Mark A. DeSorbo
SAN JOSE, CA—The last time Roger W. Welker gave his presentation on guidelines for the design and certification of tooling there were no such things as Class 10 or Class 1 cleanrooms.”I haven't had the opportunity to give this presentation since 1993,” Welker says with a laugh before taking the podium at CleanRooms West '99. “Back then, electrostatic discharge (ESD) wasn't even an issue.”
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RIGHT. When a table is placed against a wall (left), the airflow deflects off the wall and in the area where a technician would stand. When a table is placed away from a wall, airflow moves directly to the floor (right).
Indeed, a great deal has changed in seven years, as Welker, principle scientist of R.W. Welker Associates (Northridge, CA), points out. Yet, the guidelines are still applicable.
“What I have outlined here is a method, and it was just a matter of updating it,” he adds.
Like he said, ESD wasn't even an issue in 1993, but today, it is perhaps the number one problem on the minds of contamination control specialists, that, along with airborne molecular contamination (AMC), whether it is the chemical or the vapor type.
In order to achieve certification of tooling for the cleanroom, Welker recommends four stages, which fall in line with Federal Standard 209, “Airborne particulate cleanliness classes in cleanrooms and clean zones” and Mil-Std 1246, “Military cleanliness standard for product cleanliness levels and contamination control program.”
The stages are as follows:
- Stage I: After construction
- Stage II: After tooling installation
- Stage III: During full operation
- Stage IV: During energy conservation operation
“This staging technique allows for an orderly approach to the class limit of cleanroom as it is built, outfitted with tooling and placed in operation,” he says.
Ultimately, Stage II is when tooling is installed and made functional, and it is the appropriate time for a workstation wipe-down as personnel have yet to occupy the cleanroom space. “It is important that the tooling is in operation because for certification it makes up for 50 percent of the specified (cleanroom) class,” Welker says. “At Stage III, bring everybody in and generate some contamination. They are the other 50 percent of the cleanroom class. You can't just look at the tooling itself.”
It is in Stage III that the cleanroom can be calibrated for further control of ESD and AMC. Televisions and monitors, Welker says, are often sources of ESD and should be shielded or, if possible, removed from the cleanroom. For AMC, adhering to airflow requirements as well has hand-cleaning work areas are proven methods of control.
“Tool wear and parts damage are unavoidable. It is extremely important to wipe down tools because wear debris will fall off and get on to the product,” he adds.
When it comes to airflow, Welker provided a scenario he saw first-hand. “Somebody installed a box of electronic components and it had cooling fans that were blowing up, and whatever was being discharged was getting redistributed by the clean air coming from the ceiling,” he says.
But the most important aspect of tooling certification is equipment maintenance, and is, according to Welker, the most overlooked. Maintenance, he says, is often easily executed with a very simple device: A wiper, wetted with isopropyl alcohol.
“The wiper should be folded into a pad, and when the surface is visibly soiled, fold it to expose a fresh surface,” Welker says. “When you are wiping, you're getting the stuff you can't see.”
For Bob Robinson, semiconductor industry manager for Staubli Unimation, a Swiss maker of cleanroom robots, organic contaminants and the set up of the packaging are important.
“We need the type of training that [Welker] provides,” he says. “These kinds of ideas and methods are important, right down to how easy our robots are to wipe down.”