72 fpm is best

Dr. James Burnett, P.E., President

Burnett Technology LLC

Aptos, CA

Regarding your article “Cleanroom velocity not subject to outdated standards” (CleanRooms, March 1999, page 90), please be advised of the following facts:

1. The 90 feet/minute laminar velocity actually came from work at Sandia Corp., where they set that velocity so that a 10-micron steel ball would fall clear of the work on the surface of their horizontal laminar workstations. The person who was part of the Sandia Group that did this was Mr. Gordon King. Hence, the 90 feet per minute was a mis-application in vertical flow, as your article stated. This all occurred as part of Atomic Energy Commission work, before integrated semiconductor circuit manufacturing.

2. Additional information for you is that I did funded aerodynamic research work to determine what was the optimum air-velocity for Class 1 cleanrooms. This work was funded by a group of companies, including Chartered Semiconductor, National Semiconductor, Hewlett Packard, Intel, and Sematech.

My experimental work determined that vertical flow velocity is critical, with too much velocity as detrimental as too little. My research determined that the optimum velocity to (1) maintain best particle isolation in vertical flow, and (2) to control hot rising air currents from hot tool surfaces, was 65 feet per second in downward vertical flow, for a 100 percent filter coverage situation.

However, I recommend running at 72 feet per second, because that gives the Class 1 cleanroom an engineering margin of safety, resulting in a very robust condition. This work was done in about 1987, and given to the industry at that time.

About the same time, Dr. Ohmi published the same conclusions from his university research work in Japan.

If anyone wants to know more about this work, please contact me at: [email protected].