Controlling in-drive contamination
Understanding microcontamination inside a disk drive is the first step in combating it and developing a strategy
to minimize it and its effects.
By Tim Bethke, Donaldson Co.
The disk drive industry recognizes that microcontamination inside a disk drive can lead to drive failure. Ultra-low flying heights on the order of 2 micro-inches (0.051 microns) are typical today and are expected to decrease even further, as manufacturers
strive to increase areal density. This design requirement makes today`s disk drives more susceptible to microcontamination than previous generations. It applies to both traditional thin-film inductive head technology and magneto-resistive (MR) head technology. The industry recognizes that MR heads are even more susceptible to microcontamination than traditional thin-film inductive heads. This concern becomes more significant as the volume of MR heads surpasses the volume of thin-film heads.
Loss of signal amplitude from flying height changes during operation, stiction, friction, excessive wear, corrosion damage, and thermal asperities are all potential failure modes that can be caused by microcontamination. As a result, disk drive manufacturers and their suppliers are continually challenged to design products that are virtually microcontamination free.
The purpose of this article is to provide a better understanding of microcontamination, and to discuss some solutions that can aid the disk drive designer in minimizing its effects. This is not to suggest that these solutions will completely prevent microcontamination from affecting the performance of a disk drive. However, the solutions discussed in this article can enhance the performance of the drive. By incorporating these solutions as part of an overall drive design strategy, microcontamination-related issues and concerns can be minimized.
Minimizing micro contamination
Over the years, the disk drive industry has implemented a number of design practices in an effort to minimize the effects of microcontamination inside a disk drive. Manufacturing disk drives, as well as components for disk drives, in cleanroom environments is the most obvious practice that has been implemented in an effort to minimize microcontamination. Stringent cleanliness requirements are standard in the industry, both for the drive manufacturers and component suppliers.
Today`s drives and components are manufactured in some of the cleanest production environments in the world. With all this emphasis on cleanroom production for both drives and components, why is microcontamination still an issue? The reality is that sources of microcontamination will never be completely eliminated. Once a disk drive leaves its cleanroom environment at the point of manufacture, it`s fair game for microcontamination from any number of sources.
Types/sources of microcontamination
Microcontamination can be particulate or chemical in nature. Particulate and chemical microcontamination can be generated inside of the drive during drive check-out (reliability testing), normal operation, or introduced into the drive through the breather hole or through leaks (diffusion) in the drive. Internal components continue to generate low levels of microcontamination during the life of the drive.
Particulate contaminants are typically in the form of loose particles ranging in size from small sub-micron particles to large visible particles. Sources of particulate contamination include internal drive components and the ambient environment. Chemical contaminants can be in the form of organic vapors, acid gases, and water vapor (humidity). Typical sources of chemical contamination include adhesives, packaging materials, gasket materials, and the ambient (or cleanroom) environment. Additional sources are listed in Table 1.
The disk drive industry has made significant progress over the years in identifying materials, components, and manufacturing processes that minimize the risk of microcontamination inside the disk drive assembly. Much time, effort, and money has been invested over the years in understanding microcontamination, and in minimizing its effects on disk drives. Yet, microcontamination still exists. Chemical contamination in the form of organic vapors, acid gases, and humidity presents the biggest challenge to even the most experienced drive manufacturers and their suppliers.
Fortunately, there are cost-effective solutions available. One of the most practical solutions is the use of adsorbent filters inside the disk drive. Adsorbent filters have been effectively used in disk drives for several years. It is important for the drive designer to understand the benefits of incorporating adsorbent filters in a disk drive. The adsorbent filter section of this article describes the various types of adsorbent filters available, their functions, and the advantages that each type offers.
Particulate filtration
Historically, the disk drive industry has relied on two types of particulate filters: the recirculation filter and the breather filter.
Standard recirculation filters. This type of filter is located inside the disk drive assembly, typically in the air path created by the rotating disk (or disks). A recirculation filter`s function is to trap particulate con tamination that is generated inside the drive or enters the drive from the ambient environment. The performance of a recirculation filter is measured by how quickly the filter collects particulate inside the drive after disk spin-up. The shorter the particulate cleanup time, the better the performance. In addition, its efficiency and pressure drop determine its performance. The relationship between these two parameters is typically expressed as Figure of Merit (FOM). Generally speaking, when comparing two filters with the same pressure drop or efficiency, the filter with the higher FOM will have better particulate cleanup time performance.
One of the best methods for quantifying the performance of a recirculation filter is by measuring the particulate cleanup time of a given filter and comparing its performance with an established baseline as well as with other filters. In this test, a recirculation filter is installed in the slot provided by the drive manufacturer. A typical test system includes a filtered air source, particle generator, and flow meters. Sample ports are then attached to the disk drive. The test system also includes a data acquisition system, which includes a particle counter and PC. Note that this test system is custom designed by each filter manufacturer — there is no standard model available for purchase.
The drive is then filled with a known contaminant, and the concentration of particles in the drive is monitored until a steady state is achieved. Then the drive is powered on (spun up). The concentration in the drive as a function of time (seconds) is continually monitored. Each test is run for the same length of time. For comparison purposes, each filter is ranked based on its ability to collect a pre-determined percentage of particles. The shorter the cleanup time, the better the performance of the filter in collecting particles.
The most prevalent type of recirculation filter in the disk drive industry utilizes an electrostatic media encapsulated between layers of polypropylene scrim. The advantages of using a recirculation filter in a drive include quick particulate cleanup time, small physical size, and low cost.
Standard breather filters. Another type of filter in the disk drive industry is the breather filter. It is placed over the breather hole designed into the cover (or base) of the disk drive. The breather hole acts as the primary path for air to enter and exit the drive during normal operation (thermal cycling) of a drive, allowing the drive to “breathe.” The purpose of placing a breather filter over the breather hole is to prevent particulate contamination from entering the drive.
Typical materials of construction for a breather filter include high-efficiency filter media, a low-outgassing adhesive, and a silicone-free release liner. The filter media consists of a PTFE membrane layer laminated to either a nonwoven polyester or polyethylene screen support layer. A low-outgassing pressure sensitive adhesive is used to seal the media to the drive cover. A silicone-free release liner is used as a substrate to secure the filters and allow for dispensing.
The advantages of using a breather filter include high efficiency, low profile and low cost. In addition, by designing the breather filter with a pressure drop significantly lower than the leak rate of the drive, all air entering and exiting the drive passes through the breather, and not through leaks in the drive.
Breather filters are tested for efficiency and pressure drop. The design objective is to keep the pressure drop as low as possible, while meeting either a minimum efficiency of 99.97 percent on 0.3-micron particles, or 99.99 percent on 0.1-micron particles, depending on the specific design application.
Why use particulate filtration?
The application of recirculation and breather filters in a disk drive represents the most common approach to addressing microcontamination concerns. The objective is to collect particles inside of the drive (recirculation filters) and to prevent particles from entering the drive (breather filters). As a point of reference, virtually all disk drives shipped during 1996 had a recirculation filter, a breather filter, or both. Clearly, the disk drive industry recognizes the necessity of particulate filtration.
The recirculation filter is designed for optimum particulate cleanup time inside the drive. By collecting particles on the filter, rather than on heads, media, and other internal components in the drive, the performance of the drive is enhanced. Recirculation filters are designed to remove a range of particle sizes, including submicron particles. As flying heights continue to decrease, particles measuring less than one micron can trigger a head crash by becoming lodged between the recording head and the disk. Larger particles have a tendency to “bounce off” of the head.
Chemical filtration
Traditionally, the disk drive industry has focused on particulate filtration to address microcontamination concerns. As the industry augments its understanding of the requirements and benefits of using chemical filtration in their drives, a surge of interest in chemical filtration has resulted.
What types of adsorbents are available to address chemical contamination? Which adsorbent is the best choice for a drive design? How much adsorbent should be used in a drive? How do I package the adsorbent in the drive? These are typical questions asked by disk drive designers.
Before these questions are addressed, a brief discussion on adsorption chemistry is appropriate. Later in this article, the techniques available to quantify which contaminants are present in a drive, and how adsorbent effectiveness against these contaminants is measured, will be discussed.
Definitions/types of adsorbents
Adsorption is defined as the ability of a substance to hold or concentrate gases, li quids, or dissolved substances on its surface. This means that the chemical contam inant literally sticks to the surface of the adsorbent through physical and/or chemical forces. These forces are referred to as “physisorption” and “chemisorption.”
Physisorption is defined as adsorption by the attraction of a chemical to an adsorbent surface through relatively weak physical (van der Waal`s) forces. This is sometimes referred to as hydrogen bonding. Activated carbons function under the principle of physisorption. Activated carbon is the most important adsorbent used for organic vapor control because of its high surface area and relatively wide pore size distribution.
Chemisorption is defined as chemically binding one substance to the surface of another substance. Chemisorption forces are stronger than physisorption forces. Chemically treated (impregnated) activated carbons function under the principle of chemisorption.
Impregnation is the surface modification of the material to promote a chemical reaction of the impregnate and the contam inant. Chemisorption is the mechanism that bonds acid gases to the impregnate. Typical impregnates used in the disk drive industry include sodium carbonate, calcium carbonate, and potassium carbonate. Activated carbon is also used for acid gas control. When activated carbon is chemically treated (impregnated), it has the added benefit of strong chemisorption properties.
Silica gel is used to control relative humidity (RH) inside the drive. Silica gel`s function is to minimize large changes in relative humidity inside the disk drive, due to temperature and RH changes that occur outside the drive environment. Relative humidity control is achieved via diffusion of the water molecules into the silica gel. Silica gel can be packaged in either granular or sheet form. Silica gel is available in a variety of pore diameters, each with a specific adsorption isotherm (water vapor adsorbed as a function of relative humidity). The silica gel pore diameter is selected based upon the drive manufacturers desired RH operating range and specific concerns.
In order to achieve maximum contaminant control in the drive, silica gel, activated carbons, and chemically treated activated carbons should be designed into the same drive to control water vapor, organic vapors, and acid gases.
Adsorbent filter design
A frequently asked question is “How much adsorbent should be designed into the drive?” The disk drive industry expects a minimum of five years` service life from a drive. The drive designer selects the chemical contaminants of most concern (water vapor, organic vapors, acid gases, or all of the above). Then, a determination must be made as to the concentration of contaminant the drive is expected to be exposed to during its five-year service life. A comparison of the chemical adsorbent capacity of the specific adsorbent used, with the concentration expected, suggests a minimum amount of adsorbent required for that application. The final decision on how much adsorbent to use is based on a combination of adsorbent capacity vs. contaminant concentration, and available space in the drive for the adsorbent. A general rule of thumb is to maximize the amount of adsorbent in the drive, subject to the space limitations of the drive.
After the adsorbent(s) have been selected for the application, the question of how to package them in the drive must be resolved. This is where chemical adsorbent filters enter the discussion. Adsorbent filters have been designed for relative humidity control, organic vapor control, and acid gas control. Adsorbent filters are designed for controlling a single contaminant (e.g. relative humidity), and for controlling multiple contaminants (e.g. relative humidity and organics/acid gases). In addition, particulate filtration control is an inherent design feature on some adsorbent filters. Typical adsorbent materials used include silica gels, activated carbon, and chemically treated activated carbons, all of which have already been discussed.
Adsorbent filters
There are four types of chemical adsorbent filters available: adsorbent recirculation filters; adsorbent breather filters; adsorbent pouch filters; and adsorbent label filters. Each has been designed to address specific contamination issues.
Adsorbent recirculation filters. Adsorbent recirculation filters are designed to collect particulate contaminants, organic vapors, and acid gases that are present in the drive. This is accomplished by adding a layer of activated carbon or chemically treated activated carbon to a standard recirculation filter configuration. The adsorbent recirculation filter is located directly in the path of the airflow created by the rotating disk (or disks), typically in the same location as a standard recirculation filter. The most important design parameters for an adsorbent recirculation filter include its particulate cleanup performance, chemical cleanup performance, and adsorbent capacity.
Adsorbent breathers. Designed to prevent particulate contamination, organic vapors, and acid gases from entering the drive through the breather hole, adsorbent breathers have an expanded PTFE layer that covers a chemically treated activated carbon layer. A low outgassing adhesive ring is used to secure the adsorbent breather to the drive. An adsorbent breather is located inside the drive cavity, either over the breather hole, or over a diffusion channel. By placing the adsorbent breather inside the drive, the adsorbent breather also adsorbs organic vapors and acid gases that are present in the drive via diffusion. It is important to note that adsorbent breathers serve two functions: to prevent particulate contamination from entering the drive, and to adsorb chemical contaminants inside the drive. Careful attention should be given to the efficiency and pressure drop requirements of the adsorbent filter, when designing this filter into a drive. The efficiency and pressure drop requirements of the adsorbent filter must match the requirements of the drive, or the filter will not be the path of least resistance when the drive “breathes.”
Adsorbent pouches are designed with activated or chemically treated activated carbon for organic vapor and acid gas control, or a combination of activated or chemically treated carbon and silica gel. This combination of adsorbents provides organic vapor, acid gas, and relative humidity control. A high efficiency expanded PTFE membrane encloses the adsorbents while being highly porous and permeable to water vapor, organic vapor, and acid gases. An adsorbent pouch is typically located in a cavity or location fixture inside the drive. Adsorbent pouches can also be attached to an internal drive location on either the base plate or cover of the drive using low outgassing adhesive on the flat surface of the filter.
Adsorbent labels are basically low-profile versions of adsorbent pouches, with the addition of a low-outgassing adhesive layer. Adsorbent labels are typically attached to an internal location on either the base plate or cover of the drive.
Why use an adsorbent filter?
Adsorbent filters are used to control organic vapors, acid gases, and humidity levels inside the drive. The advantages of controlling organic vapors and acid gases include minimizing the risk of outgassed and corrosive contaminants from attacking primarily the disk media, but also other internal components that are prone to microcontamination. The most common problems found in disk drives today include stiction, corrosion, and media wear. All three problems are directly related to microcontamination.
The humidity level of the drive is another area of concern. It is important to minimize the effects of high or low humidity conditions and sudden temperature changes. Water vapor acts as a transport mechanism for contaminants from one surface to another inside the disk drive, and as a catalyst for corrosion. High humidity levels inside the drive have been associated with stiction and corrosion problems. Low humidity levels inside the drive is a concern because of the possibility of static discharge inside the drive, which is a potential killer for MR heads.
An ideal drive design would be one that prevents external contaminants from entering the drive. A hermetically sealed drive would address this issue, but is difficult to achieve in a cost-effective manner. In addition, any internally generated contaminants would remain trapped in the drive. An adsorbent filter designed for humidity control acts like a humidity “shock absorber,” thereby protecting the drive from rapid relative humidity swings associated with temperature and humidity changes occurring outside the drive. Silica gel provides this protection because its water vapor capacity is largely independent of temperature. Reducing the water vapor transmission rate into the drive will enhance silica gel performance significantly.
Analytical tools
Arrays of analytical tools are available to understand adsorbents and their properties. Selecting the best adsorbent for a specific drive application starts with a basic understanding of the three types of adsorbents typically used in adsorbent filters, as well as the test methods employed to quantify each adsorbent. A summary of this information is illustrated in Table 2.
In-drive contaminant identification/analysis
What tests are available to determine which contaminants are present in a disk drive? How can the results be quantified? What data is needed to determine if the adsorbent selected for the application is working?
Several analytical tools and techniques are employed to address these questions. These tools and techniques include thermal desorption/solvent extraction methods and chemical cleanup analysis. In addition, diffusion channels can be designed into the drive to slow the transfer of contaminants from the ambient environment into the drive, through the breather hole.
Thermal desorption/solvent extraction. This analytical tool involves two main steps. The first step is collecting contaminants on an adsorbent. The adsorbent can either be located inside or outside the drive environment. In the latter case, a sampling technique is utilized to collect the contaminants from inside the drive on the adsorbent material located in a sample tube. The second step involves either thermally desorbing the contaminants off of the adsorbent material, or using solvent extraction techniques to remove the contaminants from the adsorbent material. In either case, the contaminant is then injected into a gas chromatograph/mass spectrometer (GC/MS) for identification and quantification.
This analytical technique is useful in determining which contaminants are present in the drive and their concentrations. This information can then be used to select the adsorbent (or adsorbents) that will minimize the effects of microcontamination in the drive.
Chemical cleanup test bench. Another test that can be used to evaluate the effectiveness of an adsorbent involves monitoring the decay rate of a known contaminant in a drive using a test system designed specifically for this purpose. This test system is similar to the test system used to measure particulate cleanup time, which was discussed earlier, except that a chemical contaminant is used instead of a particulate contaminant. This test system consists of a standard gas source, dilution air source, mass-flow control valves, a gas chromatograph, and a data acquisition system.
This test consists of injecting a known chemical contaminant into the disk drive, and monitoring the concentration decay rate of the contaminant using either a GC/FID (flame ionization detector), or a GC/MS, depending on the type of contaminant used. This test system is primarily used to measure the cleanup performance of various adsorbent filters, such as the adsorbent recirculation filter.
Diffusion channels
Another design option utilized by the disk drive industry is known as a diffusion channel. A diffusion channel is a long and narrow air path (a tube) designed into the drive casting, or added as a separate housing over the drive opening (breather hole). The diffusion channel is located between the breather filter and the breather. Diffusion channels are effective ways of limiting the rate at which gases enter and exit the disk drive. Fick`s law governs the performance of a diffusion channel for diffusion mass transfer, which states that the mass transfer rate through the diffusion channel varies directly with the cross-sectional area of the channel, and inversely with the length:
There is, however, a trade-off between the diffusion protection of the channel and the pressure drop through the channel. If the diffusion channel is too long and narrow, the pressure drop of the channel can easily exceed the pressure drop of the breather, by an order of magnitude.
Care must be taken when designing a diffusion channel/breather filter combination to ensure that the pressure drop of this combination is significantly lower than the leak rate of the drive.
Future trends
One trend that will increase by the end of the year is the use of MR head technology over traditional thin-film head technology as illustrated in Figure 1.
A comparison of the number of recording heads manufactured in 1996 with the forecast for 1997 is illustrated in Figure 2.
Conclusions
Microcontamination is a significant challenge for today`s disk drive manufacturers — a direct result of the computer industry`s insatiable demand for more storage capacity.
Microcontamination inside a disk drive can result in drive failure, due to stiction, excessive wear, corrosion, and functional problems (thermal asperities).
Microcontamination can be particulate (loose particles) or chemical (organic vapors, acid gases, and humidity) in nature.
Even with state-of-the-art cleanroom production practices and stringent cleanliness specifications for disk drives and their components, microcontamination still exists.
Chemical adsorbent filters can control chemical contaminants.
Silica gels, activated carbons, and chemically treated activated carbons are available for the control of humidity, organic vapors, and acid gases.
Using a combination of chemical adsorbent filters and diffusion protection in the same drive helps control contaminants.
Arrays of analytical tools are available to understand adsorbents and their properties.
References
1. P. L. Kojetin, “Design considerations in computer disk drives for filtering particulate, organic vapors, acid gases, and water vapors,” (Sept 1995).
2. P. Mee, M. J. Smallen, and D. J. Vickers, “Management of disk drive component microcontamination,” IDEMA Insight, vol. IX, no. 2, pp. 1-6 (Mar/Apr 1997).
3. D. Cooper, “Near-contact recording means clean recording,” Data Storage, vol. 3, no. 8, pp. 65-69 (October 1996).
4. L. O`Brien, E. Dauber, and J. Smith, “Keeping contaminants out of hard disk drives,” Data Storage, vol. 2, no. 5, pp. 43-50 (Sept/Oct 1995).
5. Adsorbent product literature.
Acknowledgments: The author wishes to thank all of his Disk Drive Filtration Group and Corporate Technology Group colleagues at Donaldson Company for their contributions to this paper.
Editor`s Note: This article originally appeared in the Proceedings of DATASTOR Asia `97, the sister show of CleanRooms Asia `97. Both shows took place in July 1997 in Singapore.
Tim Bethke is engineering manager of the Disk Drive Filtration Group at Donaldson Co., Inc. (Minneapolis, MN).
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Figure 1: As this figure illustrates, the use of MR head technology is expected to surpass that of thin-film inductive technology during the 1997 calendar year. The disk drive industry recognizes that MR heads are potentially more susceptible to micro contamination compared to thin-film inductive heads, primarily due to decreased flying heights.
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Figure 2: It is clear that MR heads are taking on a greater role in increasing areal densities in the future. In 1996, approximately 27 percent of all recording heads produced were MR heads. In 1997, the forecast suggests that approximately 60 percent of all the recording heads produced worldwide will be MR heads. The significance of this forecast is the fact that the industry recognizes that MR heads are more susceptible to microcontamination. Serious consideration must be given to controlling microcontamination in disk drives that utilize MR head technology.
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