Recipe for making polyimide wings

Date: 5th September 2001

Description: Recipe for making light-weight polyimide wings. Each wing weighs about 0.5 mg and has an inertia of 20 mg-mm^2 about the flapping axis. The wing is made by bonding 200 micron dia x 10 miconrs thick poly tubes onto 7 micron poly face-sheet.

Step 1: Preparing the wing pattern using polyimide tubes (takes about 3 minutes / wing)

  1. Use 200 micron poly tubes from HV Tech. (labelled as 0.0077'' ID x 0.0088'' OD poly tubes). Cut them into 4 pieces of the desired lengths using the ruler made for the purpose. (The ruler is just a straight laser cut line on steel shim with markings to mark off the correct lengths). Cut them off to a slightly smaller length than the markers.
  2. Lay down the tubes in the slots of the wing mold. This is a 100 micron steel shim with slots etched in it where the tubes can fit.
  3. Cut out a small square of X0. Press down the sheet onto the tubes (after exposing the adhesive surface). When the X0 is lifted off, the tubes should come off with it held in the correct relative positions.
Step 2: Bonding the tubes to poly film (1:20 hour for each batch)
  1. Prepare a 12.5 micron SS for spin coating.
  2. Spin coat PI 2525 on it with the following settings:
    spin 1: 10 seconds at 1 oclock, spin 2: 50 seconds at 5 oclock
    This gives a 3.5 micron poly film.
  3. Softbake on hot-plate for 2 minutes at 3 o' clock. (note, 2 minutes after the hot-plate has reached a steady temperature at 3 o' clock)
  4. Repeat step 2 on the same side (which gives a 7 micron poly film, the lower 3.5 microns of which is partly cured). If necessary, reattach SS sheet onto a fresh X4 gelpak. Also, handle the sheet carefully after the first spin-coat. Do not press with hand.
  5. DO NOT BAKE! The poly tubes should be bonded onto uncured poly film. Lay down the X0 sheets with the poly tubes onto this poly film. Lightly press down to ensure that the wet poly film sticks to the tubes. A meniscus should be visible around the tubes. Each sheet can hold 2-3 wing patterns if placed well.
  6. Softbake on hot-plate at 3 o' clock for 4 minutes. This creates a weak bond between the tubes and the film makes it possible to remove the X0 sheets without lifting the tubes off the film.
  7. Remove the X0 sheets.
  8. Soft bake in the oven for 30 minutes @ 200 C. Turn on vaccum after removing the X4 gelpak from the SS sheet.
  9. Hard bake on hot-plate at 9 o' clock for 30 minutes.
Step 3: Etching (1/2 hour for each batch)
  1. Cut out wing area + a small border around the poly tubes with scissors.
  2. Place the steel pieces at bottom of an empty glass beaker.
  3. Place the plastic sieve on it and pour fresh FeCl3 on it. The sieve ensures that the wings continue to etch even after most of the steel is removed.
  4. Place beaker in ultrasonic bath. Switch on and leave for approximately 20-25 minutes. Settings: Solution Temp: 35 C. Degas Time: 25 mins.
Step 4: Cleaning (5 minutes for each batch)
  1. Remove wings from FeCl3. Handle carefully from this stage onwards. Dunk lightly in water.
  2. Repeat the etching steps above with isopropyl instead of FeCl3 for about 3 minutes.
  3. Pour out the colored iso-propyl completely and repeat with a fresh batch of iso-propyl for another 2 minutes.
  4. Remove wings from iso-propyl.
  5. Heat in oven or hot-plate for 1 minute to get dry. Note: The wings are very light and easy to lose at this stage
  6. Record the weight of the wings. Each should weigh about 0.4-0.5 mg.
  7. Visually inspect the wings for tears/FeCl3 in tubes etc.