Phototool Handling: Storage, Cleaning and Registration

Phototool handling covers everything that happens to the artwork film between the imaging department and the exposure frame, and it decides whether the pattern that reaches the panel is the pattern that was drawn. The film is a precision part that is often treated as a consumable. The film carries the geometry that every later step is judged against, so a defect in it is repeated on every panel in the lot.

Most of the loss is gradual. A film stored in the wrong humidity, wiped with the wrong cloth or left on a bench picks up a small dimensional change or a scratch, and neither shows itself until line width drifts. A film that is handled well can serve a product for years, while one that is handled badly is replaced every few months.

What Phototool Handling Covers

The sequence runs from receiving or plotting the film, through inspection and storage, to mounting it on the frame, exposing the panel and returning it to the file. Every one of those steps can alter the film. The film envelope should carry the part number, the revision and the date it was issued, so that the right tool is easy to find.

Handling also includes identification, so that the correct revision reaches the correct lot. A phototool filed under the wrong part number is a defect that no amount of process control will catch. Master films should be kept separately from working copies, so that a working film which is damaged can be replaced quickly.

Storage Conditions for Film

Film should be stored flat, in a dust free cabinet, at a stable temperature and humidity. Stacking films on a shelf without support causes them to take a set, and a curled film will not sit flat on the glass. Cabinets should be closed, because dust settles on a horizontal film and is pressed into the emulsion when it is stacked.

Humidity is the critical variable, because the base material absorbs and releases moisture. A film that has been in a dry room overnight will have shrunk slightly, and the error appears as a registration shift at exposure. Room conditions are described in imaging room humidity notes. Films should not be stored next to a heat source or a window, because the local environment around them is what matters.

Cleaning and Care

Film should be cleaned with a soft, lint free material and an approved cleaner, and it should be cleaned only when it needs it. Every wipe is an opportunity to scratch the emulsion or to leave a residue that prints as a defect. Cleaners should be checked against the film type, since a solvent that suits one base can cloud or craze another.

Contact artwork has to be dust free, because a particle between the film and the panel holds the two apart and softens the edge of every feature under it. Gloves should be worn so that skin oils do not reach the emulsion. Wiping should follow one direction rather than a circular motion, so that a particle is pushed off the film rather than dragged across it.

Dimensional Drift and Humidity

Film changes size with humidity and, to a smaller extent, with temperature, and the change is reversible only if the film is allowed to return to equilibrium. A film taken from a cold store and used immediately will be wrong. Films should be allowed to reach room temperature before they are unwrapped, because condensation on a cold film is hard to remove.

The correct practice is to let the film condition in the imaging area long enough to stabilise, which is usually measured in hours rather than minutes. Dimensional control is described in artwork film dimensional control work. The conditioning time should be written into the procedure, so that the practice does not depend on who is on shift.

Registration on the Exposure Frame

Registration is set by the tooling pins and by the way the film is located on them. The film should be placed without stretching, and the vacuum applied evenly so that the film is held flat against the panel. Tooling pins should be checked for wear, because a pin that has lost its diameter will locate every film on that frame loosely.

Registration should be verified with a test panel before a production lot is exposed rather than after. Where a direct imaging system replaces film, the same discipline applies to the data file and its revision, as described in laser direct imaging notes. Vacuum should be confirmed at the frame, since a film held only at its edges will print out of focus across the middle.

Handling Damage and Repairs

Scratches, creases and pinholes are the common damage, and each has a different effect on the image. A crease prints as a distorted line, a scratch prints as an opening, and a pinhole prints as a small dot of unwanted resist. Touch up pens should be used sparingly and recorded, because an opaque repair that is too large prints as a missing feature.

Small pinholes can be opaqued with a touch up pen, and the repair should be recorded on the film envelope. A creased or torn film should be replaced rather than patched, because the damage will keep growing with use. Films should be inspected before they are filed rather than when they are next needed, so that damage is caught while it is fresh.

Phototool and Line Width

Line width on the panel is set by the film, the exposure and the development together, so a film that has drifted changes the result without any process change. The film is therefore one of the variables to check when line width moves. Line width measurements should be taken at the same feature on every check, so that the readings can be compared over time.

Measuring a feature on the film with a toolmaker microscope takes a few minutes and settles the question. If the film measures correctly, the search moves on to exposure, development and the etch. A film that has been reissued should have the old revision withdrawn, because two revisions in circulation is a recipe for a wrong lot.

Records and Verification

The record should carry the film identity, its revision, the date of the last dimensional check and its result, together with any repair that was made. Films should also be retired on a defined rule rather than when they finally fail. The film file should be reviewed when a product is reordered, so that the tools are still available and still in usable condition.

Acceptance of the imaged panel follows the published IPC documents. A phototool that is measured and controlled is a phototool that can be trusted with a fine line product. A phototool that has been retired should be marked as obsolete, so that it cannot be returned to the file by mistake.

FAQ

Why does a phototool need conditioning? Because the film base absorbs moisture and changes size, so a film that has just come from a different environment will not register correctly.

Can a scratched phototool be repaired? Small pinholes can be opaqued and recorded, but a crease or a tear should be replaced because the damage grows with every use.

How is a phototool checked? By measuring a known feature on the film with a microscope and comparing it with the drawing, and by registering a test panel before production.