Particles in mirror are bigger than they appear
05/01/2001
WHETHER IT'S IN PAINTING OR AIRBAG MICROELECTRONICS, RELIABLE CONTAMINATION CONTROL IN AUTOMOTIVE CLEANROOMS KEEPS INDUSTRY CRUISIN'
by Hank Hogan
In the week ending March 17, 2001, more than 310,000 cars and trucks rolled off North American assembly lines. Year-to-date, manufacturers produced nearly 3.2 million vehicles. However, each time a satisfied customer drives away in a perfectly operating vehicle, the manufacturer has contamination control efforts to thank.
 Photos contributed by Gentex, Delphi Automotive, and Contec. |
In the auto industry, contamination causes problems that range from outright failure to cosmetic defects. The latter, while often only paint deep, can lead to consumer reactions just as severe as those caused by a dead, electronics-laden engine control module. For car and truck buyers, the demand for beauty and performance is driven by economics.
"You just spent $20,000 on an automobile, you should expect a level of perfection," acknowledges Jeff Rose, director of paint material and process engineering at General Motors' Warren Technology Center in Detroit.
Unwanted bumps and dips caused by fibers and other contaminants lead to visual defects in paint.
Similar problems crop up in other areas, such as mirrors. At the same time, the semiconductors and micromachines in automobile electronics can fail because of microscopic dirt.
 Gentex rear-view miror: painted in a cleanroom. |
To combat these unwanted intruders, the auto industry employs contamination control techniques similar to those used elsewhere, including such tools as lint-free garments and filtered, laminar air flow. At the same time, there are special contamination circumstances that confront the industry, such as the need to paint large pieces of metal in hot, humid enclosures. In some cases, there's a need for an almost absolute elimination of contamination-induced failure.
"In our sensor facilities, if one of our devices fails, your air bag doesn't go off, and that has very grave consequences," explains John Weaver, manager of contamination control at the Delco Electronic Systems unit of Delphi Automotive Systems (Troy, MI).
Paint by small numbers
The consequences aren't as severe in the painting of automobile body parts. No one dies because of a blemish. Nevertheless, the task of painting defect free is challenging from a contamination control perspective. Because the paint is often one of the first things a consumer sees, a perfect coating to the unaided eye is important.
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In a typical paint-spray process, parts enter a paint booth and undergo initial preparation. This involves a phosphate clean and then electrochemical coating of the bare metal. The coating is sanded down, and then the part is covered with a primer. After further sanding, the part is topped by a final color coat and clear coat. This is done in a multi-story facility, with an overall size of perhaps 850,000 square feet. Within the facility, there are robots, people working, as well as ovens for paint drying and tools to move the parts from one station to the next. There may be 30 or so bays in the typical paint facility of an original equipment manufacturer.
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As for the size of the contamination of concern, that depends on the eye of the beholder.
"We try to, on the vehicle itself, have nonvisible defects. Now, typically, that's in the 20-micron range, but anything below 17 you can't see," says GM's Rose. The cut-off number for particles differs from one manufacturer to the next, but usually it's in the five to 10 micron range.
According to Rose, the majority of contaminants, some 60 percent, are generated by people. The rest come from the process.
Of the people-generated contaminants, many originate in clothing. With this in mind, the industry has gone to lint-free garments. In a procedure analogous to that found in electronics manufacturing, paint-spray booth workers will enter the building and change into lint-free clothes, complete with head covering, followed by an air shower to blow off particles, with sticky mats and the like removing contaminants from feet.
Paint booths follow the classic contamination control strategy of separation and isolation. The most contaminant-sensitive areas are divided from the rest of the process by a series of barriers. For instance, the metal surface preparation areas, at the beginning of the process, are at a lower air pressure than the top coat, one of the final stages. Thus, the positive air pressure forces air, and airborne contaminants, out from the most defect-sensitive areas of the painting facility. The air is filtered to remove what the industry labels as dirt.
In style and comfort
However, paint booths are large areas with bulky pieces of metal moving through them. The process itself involves spraying a wet substance onto the metal, with some chemical bonding taking place. So, paint booths don't fit the traditional cleanroom classifications.
"We've done particle counts in booths for various customers of ours, and you're typically finding a Class 10,000 [ISO Class 7] plus type range. It's difficult to classify. Just the amount of air movement within a booth, the amount of turbulence as a result of that velocity and things of that nature can make it difficult," notes Robert Nightingale, president of Toronto-based Cleanroom Garments, a division of Nightson Products, Ltd. Cleanroom Garments specializes in lint-free apparel for the industry's workers and contamination-free covers for the robots used in painting.
White Knight Engineered Products Inc. (Charlotte, NC) also makes paint-spray booth garments. The company has just introduced a laser-based process that simultaneously cuts and seals fabric to within ±0.03 millimeter. The company claims the laser sealing virtually eliminates loose fibers and subsequent fabric unraveling.
On the other hand, according to Donna Bass, White Knight's Midwest regional sales manager, painting applications are typically done without controlling either heat or humidity. The paint-spray process may tolerate larger contaminants than either electronics or pharmaceuticals, but worker comfort demands garment designs that may actually produce more fibers. There may be slits, or pass-through pockets or an open weave on the back. The fabric itself is also a bit more relaxed as compared to what's typically found in cleanrooms, according to Bass.
"You can get by with a little looser weave fabric so that air goes through," says Bass. "The environments are a lot hotter, too, so comfort is a bigger issue, whereas in electronics or pharmaceutical you usually have controlled temperatures."
Location, location, location
As for the non-human contributors to paint booth contamination, painting is often done with robots. These are large pieces of machinery, which run on compressed air and can move very quickly. Controlling the process-generated contaminants involves the use of filters on compressed air, covers on robots and filtration of incoming paint. However, a complicating factor is that in some paint the color-carrying pigments themselves are the size of contaminants.
"If you have an aluminum mica containing base coat, you can't filter it to five microns because you pull out all the pigments in the paint. So we look at each color and typically have specifications for each color or each type of color," comments Rose of GM.
Chuck Berndt, president of contamination control specialists C.W. Berndt Associates Ltd. (Highland Park, IL), notes that this selectivity also extends to the location of the contaminant. Since it's easier to spot a paint blemish on a large, horizontal surface, there is typically one specification for the hood and trunk of a car while there's a somewhat looser one for fenders or doors.
Berndt, along with Nightingale and Bass, sat on the Institute for Environmental Sciences and Technology's (IEST) Working Group 29. Berndt was the committee chair. The group, which included representatives from the automakers, produced a recommended practice for paint-spray booths, IEST-RP-CC029.1. According to Berndt, the document has been quite successful. Manufacturers worldwide contributed to the recommended practice and have put it to use. Berndt notes that an enhancement to IEST-RP-CC029.1 is in the works, with publication expected in the next year or two.
In the rear view mirror
Paint isn't the only automotive area where looks are important. There are also mirrors, which, like paint, are subject to visible imperfections due to contamination. The criteria is about the same as paint, with defects being considered five microns and above in size. So, part of the manufacturing for mirrors is done in cleanrooms or controlled environments. The challenges are different from those found in paint-spray booths, because mirrors are much smaller than hoods or doors. However, the need for a clean surface prior to film deposition is the same.
Gentex Corp. (Zeeland, MI) makes mirrors for both automotive OEMs and the aftermarket. The company's specialty is electrochromatic mirrors. These are constructed by sandwiching and sealing an electorchromatic gel between two pieces of glass. When a voltage is applied, the optical characteristics of the gel changes. The gel can be made lighter or darker on demand. So the reflectivity of the mirror can be controlled electronically. Gentex uses this in a line of Night Vision Safety mirrors that automatically adjust to and reduce glare.
According to Kurt Brouwer, corporate facilities manager at Gentex, the company runs three cleanrooms for its manufacturing and product development. Glass cutting is done outside of the cleanrooms, with final surface preparation and deposition of the gel taking place inside. The mirror is also sealed inside the cleanroom. The product is completed by including electronic circuit boards and light sensors.
All of the Gentex cleanrooms are in the ISO Class 7 to ISO Class 6 (Class 10,000 to Class 1,000) range, with overhead HEPA-filtered laminar flow. Inside the cleanrooms, workers wear lint-free garments. The company started with a half-million-dollar, largely traditional cleanroom in 1988. However, over time, the cleanrooms have evolved into something much more flexible and manufacturing friendly, notes Brouwer.
"We've gone with soft walls, curtain walls, that we've put up around the actual cleanroom. It's created much more flexibility for us," he says.
As is the case in many other industries, most of the contamination comes from the people inside the cleanroom. A study undertaken by Gentex several years ago showed that 80 percent of the unwanted particles were fiber, skin flakes and other human-produced contaminants. To suppress and control this, the company has adopted the use of lint-free gowns and gloves. Employees have to cross a sticky mat before entering the cleanroom. However, Gentex has not installed air showers, because the company's products can meet specifications without them.
But Gentex is looking into the effects of air flow. In particular, the facilities group is investigating what happens to a contaminant when it is dislodged during, say, a cleaning operation. The plan is to arrange the cleanroom so that these cast-offs are forced away from the product. The soft wall approach helps when rearranging the layout to improve cleanliness.
"We've also learned that certain heights of the cleanroom ceiling work better than others. There's a lot of different things we've been working on in terms of the ceiling heights and the diffusers, areas of concentration in terms of where we need more air, more clean air," says Brouwer.
Avoiding the bleeding edge
Finally, there are automotive components manufactured in near state-of-the-art cleanrooms. Delphi Automotive Systems, for instance, makes both semiconductors and micromachines for automotive electronics. The semiconductors are manufactured in a mixed ISO Class 3/4 (Class 1/ Class 10) cleanroom with a sub-micron process. According to Delphi Delco's Weaver, the process is not as fine a line width as is found in the most advanced semiconductors, but the manufacturing requirements can be quite strict. If a chip fails in a PC, the computer may crash, and the user may be upset. In an automobile, the consequences of a defective device are different.
"In the auto industry, if one of our ICs fails, your car doesn't run, and you walk," says Weaver.
Other challenges involve the device working environment. A car vibrates, is full of dust and oil, and works in everything from sub-zero to plus 100 degree temperatures. That all translates to a need for greater reliability, and that, in turn, leads to a need to control contaminants. That's one reason why Delphi Delco's cleanroom is state-of-the-art, even if its semiconductors aren't.
The company also runs several other cleanrooms for its micromachined sensor and discrete component business. These are mixed ISO Class 5/6 (Class 100/Class 1000) cleanrooms. The sensors are used for such things as accelerometers, which detect collisions and fire airbags. The discrete components are used in all the other electronic innards of modern cars.
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Right: Contec's SATWIPES wipers are used for product wipedown.
Constructed using techniques developed originally for integrated circuits, micromachined accelerometers often work by creating a thin silicon bridge. A mass of some kind is suspended on one end of the bridge. With a change in direction, the bridge flexes, and circuitry picks up that movement. That is then translated into an acceleration. In the event of a crash, cars rapidly come to a stop and so undergo high acceleration. That is what the accelerometer detects, and that is what's used to decide when to fire an airbag.
However, contaminants and particulates can severely impact this.
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"If you have an inertial mass that's moving in an accelerometer, a particle will prevent that movement, or dampen that movement, which would give a false signal," remarks Weaver.
Right: Contec's DetaminatR wiper removes particles in wet sanding applications.
The challenge in this case is that there must be essentially no particles above a certain size. The exact number depends upon the construction of the micromachined sensor, but typical values are 80 microns and above. That's one reason why Delphi Delco uses a much tighter class cleanroom than would ordinarily be needed.
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Right: A Contec Quiltec wiper is used to clean a bellringer.
As for the future, expectations are that allowable contaminant sizes will continue to decrease. In the case of paint, one driving factor is that paint coatings are getting thinner. Consumer expectations for a perfect finish, on the other hand, aren't changing. In the case of mirrors, there's a push to convert them into purveyors of such information as location and driving directions. That will involve more electronics and display capabilities. Both will require more contamination control. Semiconductors and sensors are shrinking in line width, with the attendant decrease in defect size.
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The auto industry is also expanding the use of cleanrooms and controlled environments. For instance, there is a trend toward doing metal machining, with oil mist in the air and particles being generated, in a controlled environment. In such cases, the very definition of contamination changes. In most cleanrooms, oil is a contaminant. In machining, an oil mist actually suppresses particle production.
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Top and right: Contec's SatWipes are used for final wipe-down prior to painting in many automotive assembly plants.
As Weaver explains, "The automotive industry has really been pushing cleanrooms and controlled environments in a bunch of different directions for what we would call non-conventional applications, and it's been quite an opportunity to work on cleanrooms like these where you would never expect to do things in a clean manner."