliers but the best prices are clearly from firms that deal in electronics excess materials (suggest All Electronics.)
- The only photoresist I've found is in the form of a spray can from GC Electronics. (GC Electronics-- PCS Positive Acting Etch Resist Sensitizer. Few distributors stock this: suggest Mouser Electronics, price was $12.50 in 1997.)
In practice it is much simpler and reasonably inexpensive to buy precoated boards. These notes refer specifically to --- boards
The manufacturer is Kepro and the board are designated as .....
- These are available from most distributors.
- A typical package contains one 3 x 4 board (single sided) for about $xxx
We generally cut these into smaller units (1"x2") for student use
- This reduces cost and allows more students to use the light source at one time
  - The best technique is to use a sheet metal break in the shop. (Shear in the envelope and place into a covered box to minimize exposure to light.)
  - Saws and large sheet metal shears will work, but they are slower and leave rough edges.
  - Ideally the material is kept in the dark or viewed under a weak yellow or red light. There is little problem if room light exposure is kept under 1 minute.
We have a collection of Kodalith negatives that we use for this exercise
- We've tried laser printing or xeroxing on transparent sheets but the contrast is not high enough to be successful.
- some of our images are actually printed circuit board layouts
- some are images of leaves, animals, letters, symbols (The dingbats and Botanical Fonts available with Windows generally make nice patterns.)
- we favor relatively bold details here (under good circumstances the board material will reproduce 0.5mm lines, but in the hands of beginners such details may be lost.)
- This is a negative?? material so you end up with copper only in the clear areas of the negative.
- There is, of course, nothing wrong with an image that is mostly copper with the image being the areas where copper is removed.
We use a commercial UV contact printer to expose the materials.
- this has a bank of UV tubes
- it has a built in timer and we use exposure times of about 4-6 minutes
- remember that the material is really only sensitive to the blue/UV end of the spectrum
- A printing frame can also be used with either sunlight or a Quartz-Tungsten Halide Lamp (We've used a 150-200 watt work light from a hardware store.)
The exposed material is developed in a solution of Sodium Carbonate (supplied with the board.)
- Important-- first peel off the thin upper film (or developer won't reach the resist.)
- Development in room light is satisfactory (avoid direct sun or prolonged exposure)
- Development takes 1-2 minutes; the image can be clearly seen an a dark red/puirple color
- The process is completed by rinsing the board under a faucet in warm (not hot) running water. A healthy flow of water is important. The manufacturer supplies a small sponge to gently swab the face of the board.
Ideally the board should be cured (dried overnight or at 75 C for about 20 minutes)
- we've often made some samples ahead of time to avoid this delay
The boards are normally etched in hot Ferric Chloride with agitation.
- Etching takes 10-30 minutes at xx C
  - Frankly, ferric chloride is pretty messy.
  - The solution is too dark to see the process
  - Spills produce ugly rust stains everywhere
- We generally use Ammonium or Potassium persulfate as the etchant
  - concentration-- temperature-- time--
- Disposal-- the developer is simply a weak base (drain)
- The etchant solution contains copper and should be treated to remove excess oxidizing material and treated as a heavy metal waste disposal solution.
Total time--
- about 5-10 minutes to prepare and expose (can do other tasks during exposure)
- 1-2 minutes to develop
- 10-20 minutes to etch (can do other tasks)

Electroforming Exercise

We have a collection of 1"x2" sheets of stainless steel. (These have been sheared from larger stock and the edges have been treated to avoid sharp surfaces.)
We coat the boards using the GC Positive Acting Resist listed above.
- It is impractical to have students coat their own material.
  - It takes some experience to get a good coating
  - The coating should be done out of daylight (use a bug lamp)
  - The coating should be done with good ventilation (solvent must evaporate); a darkroom is not a good location.
  - The coating should dry overnight before use
  - A spray can is best used to coat a large number of objects at one time.
  - We generally stand the metal pieces at about 45 degrees against a sheet of newspaper and spray an entire row at one time. After a few minutes we apply a second (relatively light) spray. (most of the pray ends up draining off into the paper.)
We provide a collection of Kodalith negatives for student use
- Of course it is important that the negative produce a single interconnected pattern
- remember that metal will be formed in areas that are clear on the negative
- we favor lines that are at least 1 mm wide.
We use the same UV contact printer above with exposure times of 5-6 minutes.
The sheets are developed in a tray of solvent; we use Xylene
development time is about 1.5 minutes (there is no apparent image)
The sheet is then placed in a moderately stream of water from a faucet. (A weak flow will leave resist particles n the corners)
In about a minute the pattern can be seen, mostly by reflected light.
Again, the developed image should be permitted to cure
We supply some previously developed images at this point.
The bottom surface of the sheet needs to be made nonconductive.
We've used spray paint, varnish, wax (obviously you can't coat before developing in Xylene)
In practice, a layer of contact paper from the grocery store does an excellent job.
We then electroplate this piece of metal
We use a Watts Nickel plate Solution (Nickel sulfate, chloride)
We use a Nickel anode
We use a variety of DC supplies-- one particularly efficient approach is to use a single power supply and a series of simple current regulators for each electroplating cell.
- A simple LM7905 ($1) and a 50 ohm/ 1 Watt resistor will provide 100 ma of current
- connect pin 1 to the dc supply (8-25 volts)
- connect pin three to the resistor
- the other end of the resistor goes to both the cell's anode (+ terminal) and to pin 2 of the regulator.
- The negative lead on the power supply is connected to the cathode (object to be plated)
- The recommended plating rate is about 100 ma per square cm
- A plating time of 30-45 minutes gives a reasonably good film; frequent stirring is desirable but not essential.
- On occasion the plating begins to peel from the edge and may begin to form two thicknesses; it makes an undesirable product but we just live with it.
(We have found that computer generated grids, rectangular and circular, make good images. We have also found that leaf many leaf patterns and large letters make good solid electroforms.)

Sandblasting Images into Glass

Hobby stores often carry glass etching kits, with transfer decals that serve as stencils and a creme that serves to etch the glass.There are usually books with patterns (positive or negatives) that are easily photographed or copied on an office copier. Most etching creme is fluoride based and I'm uncomfortable using such material in an introductory lab (even though it introduces a wider range of chemistries.) Some recent ads describe a fluoride free etching solution which might be worth investigating.

We also post copies of the article and manufacturer's literature describing the fabrication of Micro Joule-Thompson Refrigerators. I also try to discuss some of the recent developments in miniaturized GC and HPLC systems; these are generally formed by etching channels into a silicon wafer.

We initially did this using a dichromate - gelatin coating, but found this to be an unreliable process if conducted on an occasional basis. Coating thickness is an extremely important variable, difficult to control.
We now use a commercial product designed for professional artists. Our source is a firm called Reactive Metals Studio which will sell small quantities of the material. (incidentally, this is an excellent source for Titanium and Tantalum samples for an anodizing experiment.)
web site:
The manufacturer is a firm called PhotoBrasive Systems (Web site available). The product is a film called ImagePro Super. It costs about $13 for an 10x12 inch sheet. You also need a bottle of the glue (minimum is 1 quart at about $22.50.) Prices are 1996 catalog, small quantities.
We generally have students use samples that are about 1-2 square inches to keep cost down and to greatly reduce the time required to sandblast the final image.
For this material we have found that overheads prepared on an office copier have adequate contrast to produce good images. We also draw on a stock of Kodalith images.
The glass object is coated with a thin layer of the glue using a small artists paint brush.
When the glue is dry (about 15 minutes), the film is placed over the treated area and carefully burnished to remove any air bubbles. (Be careful to put the film correct side down-- see directions with the film.)
If the glass object is flat, we use a contact printer to expose the film. Our printer requires about a five minute exposure.
If the glass is curved, the negative can be taped tightly over the film and exposed to light. This is more difficult to time and control.
After exposure the negative is removed and the film is placed under a faucet and treated with a steady stream of warm (not hot) water.
Development in the water should continue for about two minutes until the film is fully developed. The masked areas will remain dark red while the exposed areas will become completely clear.
The film is then allowed to dry for about 15 minutes.
We sandblast using a Pasche miniature sand blast gun (it's a modified air brush) using fine (200 grit) Aluminum oxide abrasive. Approximately 60-70 psi is required for efficient etching.
The sand spray is played across the resist mask until the pattern is clearly etched. For small pieces like this, etching takes 2-4 minutes.
The piece is removed, immersed briefly in boiling water to soften the glue, and the mask is removed by a mild scrubbing.
Some important precautions with respect to sandblasting
- The dust generated can be quite harmful if inhaled. We use an enclosed sandblasting cabinet with air filters.
- If the skin is exposed directly to the sandblast, alumina particles and air will be injected under the skin causing serious injury. We require users to wear heavy rubber gloves and we do not permit the work piece to be hand held during the sandblasting operation.
- If the gun must be cleaned or refilled the air supply must be shut off and the pressure released to prevent accidental discharge.

If you want to try the gelatine - dicrhomate system, heres my best recipe.

Heat 100 ml of water to nearly boiling on a hot plate/stirrer; avoid boiling.
- While stirring strongly, slowly add 2 grams of gelatine (we use Knox unflavored from the grocery store.)
- Stir until the gelatin is fully dissolved and allow to cool but not to set.
- Add 0.25 gram of potassium dichromate (This solution will keep for about a week if stored in a dark location; keep it covered to avoid drying out.).
clean the glass samples very well so water runs freely; then dry (an alcohol rinse speeds the process)
- This only works well with glass plates (flat areas)-- microscope slides are a reasonable size, but the materials are cheap enough to make much larger samples. If you cut ordinary sheet glass, be sure to sand the edges and corners to avoid sharp surfaces.
Coat the glass samples in a darkened area, using a red or orange safelight (orange bug lamp is ok); try to avoid fluorescent lights.
Start with cooled gelatine mixture around 30-35 C (still liquid); warm it if it has begun to set.
It is helpful to slightly warm the glass plates (using a hot air gun or hair drier)
Place a few drops on the plate and use the edge of a stirring rod to draw the liquid across the surface. (this can get messy so put a few layers of newspaper or paper towels underneath.)
- The film should be thick enough to be evident, free of bubbles, but you want to avoid heavy coats. (In a heavy coat the bottom layer will not be irradiated and the entire film will be rinsed away later.) YOu can see a light orange color in the dried film, but it is not dark.
- Allow the coated fims to cool and set for at least 2-3 hours; If stored overnight it may be desirable to place them in a plastic bag or large closed container along with a piece of wet towel to avoid serious evaporation and drying.)
These films require about 6-7 minutes of exposure in our UV printer.
The films are developed by rinsing under a heated water stream (should feel hot but not uncomfortable on your fingers); a forceful stream can remove all the material (undesirable) but a weak stream merely softens the gelatine without actually removing it. development takes about two minutes.
- The pattern can generally be seen by reflected light; it is not colored so the pattern is quite faint. (You could probably dye the pattern at this stage.)
The gelatine layer must now dry. This is faster in a warm (not hot) oven; it needs 2 hours at room temperature.
The pattern is now ready to sandblast
The remaining resist can be stripped following a brief immersion in boiling water; it rubs off with a paper towel.
Concerns: Dichromate is a suspected carcinogen; clearly gloves should be used in handling the material, especailly in the messy portion of coating the sheets. The unused resist should be treated as a hazardous material.

Less clear is the proper treatment of the rinse water used in developing the image. The disposal of Cr(VI) is controlled. W e've tended to collect the first 1-2 liters of rinse water and treated it with a reducing agent before putting it into the drain (as Cr(III).) These are very thin films with very low levels of dichromate so there may not be serious restrictions.