Crystal clear reproductions: optimizing process controls for the next generation diazo phototool films.

A FEW YEARS ago, the market offered a variety of diazo emulsion designs. The diazo films then were commonly referred to as standard and double-sided emulsions. Diazo film was designed for silver film duplications, a matte emulsion version and clear emulsion, and the first generation of scratch and chemical resistant emulsions.

Today, the needs and requirements of end users have changed as the U.S. and European PCB production models have moved from a blend of prototype, short production runs and "long runners" to primarily prototype jobs. China's demand for diazo films for long runners is growing and it's used in both primary imaging and LPISM applications.

This transition has pushed diazo manufacturers toward refining their product offerings to a single design that features an improved blend of a chemical and scratch resistant emulsion. Diazo is the increasingly popular choice as the working photo mask (phototool) for liquid photo imageable solder inks allowing for easy registration to the panel.

Also, diazo has a home in both the outer and inner layer imaging areas because of its outstanding ability to reduce both artwork and board touchup/rework/scrap associated with handling scratches during the production process. The new and improved chemical resistance features of the diazo film allow for easy cleaning and removal of soldermask inks from the film.

This same ability to resist chemical degradation also prevents damage to the actual image on the film. Silver film gelatin emulsions are easily attacked and permanently damaged by many of the liquid soldermask inks on the market. Finally, diazo films' scratch resistance provides superior protection when compared head to head with silver phototool film on the production floor.

Superior resolution properties are the result of reducing the matte particulates while still allowing for air evacuation during the contact exposure stage, both from the silver master to the working diazo phototool and during the imaging step for on the production floor. The medium-orange tinted emulsion was specifically designed to fall between the darker style emulsion diazo films and the lighter style diazo emulsion films for ease of registration to the panel.

The actual color of the film has no relationship to its potency at blocking UV light. Manufacturer D-Max release specs are in excess of 4.5 D-Max--more than adequate for any and all imaging applications--from dry film photo resists to liquid photo imagable soldermask inks. Actually, the mid-range color is a significant benefit on the production floor and assures easy visual registration verification on the panel.

On the processing side, it is pretty much business as usual. You expose, develop and touch up in a similar manner as with past diazo films. However, you should take these steps with care. Only then can you achieve optimum results.

Let's review the entire process. In the artwork generation/photo lab, inspect the diazo or silver master for any damage and touch up any pin holes or scratches. Verify image acuity/sharpness and check for visible damage. Re-plot your silver master if you encounter any fuzzy/blurred lines, creases or dents on the film.

Next, clean the master with the appropriate film cleaner. Clean the exposure unit to reduce any imaged-on defects caused by dust, dirt, lint or other influences. Center the films in the contact vacuum flame emulsion to emulsion and start the vacuum drawdown. I recommend increasing the drawdown time to at least 60 seconds, preferably 120 seconds.

With less matte in the emulsion, air evacuation during this drawdown stage requires a little additional time. Check for Newton rings over the entire film surface--this ensures a good contact and produces a resulting image that has crisp sharp line edges on the diazo duplicate. If you are using a Stouffer 21 step wedge to verify the correct exposure time you want to see a clear step I and 2 with a visual step 3.

Once in the development stage, make sure the developer has had the proper warm up time of approximately 30 minutes and then verify the developer "film plane" temperature (what the film actually sees going through the developer). You can verify this using temperature tapes; disregard the thermometer on the side. Often the thermometer on the developer only indicates the tank temperature and not the film plane. You want a minimum of 155 [degrees] F and a maximum of 165 [degrees] E

Verify that the metering pump is delivering 15-17 cc/ml in 10 minutes. I recommend using 28-30 "baume A" ammonium hydroxide, not technical or diazo grade. Now pass the film through the diazo developer. With the improvement to the diazo chemical resistance feature, I recommend a minimum of four passes through the developer to ensure complete development. Remember that unlike a silver film, diazo cannot be overdeveloped. So if you have any doubt whether the film is completely developed, just pass it through the developer a few more times. If you are using a densitometer to verify D-Min and D-Max, always first perform a "burn back" on the diazo by placing the film base toward the exposure lamp and exposing at two times the original exposure time setting before taking the density readings.

Next, visually inspect the film for any image defects and touch up accordingly. Clean the film and place it in a protective sleeve. You are now ready to send the film to the production department.

JEFF JARVIS is global business manager for InteliCoat Technologies. He can be reached at [e-mail].