Engineering News 5-4-98Engineering News 5-4-98

May 4, 1998

7 Min Read
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May 4, 1998 Design News

ENGINEERING NEWS

Standards take aim at microcontamination

The hard disk drive industry is taking steps to counter the risks incurred with technological advancement

by Deana Colucci, Associate Editor


Newton, MA--Better, faster, cheaper. Like most everything, attaining this goal comes with a price. Take the computer hard drive industry for example. Technological gains in recent years--such as the use of magnetorestrictive (MR) head technology--have enabled greater memory capacity. The tradeoff: increased potential for contamination. (see sidebar)

Prompted in part by a growing industry concern with microcontamination, the Santa Clara, CA-based International Disk Drive Equipment and Materials Association (IDEMA) is establishing standards aimed at eliminating microcontamination in hard disk drive (HDD) assemblies.

IDEMA's Microcontamination Committee, in existence for two years, receives input from members and volunteers in the hard drive industries in Japan, Singapore, and the U.S. The Committee comprises two sub-committees--one focused on terms and definitions, another on test and methodology.

"You have to be realistic when setting standards," says Arnold Toxen, consultant and Chair of IDEMA's Terms and Definitions Committee.

"It's important to define what 'contaminant' means, types of contaminants, what harmful effects they might have, and units. Plus, specifying contaminant levels means nothing unless you also define the test procedure by which you make the measurement," he says.

The Committee currently has several proposals addressing such issues up for vote. Down the line, IDEMA hopes to set specifications on specific contaminants. "It's an on-going process," acknowledges Toxen.

One thing is certain, microcontamination is not new, and it isn't disappearing any time soon. "As disk drives get more sophisticated, the problems get exacerbated," Toxen says. "And as microcontamination becomes more of a problem, disk drive manufacturers will set more stringent specifications on products made for them--from disks to motors."

Moving ahead. Even without definitive standards, some suppliers are already taking steps to end microcontamination. Take the Glen Rock, PA-based adhesive manufacturer Adhesives Research Inc., for example.

The company's pressure-sensitive adhesives (PSAs) are widely used to bond components on or in HDD assemblies. Components fabricated with PSAs include environmental seals, such as warranty and clockhead seals; identification labels; and bar code parts. PSAs also bond various filters within the hard drive housing, as well as flexible circuit cables, vibration insulators, and magnets.

Industrial-grade pressure-sensitive adhesives have performed satisfactorily for many years in HDD applications. Now, "cleaner" adhesives are required.

Specifically, very low outgassing characteristics and minimal ion contamination are musts, says William Stratton, market development manager at Adhesives Research. The problem: industrial-grade PSAs typically contain between 0.5 and 2.5% volatile organic content by weight, which contributes to high outgassing, which in turn leads to corrosion.

In addition, many industrial-grade PSAs use acrylic acid in the adhesive polymer backbone. It increases adhesion to polar surfaces such as metals and glass, and can be used to cross-link the adhesive polymer to increase the cohesive strength of the PSA, Stratton explains. Despite its benefits, however, acrylic acid causes head corrosion and media fogging. Plus, some industrial-grade PSAs incorporate release liners catalyzed by organotins, which have been cited as a cause of hard drive failures.

Potential solution. Adhesives Research has developed an acrylic acid-free, low-outgassing PSA technology that employs a PSA polymer known as AS-123. A proprietary alternative chemistry is used to achieve peel adhesion and cohesive strength. And the polymer contains minimal anion content, reducing potential corrosion.

Pressure-sensitive transfer adhesives and label stocks that use this new acrylic acid-free polymer typically exhibit outgassing values of 0.15 to 0.20% when measured after a 2 hour exposure at 100C in a vacuum oven. "This compares favorably with industrial-grade PSAs often specified into hard drive assemblies," says Stratton.

In addition, transfer adhesives and label stocks manufactured using the AS-123 technology use no organotin catalysts in either adhesive or release coating curing. And the technology withstands the environmental conditions associated with electronic applications with no degradation in performance, Stratton says.

Overall benefits, he claims: Eliminating acrylic acid from the entire PSA manufacturing process reduces potential media fogging and head corrosion. Minimizing volatile organic content decreases failures related to chemical contamination due to outgassing. And the use of chemistries with minimal ion content and no organotin prevents other sources of chemical contamination from affecting drive performance. "Each of these factors helps extend the Mean Time Between Failure for the HDD assembly, increasing reliability," Stratton says.

So far, the AS-123 PSA technology has been used to reduce microcontamination hard disk drives that use MR head technology. Applications include tape seals and HSA barcode labels. It is currently being evaluated for flexible circuit bonding and filter bonding inside the HDD assemblies.

While the PSA technology is a step in the right direction, IDEMA knows it has a long way to go. "Everyone in the industry is aware of the problems. Once there's a sanctioned standard, there will be more pressure on them to conform," says Toxen. "For now, I think we're heading in the right direction."

Technology vs. contamination

Hard disk drive assembly manufacturers are striving for increased memory capacity for hard disks, but they don't want to sacrifice reliability. Unfortunately, better technology has also brought an increased potential for contamination. Here's an overview of the major contamination issues facing hard disk drive assembly manufacturers.

Erratic flight height. The "slider," or head, "flies" just microinches above the disk reading or writing data. Any material on the disk--such as dust particles--will interfere with the slider and change it's flight path. The result: errors in reading or writing data. "What to a person is a speck of dust, to this head is a skyscraper," says Arnold Toxen, consultant and Chair of the Terms and Definitions Committee of IDEMA's Microcontamination Committee. The severity of this problem increases as the hard drive industry strives to write data to disks in higher density because the slider will have to fly closer to the disk.

  • Head crashes. Contamination on the disk will eventually cause corrosion of the magnetic layer. Due to the minute distance above the disk at which the head flies, the head will often crash into the corrosion product. Corrosion on the surface of the disk also causes interference that eventually leads to disk failure.

  • Stiction. In many HDD assemblies, the read head comes to rest on the disk when the drive is turned off. The presence of certain organic materials on the disk may prevent the head from taking off again. Why? They're sticky. Some corrosion products can cause this stiction as well.


10 questions to ask about PSAs for hard drive assemblies

With the current trends in hard disk drive design, contamination control of all components, including pressure-sensitive adhesives (PSAs), is more critical. Design engineers must address several key questions when specifying a PSA into components for hard disk drive assemblies. William Stratton, market development manager at Adhesives Research Inc. (Glen Rock, PA) offers these guidelines:

What to ask

Why it's important

"Cleaner" adhesives are now required. Low outgassing and minimal ion contamination are musts. High volatile organic content by weight contributes to higher outgassing, which leads to corrosion. Acrylic acid, causes head corrosion and media fogging. Plus, release liners catalyzed by organotins, have been cited as a cause of hard drive failures.

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