Photoresist Image Quality

Author: Robert Gaertner, PhotoBrasive Systems

Posted:

Revised: August 03, 2005

For the uninitiated, I will give a brief explanation of photo resist. Photo resist is a light sensitive photopolymer emulsion that has been coated onto a clear carrier sheet. This emulsion is water soluble before exposure to light; it is no longer soluble after exposure. This technology allows for the creation of a stencil (mask) by simply exposing certain portions of the emulsion to ultra violet light. The emulsion is then washed with a high pressure stream of water. Any emulsion that has NOT been exposed will dissolve, leaving an open area. This area will ultimately be etched on the glass. If the photo resist is a non-washout variety, the black area of the artwork will represent the unetched area of the glass.

The previous paragraph holds the key to the importance of image quality in this process. GOOD IMAGE QUALITY IS THE FOUNDATION FOR A GOOD STENCIL. If the artwork is not created correctly, the processing of the mask will be difficult if not impossible.

Quality images have: high resolution, high density (printed area), and low density (unprinted area). Definitions and descriptions of these terms follow:

High resolution-This describes the quality of the printed image. A high resolution image will have very sharp definition., with no jagged or “stair stepped” edges. This is important whether the image is a line that is .003” wide or a letter that is 2” tall. A photo resist that has been processed properly will provide an exact “copy” of the artwork, so cleanliness counts!

High density (printed area)-. Users of photo resist often assume a high resolution printer will provide high density, but this is not always the case. High density describes the ability of the printed area to block ultra violet light. If the printed area does not block light well, the resist will be exposed in areas it should NOT be.

If the resist is the washout-type, this unintended exposure will cause partial crosslinking. This will make washout difficult since the emulsion will not dissolve as quickly as it should. Overwashing the film may cause a loss of resolution or reduced ability to resist the abrasive blast.

In the case of a non-washout film, unintended exposure will cause the mask to lose adhesion. The end result will be a resist that blows off during blasting, or a mask that will not adhere to the glass at all.

High density can be achieved several ways. If the artwork is to be printed in a laser printer, the medium of choice is either vellum or drafting film. If the artwork is to be printed on an inkjet printer, a suitable inkjet film must be chosen.

Vellum is a paper that can be compared to tracing paper. The key in selecting a vellum is to obtain one with low density. It is entirely possible to have two identical weight/identical appearance vellums that are very different in their ability to pass ultra violet light. For best results, purchase a vellum that is intended for use in UV exposure. Vellum normally has a resolution limit of approximately .013”. The minimum density of a vellum should be close to 0.380. The printed density of a vellum will usually not exceed 1.60.

Drafting film is a plastic-based material that will withstand the heat of a laser printer’s drum. It has the appearance of acetate that has been frosted. Drafting film is usually considered to have a resolution limit of .006”. Although the resolution of drafting film is higher than vellum, its density (printed or unprinted) is similar to that of standard vellum.

The density of laser-printed vellums can be enhanced with spray treatments, but the density can rarely meet that of film positives or certain inkjet films.

Inkjet film is a chemically treated film. The chemical treatment allows the ink to dry instantly. The combination of ink and the chemical coating makes for a very dense printed area. A density example of an inkjet film would be a low of .058, and a high of 4.2. This density rivals a genuine film positive.

Low density (unprinted area)- This describes the ability of the artwork medium to pass ultra violet light. This is important because some areas of the emulsion must be exposed to UV light so it will not dissolve during washout. Low density can be achieved by the correct selection of the artwork medium. A vellum will usually have an acceptable minimum density, whereas a bond typing paper will not. It is interesting to note that the density of a vellum will actually decrease if treated with a toner-enhancing spray.

If you are planning to purchase a printer for artwork production, I would recommend testing several models to see which one can produce the most dense image. It may even be necessary to sacrifice some resolution for density. Considering that poor density can cause film failure while poor resolution cannot, it may be a worthwhile trade-off.

Can you imagine trying to use a camera that has a hole in the shutter? How about a spray paint stencil made with window screen? NO? I wouldn’t either. That’s why I do my best to make my artwork as dense as possible. It’s not worth the risk of wasting photo resist to save a little time or money on artwork.