Roll Lamination of Dry-film Photopolymers - Photoresist and Soldermask
Photosensitive polymer films (photopolymers) are very similar to roll form adhesives in that they tend to be quite sticky and can be frustrating to load and use properly. As with the adhesives there are a number of simple techniques that will minimize the difficulty in learning to use photoresists and soldermasks. The following procedure assumes that the photopolymer of choice (photoresist) is ready to be loaded into your laminator. that the unit is up to operating temperature, and that you will be laminating one side of the copperclad at a time.
Before using your laminator, review the safety notes at the beginning of this chapter.
- Mount the photoresist (or soldermask) onto the top supply shaft with the film feeding down off the back of the roll. Center the roll on the supply shaft.
- Mount a roll of lint-free backing paper on the bottom supply shaft paper feeding up off the back of the roll. Center the roll on the supply shaft.
- Pull a length of dry dry-film out and drape it over the backing paper. Assuming that the roll of dry-film is the same width as the backing paper, the edges should be in near perfect alignment. If this is not the case, move the paper roll to the left or right until acceptable alignment is achieved.
- After feeding the dry-film under the top idler, strip away the polyolefin release liner (peel sheet), feed it back over top of the dry-film supply roll (top supply roll), and tape it to the take-up reel.
- Feed the backing paper around the bottom idler and drape back over top of the backing paper supply roll (bottom supply roll).
- Drape the dry-film in front of the hot shoes, then bring the backing paper up, carefully align the edges with the dry-film and press the paper against the exposed tacky surface.
- Turn the laminator's drive motor on, engage the clutch (if applicable) and use a piece of scrap copperclad (the feed-board) to thread the paper/dry-film through the rollers.
- To minimize film distortion and stretching, the web tension adjustment should be tightened to the minimum necessary to remove any wrinkles from the film as it feed between the rollers. Both the film and the paper must feed through the rollers without folds or wrinkles to insure smooth, uniform lamination.
- After the feed-board exits the rear rollers, turn off the motor drive (or open the clutch) and tear off the excess film.
- Turn on the shoe (or roller) heaters and allow the unit to come up to operating temperature (approx. 110 +/- 5 degrees Celsius for most popular dry-films).
- Your laminator is now ready to use.
Things to remember
- Make the grit disappear - Dust and grit that is sticking to the rollers can leave deep impressions in the film that will almost certainly show up in a critical circuit feature and ruin your entire board. A useful trick for cleaning the rollers is to make a "tack board" by laminating both sides of a piece of clean scrap copperclad with soldermask. To clean the rollers, remove any film or backing paper that might be loaded and allow the laminator to come up to operating temperature. Strip the cover sheet from the tack board and feed through the laminator. Soldermask gets incredibly sticky when it is hot and is thick enough to allow virtually any grit that you are likely to encounter to embed itself into the bulk of the film to be stripped away as the board passes on. If you are careful to replace the cover sheets as soon as the board exits the rear rollers, you should get at least 10 uses out of the tack board before it should be stripped and re-coated. Tacky boards that are coated with a very aggressive pressure sensitive adhesive are available for cleaning rollers whether they are hot or cold.
- Peel sheet blues - Unless you have razor sharp nails, separating the thin polyolefin release liner during loading can be difficult at best. It is however, very easy if you use two pieces of sticky tape to pull the film apart. Stick one piece of tape to the peel sheet in one of the corners. Being careful not to touch the two pieces of tape together, stick the other piece of tape to the cover sheet in the same corner as the first piece. You can then pull the two pieces of tape apart to de-laminate (separate) the film. Luckily the cover sheet sticks to the photopolymer far better than the peel sheet so it is just about guaranteed that pulling the pieces of tape apart will strip the peel sheet as desired.
- Too hot to handle - Loading a dry-film laminator while it is hot can make it very difficult to position the film in good registration to the backing paper without massive wrinkling and waste of film. If you cannot wait for the laminator to cool down, and your laminator has a "loading / unloading' lever to separate the top and bottom rollers, you can load the film in good registration using the following technique. Use a threading board to thread the backing paper through the rollers first. Take a 12" or so piece of scrap backing paper and stick it to the adhesive side of the photopolymer (after peeling away the release liner). Disengage the clutch to separate the rollers and manually feed this leader through the rollers and pull it tight as you close the clutch. Inspect the alignment of the film with the backing paper. If they are out of registration, move the roll of backing paper in the direction of the misalignment, open the clutch, pull the film and the paper tight, close the clutch and again inspect the registration. Continue this process until the two are in perfect registration.
- No wrinkles on this stripper - The peel sheet (release liner) will strip away from the film more reliably if you make sure that it is winding up on the take-up reel wrinkle free. To accomplish this, fasten the liner to the reel by putting a piece of tape in each corner and evenly tensioning the film as you lock it in place.
- Board tricks - Use a piece of scrap copperclad as a feed-board for film loading. It is very stiff and will track very accurately through the rollers. Before using copperclad that has been sheared or saw-cut, deburr all edges to prevent injury to yourself and your lamination rollers. Be sure to clean off any photopolymer that might stick to the board after each use.
- Thick as a brick - Laminating thick substrates can present special challenges to many roll laminators. This simple technique will alleviate most of the problems encountered and minimize the wear and tear on your machine.
- Place your laminator on a table (or any other stable horizontal surface) such that there is enough room behind the unit to accommodate the longest substrate that you intend to laminate. Make sure the tabletop is clean and ready to use.
- Inspect the unit to ensure that the lamination temperature is properly set for your application. If you are not sure what the proper temperature is, check the application note that came with the photoresist (or soldermask) or call the film manufacturer. Using the right lamination temperature is absolutely essential. Always allow at least 10 minutes after changing the temperature setting before attempting to laminate your board.
- Prepare the copperclad for lamination by carefully cleaning both sides.
- Very slightly wet one side of the board with water using a fine spray plant mister. If the water beads up from the surface, the board is not clean and you should re-clean the board.
- Pass the board through the laminator with the wet side facing the photoresist (or soldermask). A small amount of water may drip off of the trailing edge of the board as it passes through the rollers but this is normal and no cause for concern.
- After the board exits the laminator, use a razor knife to carefully trim away all of the excess film hanging over the edges.
- Carefully inspect the dry-film for voids or signs of delamination. After lamination, the adhesive layer of the photopolymer should be in very intimate contact with the activated copper surface resulting in a strong mechanical bond.
As a test, you can try to peel away one corner. If the dry-film separates from the copper without tearing, it is not properly adhered. Your only option is to strip the board in developing solution and return to step 3. Possible causes of inferior adhesion include; poorly cleaned/activated copper surfaces, lamination temperature too low, lamination speed too high, lamination pressure too low.
Assuming that the film on the first side passes the peel test, you may proceed.
- Repeat steps 4 through 7 for the second side of the board.
- Let the board sit for about 30 minutes in a dark place to allow the dry-film to totally absorb any residual water before imaging.
- The board is now ready for imaging and further processing.