Ač to zní komicky, nejsnažší je asi nanést suchý fotorezist (nepájivou masku) mokrou cestou. Tento či velmi podobný postup znají všichni, kdo pracují se samolepícími foliemi. Tento článek je kompletně převzat včetně obrázků od Adama Seychella. Ani jsem ho nepřekládal. Pro první seznámení s touto technologií je dle mého skromného názoru dostatečný.

Introduction

The most common method of applying dry film photoresists are with hot roll laminating machines. A variation to this method, called wet lamination, covers the copper substrate with a film of water while being feed to the hot roll laminator. People in the industry will use wet lamination in order to improve dry film photoresist conformation onto slightly irregular surfaces [Ref. 1]. Part of my hobby includes the use of dry film photoresist to fabricate printed circuit boards. I therefore have a desire to laminate dry film photoresists onto circuit board material (copper cladded fiber glass laminate). In my quest, I discovered wet lamination had other advantages for the hobbyist. First, was to utilize water as a heat source and therefore eliminate the need for a hot roll laminating machine. The second advantage came with water's ability to wash away particles and other surface contaminants prior to lamination. Over the years I had experimented with many wet lamination techniques and as typical of anyone "exploring new territory" I ran into many problems before finally reaching a method that worked to my satisfaction. This web page describes my alternate lamination method to the hot roll laminator.
The diagram below illustrates the concept of the wet lamination method described. The dry film photoresist is applied by a special squeegee to a heated and wet copper substrate. The photoresist film is initially placed on a wetted squeegee board and remains held on by water tension. Photoresist film is positioned such that one edge slightly overhangs the rubber strip of the squeegee. This edge will then make contact on the far end of the copper sheet. A downward force is applied to the squeegee which causes the photoresist and copper to bond directly at the squeegee tip. When the squeege is dragged over the copper sheet, the photoresist slides off the squeegee and transfers to the copper.

Equipment List

The equipment is simple and low cost but requires a special type of squeegee, which I call a "squeegee board". Below is a simplified list of tools and equipment required.
  • squeegee board
  • polystyrene foam block
  • large plastic tray
  • anti-slip mat (large)
  • anti-slip mat (small)
  • tweezers
  • kettle of boiling water
  • 3M Scotch-Brite pad or waterproof abrasive paper grit 400 to 800
  • plastic ruler
  • large scissors
  • water spray bottle
  • latex gloves
  • alkaline degreaser (optional)

Squeegee Board

The squeegee board is a tool use for applying the photoresist. A mechanical drawing of an example squeegee is shown below. This design is based on a 3mm acrylic sheet with a rubber strip mounted flush on one edge. The 3mm acrylic is flexible enough to give some visual feedback of the applied pressure, and also makes it light weight. The 8 x 10mm acrylic strip stiffens the edge to provide a more uniform lamination pressure over the length of the rubber strip. The material can be of any common hard rubber such as neoprene, EPDM, or natural rubber. The edge finish of the rubber should be a straight cut with sharp corners. A small overhang of up to 2 mm provides added flexibility but not too much to reduce the lamination pressure. The dimension Lshould be at about about 100mm greater than the longest sheet of photoresist to be laminated. The dimension W should be about 20mm greater than the widest sheet of photoresist to be laminated.

Procedure

Step 1: Cut photoresist to size

Remove photoresist film off the roll. Cut with scissors or knife. To allow some freedom in positioning the laminated photoresist, the cut sheet should be oversize by at least a 10mm boarder to the actual exposed artwork area.

Step 2: Clean substrate and roughen surface.

Scrub copper surface clean using Scotch-Brite pad or waterproof abrasive paper grit 400 to 800 (wet and dry ) . To help the scrubbing, place copper on a flat ridged surface such as a piece of glass and wet scrub inside a tray or sink. Once the copper is clean, remove and rinse thoroughly under running water.
Notes:
  • Good photoresist adhesion requires clean and oxide free copper surface. To guarantee the surface remains clean you should always wear gloves while handling the copper.
  • After cleaning there should be a continuous film of unbroken water over the copper surface. (see photo of cleaned copper)
  • If the copper is particularly dirty with organic oils and contaminants then add a strong alkaline degreaser to the water during scrubbing. Rinse thoroughly. I may be of benefit to neutralize with a weak acid such as 5% acetic acid (distilled vinegar), citric acid or dip in ammonium persulfate etchant for a few seconds. Rinse once again.

Step 3: Place foam block, copper sheet, and squeege board into tray

Place the plastic tray on a bench with the anti-slip mat. Inside the tray, place foam block, followed by the smaller anti-slip mat and copper substrate. The squeegee board is placed above the copper with rubber strip facing up as shown in the photo. The squeegee should not be touching the copper.
Notes:
  • After placing the copper, its always a good idea to drag the squeegee over the copper surface to mechanically remove any stubborn particles that may not of rinsed away freely.
  • Visually inspect the copper and squeege board surfaces for foreign particles. Remove particles with the spray water bottle.
  • Always use a anti-slip matt or similar soft material between the foam block and substrate when doing double sided lamination. Photoreist on the under side will be soft during lamination due to high temperature. A soft mat protects thebetween the

Step 4: Place photoresist

Wet the squeegee board with water bottle. Place the piece of photoresist film with protective polyethylene backing material facing up. Position the film so one edge is overhanging the rubber strip by a few millimeters. Next, take the plastic rule and scrap the photoresist film from one end to the other to remove large air bubbles trapped between the film and acrylic (see photo).
Notes:
  • Be careful not to mistake the much thinner polyester film on the exposure side with the polyethyleneprotective backing film.
  • The photoresist may not sit completely flat at this stage, and curl up. This is acceptable, provided the bulk of trapped air was removed.
  • Any visible particles on the photoresist should be rinsed off with the spray bottle.

Step 5: Initiate backing film separation

Using pointy tweezers, poke or scrap a small area of the backing film at a far corner. You should see backing film start to separate. Use the tweezers to peel away a few centimeters from this corner. If the film is proving difficult to remove you may be peeling the polyester protection film.

Step 6: Peel backing film

Remove the backing film by pulling the corner with your fingers while holding down the photoresist edge with a plastic ruler or similar object. As more of the backing film is peeled the water tension will help prevent photoresist from lifting off the squeegee board and lay perfectly flat. Discard the backing film.

Step 7: Heat copper

Temporally move the squeegee aside. Using the kettle, slowly pour very hot water ( ? 80^(o)C) over copper for at least 5 seconds. This will assures the copper is properly heated. It also gives the copper one final rinsing.
Notes:
  • It is important that the hot water does not contain dirt or particles.

Step 8: Heat squeegee

Return the squeegee with photoresist facing up and the end resting on the foam block as shown in the photo. Pour more hot water directly over the photoresist and squeegee bring it up to temperature. The plastic squeegee will bend up in the middle with raped heat change. Wait for 10 seconds in order to help equalize the temperatures of copper sheet and squeegee. At this stage the water temperature at copper surface should be between 50 and 70°C.
Notes:
  • If the temperature of the squeegee is cold or is more than about 40°C below the temperature of the copper, then the temperature increase of photoresist film during lamination will cause excessive expansion and result in thermal wrinkling (see troubleshooting).
  • No damaged is caused by pouring boiling water directly over the photoresist film.

Step 9: Prepare for lamination

Take the squeegee board and flip to opposite side. Hold it 45 degrees to the copper and carefully lower the rubber edge (with overhanging photoresist) onto the far side of the copper edge as shown. Press down, and hold for a couple of seconds.

Step 10: Lamination

Using both hands, apply generous force to the squeegee while dragging towards you keeping the angle approximately 45 degrees. The photoresist will slide off the squeegee and transfer to the copper. As a very approximate guide, I have found the drag speed should be 3 to 8 cm/second. Excessive slow drag speeds may cause rippling in the photoresist as it becomes soft with the higher temperature and then flows by the pressure of the rubber strip. Slow speeds also allows temperatures to drop. Very fast drag speeds may trap water or lead to wrinkling.
Click in images below to see lamination results

Step 11: Prepare for exposure

If the opposite side is to be laminated then flip the copper substrate and goto step 3 and repeat. When lamination is complete, trim overhanging photoresist. Dry with paper towel followed by warm air (if available). Let stand for over 5 to 10 minutes until the copper has completely cooled to room temperature.

Troubleshooting

The following table list common symptoms and possible causes resulting from the lamination process.

Wrinkles

Lamination wrinkles can be due to one or a combination of:
  • Cold squeegee temperature relative to copper. A large temperature difference (above approximately 40°C) between the squeegee and copper will cause the photoresist film to expand and wrinkle at the lamination pressure point.
  • Photoresist film not sitting flat on squeegee board prior to lamination. Excessive large trapped bubbles between squeegee board and film may increase risk of wrinkles.
  • Insufficient squeegee pressure. Any wrinkling of the film on the squeegee board is less likely to be flattened during lamination if the squeegee pressure is too low.
Thermal expansion induced wrinkling.

Poor photoresist conformation

Poor photoresist conformation is a subtle problem that can occur if the lamination temperature is too cold (< 30°C ) or insufficient squeegee pressure is applied. The photo below shows the effect of a sudden loss of squeegee pressure. The left side has correct pressure, and the surface topography of the underlying woven fiber glass is visible through the photoresist. The right side has insufficient squeegee pressure and shows less surface features plus being a lighter color due to the increased water content.
Good and poor photoresist conformation example.

Trapped particles

Trapped particles usually occur from insufficient rinsing of the copper substrate and/or squeegee board. The use of a spray bottle helps force particles away.
Example of trapped particle

Poor adhesion

Poor resist adhesion causes areas of exposed photoresist to lift off the copper after developing. Damage of fine traces and artwork features are especially susceptible to adhesion problems. Below is a list of typical causes.
  • Contaminated copper surface is often a cause of poor adhesion. A properly cleaned copper surface should form a continuous break free film of water. Grease and oil contamination drastically reduces adhesion.
  • Low temperatures or insufficient pressure during lamination can cause increased trapped water under photoresist.
  • Inadequate dwell between lamination and developing stages. Laminated dry film photoresists will tend to increase bond strength over time.
Photo showing both a contaminated (left side) and clean (right side) surface. Note the water beading on the contaminated side.

Photo showing damaged resist after developing due to contaminated copper surface as in above photo. Only the left side was contaminated and shows damaged photoresist. The right side of the photo was properly cleaned and shows no damage. The copper is also slightly discolored on the contaminated side.

Photo of damaged photoresist due to cold lamination and/or very short dwell time < 30 seconds) before developing.

Where to Buy

Dry film photoresist is an industrial product and therefore normally only sold in quantities too large for hobbyists. Standard packaging for photoresist is in boxes of 2 rolls, each 500 foot in length. Roll widths range from 12 inches and up. There are a two places I know that sell photoresist. In the USA small quantities cab be purchased from http://www.pcbhobbyist.com. Another mob selling dry film is Think&Tinker, although the quantities are not what I call 'hobbyists' sizes.
If there is sufficient interest I can sell a dry film photoresist by the meter. It will be supplied on rolls suitable for permanent storage of the photoresist and yet quickly accessible for usage. I can also sell squeegee boards similar to one demonstrated in this page.

References

1. Tech Talk: Fine Lines in High Yield (Part LVII): "Wet Lamination of Dry Film Photoresist", http://www.circuitree.com/Articles/Column/ac577d14667d7010VgnVCM100000f932a8c0____