Hard Disk Drive Reliability Modeling and Failure ...

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Hard Disk Drive Reliability Modeling and Failure Prediction Brian D. Strom, Member, IEEE, SungChang Lee, George W. Tyndall, and Andrei Khurshudov

Abstract—A reliability model for the hard disk drive (HDD) is developed, focusing on head-disk separation as the primary independent variable. The model is structured to incorporate the theoretical effects of environmental factors, plus empirical dependence on the product operating mode. An experimental method based on magnetic spacing loss theory is used to characterize the head-media separation as a function of temperature, altitude, humidity, and HDD operating mode. A statistical model based on these empirical data is developed to predict HDD reliability for various operating conditions. The predictions of the model are verified experimentally through comparison with HDD product reliability test data. Index Terms—Magnetic hard disk drive, HDD, head-disk clearance, reliability model, failure prediction. I.

T

INTRODUCTION

HE HARD DISK DRIVE (HDD) is a highly complex, massproduced, electro-mechanical device that utilizes principles of magnetic recording for data storage. As fundamental elements of modern computer systems and consumer electronic devices, HDDs have managed to combine a steady increase in storage density and capacity with a concomitant decrease in the cost per megabyte. The HDD combines the most recent achievements in the science and technology of magnetic recording, material science, and digital signal processing. The slider, read/write heads and magnetic recording media are the key components of the HDD and form together the head-disk interface (HDI), illustrated in Fig. 1. These heads are integrated into the ceramic slider, which includes an air bearing surface (ABS) formed in relief on its surface facing the disk (Fig. 1a). Air entrained between the ABS and disk generates lift by virtue of the viscous properties of air being squeezed through the gap. The flow of air is guided by the ABS to control the head-media spacing within close tolerance (± 0.1 nm). FIG. 1 HERE

The separation distance between read/write elements and Manuscript received December 15, 2006. B. D. Strom, S. C. Lee, and G. W. Tyndall are with Samsung Information Systems America, San Jose, CA 95014 USA (phone: 408-544-5701; fax: 408544-5924; e-mail: [email protected]). A. Khurshudov. was also with Samsung Information Systems America. He is now with Seagate Technology, 389 Disc Drive, Longmont, CO 80503 USA (e-mail: [email protected]).

the disk directly affects both signal strength and resolution, and istherefore critical to the recording density of the HDD. As magnetic recording densities increase, the magnetic spacing and hence the head flying height must decrease correspondingly. Today, only a few nanometers separate the slider from the disk surface moving at 30 m/s. A. Definition of head-disk clearance We define clearance as the difference between the flying height and the glide height avalanche (GHA), which is the level of highest detected disk asperities. The distribution of flying height in a slider population is typically normal, characterized by its mean (FH) and standard deviation. In a well-designed interface, the lowest-flying slider of the total population will fly significantly above the GHA, eliminating the possibility that any heads drag on the disk at high speed. Fig. 2A illustrates such a condition for an exemplary product operating at room temperature: it has mean clearance 5.3 nm and standard distribution 0.8 nm, thus operating at a 6-sigma level ensuring positive clearance. FIG. 2 HERE spanning two columns The demand for HDD products with higher areal densities, coupled with the cost pressures of a mass-production environment where inspection of all incoming components is not practical, can result in HDD product populations having some finite “interference” of the FH distribution with the GHA. This interference can be exacerbated by the environmental conditions in which the HDD is operated. In Fig. 2B, we illustrate the case of “minor interference” where the tail of the flying height distribution overlaps the disk GHA level. Fig. 2C shows the case of the major interference when exactly 50% of the population of the heads interfere with the disk and the clearance as defined previously is less than zero. We describe below why in this case as many as 50% of the population will eventually fail. B. Clearance as a critical factor for HDD reliability Experience has shown that a conventional HDI will fail if clearance is less than zero. Attempts to eliminate the headmedia spacing through the design of contact recording systems have not emerged from the laboratory as products, for want of both performance shortfalls and reliability concerns [21]-[25]. A robust, long-lived HDI requires sufficient clearance to prevent the heads and slider from contacting the

AA-2 Asia Pacific Magnetic Recording Conference media during operation. The hatched regions of the distributions in Figs. 2B and C represent the portion of the HDI population having negative clearance. The heads in this portion of the population are rubbing against the disk and will likely fail. Following this failure criterion, Fig. 3 summarizes the relationship between the normalized clearance and the cumulative failure rate of the HDD population. As the clearance decreases, the rate of cumulative failures increases rapidly. Also, for a given FH probability density function, the cumulative failure rate increases with increasing number of interfaces, which is a common way to increase the HDD capacity. This analysis illustrates how rapidly the HDD failure rate can increase with decreasing clearance. FIG. 3 HERE FIG. 4 HERE Moreover, even as recording densities have increased at an average annual rate of 60% [1], the requirements for HDD reliability, such as the Mean Time to Failure (MTTF), increase as a result of the competitive pressure from inside the disk drive industry and from other data storage technologies, such as flash memory. For example, HDD designers have managed to increase MTTF ratings from 100,000 hours to over 1,000,000 hours in the span of the last 10 to 15 years (Fig. 4, [1] ). It has proven extremely challenging to simultaneously reduce the head flying height (and clearance) while enhancing the reliability of the head-disk interface. Looking forward, this challenge will become more severe as the HDI is exposed to more contact than before and the thickness of the protective films in the HDI is minimized. Continued progress will be facilitated by increased understanding of the factors that impact slider-disk clearance and by our ability to manipulate these factors. II. THEORETICAL CONSIDERATIONS Over the past 20 years of HDI technology development, clearance; surface roughness; film thicknesses; and applied loads have all diminished considerably. Consequently the significance of nanometer scale object dimensions and intermolecular forces has increased, and the theory of HDI behavior has evolved accordingly. Significant factors that impact the HDI clearance and reliability include properties of: a) the disk, such as roughness and lubricant, b) the head, such as dynamic pitch, as well as c) the environment. A. Disk Factors Disk roughness can affect clearance through two different mechanisms. First, as is apparent from the illustration in Fig. 1b, the slider must fly higher than the ridges and asperities on the rough disk to maintain positive clearance. In this regard, higher disk roughness produces higher disk glide avalanche (GHA) and reduces clearance. This consideration has driven technology for ever-smoother disk surfaces over the past 20

2 years. Secondly, disk roughness can significantly affect the clearance by attenuating the intermolecular forces (IMF) between the slider and disk. When exposed to the IMF, the slider flies lower and the clearance is reduced. The nonretarded, long-range IMF (predominantly the van der Waals forces) increase as a square of the decrease in separation distance. It follows that increasing the surface roughness results in an overall decrease in the IMF. Thus, experience of a decade ago showed that lower roughness produced higher clearance because the GHA was reduced. But at today’s slider-disk clearance, the influence of IMF is more significant, so that lowering roughness increases the effective interaction area, thereby strengthening IMF and decreasing clearance [2]. The lubricant film is another important component of disk design, and has become increasingly important as the clearance has been reduced to a few nanometers. This film is typically the weakest mechanical element in the HDI, in that it moves under the influence of air bearing pressure, disjoining pressure, and intermolecular forces. In the conventional HDI, peak air bearing pressure can reach 20 atm, and the lubricant tends to be squeezed thinner as a consequence. The lubricant is displaced until the disjoining pressure, which resists film thinning, comes into hydrostatic equilibrium with the ABS pressure [3]. This lubricant thinning has a relatively minor effect on clearance. More significant effects on clearance are produced by intermolecular forces acting between the slider and disk, which weaken the adhesion of the lubricant to the disk [3]. Under this effect, the lubricant expands towards the slider and thus reduces the slider-disk clearance. (Should this effect be sufficiently large, the lubricant ‘jumps’ between the disk and slider surfaces thereby momentarily reducing the effective clearance to zero. This ‘soft’ contact can cause errors in writing and reading data, or even failure of the HDD.) The magnitude of this effect depends on the initial clearance as well as the lubricant structure, thickness, and molecular weight. For example, since the lubricant disjoining pressure decreases with increasing thickness [4], the IMF generated between the flying head and rotating disk will have a larger effect on thicker lubricant films compared to thinner ones. Experiments involving the common lubricant Zdol have also shown that clearance decreased as molecular weight increased, approximately as the square root of the lubricant molecular weight [5],[6]. The longer the lubricant molecule, the greater its vertical expansion is when affected by IMF. In summary, clearance will generally be enhanced by the use of a thin, low molecular weight lubricant film. B. Head Factors As described in discussion of the effects of disk roughness, the roughness of the slider affects clearance through two competing mechanisms. For the conventional HDI, the effects of IMF are most significant, so increased roughness reduces interaction forces between the slider and disk with consequent

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benefits for HDI reliability [7]. The air bearing is a critical technology for maintaining consistent clearance throughout the HDD operating life, despite rapid changes in radial position on the disk, changing environmental factors, and manufacturing tolerances. One consideration in the design of air bearings is the dynamic pitch angle, defined as the angle between the plane of the disk and the flying slider surface. The pitch angle is largely responsible for establishing the air squeeze film which produces lift. Recently, because intermolecular forces have become stronger at lower clearance, these forces are an important consideration in the design of pitch angle. At lower pitch angles, a greater portion of the slider is close enough to the disk for IMF to be significant, and clearance is consequently reduced. Conversely, higher pitch angles can increase the effective slider-disk clearance. Larger pitch angles have consequently been associated with greater reliability through on-track durability tests [8]. C. Environmental Factors Temperature, air pressure, and humidity are the environmental parameters that commonly affect HDI clearance. Temperature affects the shape of the slider and disk as a consequence of thermal expansion of materials. The viscosity and density of air, and the energetics of the lubricant also are affected by temperature. The typical net effect is a reduction of clearance with increasing temperature [9]. Significant changes in air pressure most commonly occur because of a change in altitude. Air entrained between the head and disk generates lift that keeps the head and disk separated (Fig. 1). If ambient pressure decreases, then air density decreases, and the lift provided by the entrained air weakens. Clearance consequently decreases, creating a thinner squeeze film to compensate for the lower ambient air density. 1) The role of water HDD reliability can be severely reduced when operated under conditions of high humidity and high temperature. As an illustration of this effect, Fig. 5 the cumulative failure rates for a HDD product as a function of relative humidity (RH) at high temperature. The reason for increased failure rate at high RH has in the past been attributed to contact tribology issues associated with starting and stopping the HDD [10],[11]. But the reliability of today’s HDD products is significantly affected by RH even when operated continuously. As described below, the increased failure rate with RH results because water vapor is largely ineffective at supporting the air bearing. FIG. 5 HERE Relative humidity is a measure of the water concentration in the air at a given temperature. Mathematically we can express the relative humidity (RH) as:

% RH =

P *100 Po

(1)

where P is the partial pressure of water in the air and Po is the saturation vapor pressure. The saturation vapor pressure is the maximum amount of water vapor that can be supported by the air at a given temperature. An empirical expression relating these two quantities can be written as:

P o = 0.00611e (17.5 xT ) /( 241+T )

(2)

where Po is given in units of atmospheres, and T is the temperature in degrees Celsius. Inspection of this equation, a psychrometric chart, or chemical table [14] reveals that the vapor pressure of water in saturated air increases dramatically with temperature. By reference to (1), the partial pressure of water at a given RH shows a similar sensitivity to temperature. Should the water partial pressure exceed the saturation vapor pressure, then by definition there is more water in the air than is thermodynamically stable, and water condenses. We have recently found that the water vapor in a HDD is routinely compressed beyond the saturation vapor pressure when subjected to the high compression in the squeeze film of the air bearing (>10x compression is typical). Consider for example a HDD operating at ambient pressure (1atm), 50C, and 50%RH. The saturation vapor pressure of water at T = 50C is Po = 0.122 atm. At 50% RH, the partial pressure of water will be, Pwater = 0.5 x 0.122 atm = 0.061 atm. Following Dalton’s Law, the total pressure can be written in terms of the partial pressures of all the gaseous constituents: P = P1 + P2 + P3 + … = ΣP

(3)

If we approximate dry air to be comprised of 80% nitrogen and 20% oxygen, then we have the partial pressures shown in Table 1. Considering a 10x compression typical for a common air bearing, and assuming the saturation vapor pressure at a given temperature is independent of the external pressure, it follows that the water vapor in the compression zone becomes supersaturated. Water molecules flowing under the slider are therefore thermodynamically driven to coalesce until the partial pressure of water in the compression zone is reduced to P = Po. In the current example, coalescence of the water vapor results in a 5% drop in the total pressure (Table 1). This condensation process results in lower clearance, according to details which are published separately [12],[13]. III. EXPERIMENTAL The theoretical considerations described above predict the effects of HDI design and environment on HDD reliability, through their effects on clearance. Many of these theoretical relationships have been established through careful study of the behavior of HDI components, or of model systems, not of complete HDDs. To verify the validity of these predictions, we measure the product clearance and reliability independently and on a statistically significant number of HDD samples. The discussion below concerns the methods used to collect these data.

AA-2 Asia Pacific Magnetic Recording Conference A. Clearance measurements Several methods have been proposed to measure clearance [8],[15]-[19]. Some require an external sensor such as an acoustic emission sensor; others employ transducers internal to the HDD. Our chosen method bears some similarity to that employed in [8], and uses an internal sensor (the data reader) and an external instrument for signal processing (a digital oscilloscope). FIG. 6 HERE FIG. 7 HERE Our HDD clearance measurement system, called the Altitude Clearance Tester, includes the following hardware: a general-purpose computer with ATA interface card; a digital oscilloscope; differential probe; vacuum chamber with controller; and the HDD sample itself (Fig. 6). Most of the software controlling the HDD, oscilloscope, and vacuum chamber controller resides on the computer. Communication with the HDD is accomplished through its ATA interface, while communication with the oscilloscope and vacuum controller are accomplished through TCP-IP and RS-232 interfaces, respectively. The process flow for the Altitude Clearance Test is shown in Fig. 7. First, the temperature and humidity conditions of interest are established in the chamber containing the HDD. The clearance measurements proceed by first writing a regular, short-wavelength (about 0.15 µm) data pattern to the media using a specified head at the radial location of interest. The head’s reader is then positioned over this track, and the read-back amplitude is probed directly downstream from the HDD’s preamplifier and processed by the oscilloscope. Subsequent measurements are conducted at successively lower air pressure. The air bearing response to decreasing air pressure results in lower clearance following the mechanism described in Section 2.3. This smaller head-disk spacing is characterized by higher signal amplitude in accordance with the Wallace spacing loss equation [20], A = d exp -(2π h / λ ) (4) where h is the head-media spacing, λ is the wavelength of the recorded data pattern, A is the signal amplitude, and d is a constant value that depends on static properties of the recording system. Applying (4) to two operating conditions and computing the difference in head-media spacing for each leads to the relation ∆h = ln (A2 /A1) · λ / 2π (5) which allows us to compute the change in spacing between any two pressures. FIG. 8 HERE FIG. 9 HERE Fig. 8 shows a typical pattern of head-media spacing

4 changes in response to pressure changes for one HDD having six heads. The magnetic spacing decreases from its zero reference at 103.1 kPa air pressure to as low as -6 nm, at which point the heads contact the disk and the signal cannot be properly resolved. From this graph is obtained the clearance altitude sensitivity (0.13 nm/kPa in this case) as well as the total clearance to disk contact (5-6 nm, depending on the head). We measure the effect of temperature on clearance by repeated measurements of total clearance as described above but at several temperatures. Fig. 9 shows the temperature sensitivity of total clearance for several heads from similar HDDs (-0.035 nm/C). Many temperature effects combine to produce this overall result: lubricant response, slider deformation, disk distortion, etc. B. HDD Reliability Tests Several HDD’s are installed in an environmental chamber, each HDD being connected to a central controller providing both power and data communication. For this study, two test sequences were used to characterize the reliability of a HDD sample population: Operational Altitude Test and CSS test. The Operational Altitude Test comprises a sequence of environmental conditions and operational tasks described in Fig. 10. A sample population of HDDs is subjected to ever lower air pressure (thus simulating higher altitudes) while continuously reading and writing at randomly chosen portions of the HDD’s storage capacity. As the environment steps to higher altitude levels, clearance decreases, increasing the likelihood of failure for HDI’s having the lowest clearance in the population. Typically, an increasing number of HDDs fail as altitude increases, as described in Section 2.3, and as illustrated in Fig. 16. FIG. 10 HERE FIG. 11 HERE The CSS (Contact Start Stop) test, described schematically in Fig. 11, includes power cycle stress as well as sequential read and write tasks. After first collecting performance data at moderate environmental conditions, the environment is changed to a more extreme condition (e.g. 70C, 80%RH) for extended power cycling interrupted by occasional read and write tasks. In this test, stress remains constant over time; HDI failure occurs as a consequence of extended exposure to the stress. IV. A MODEL FOR HDD RELIABILITY Our primary objective is to develop a statistical model of the HDD reliability under the influence of all significant environmental and operational parameters. The benefits of such a model are: improved diagnosis of product reliability problems; improved capability to design for reliability using conventional technology; and the ability to evaluate candidate HDD technologies for improved reliability through a

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simulated technology integration. A. Structure of the Model Since clearance is the key factor determining the mechanical reliability of the HDI, we place population clearance statistics at the center of the model. We begin by specifying an initial clearance distribution for the population, characterized for fixed, moderate environment and operating conditions. This initial clearance distribution is represented by sample population data of the type shown in Fig. 12. FIG. 12 HERE

FIG. 15 HERE

FIG. 13 HERE Each member of the HDD population operating under these conditions is characterized by initial clearance z0. Any deviation in environment or operating condition from its initial state results in a new clearance, z, such that z = z0 + a∆T + b∆P + c∆Pw +f(S)

disturbances could include operating vibration, lubricant transfer between slider and disk, or others. The threshold for failure will depend on the design of the head-disk interface, and may also depend on environmental conditions. Ultimately, the failure threshold must be determined by reconciling the model results with HDD reliability test data. For that purpose we consider reliability test data acquired through the Operational Altitude Test (described in Section III.B), conducted at different temperatures. We apply our model to the same environmental conditions as used in the test and determine the clearance level associated with failure.

(6)

where T, P, and Pw are temperature, total air pressure, and water partial pressure, respectively, and f(S) is a function of the HDD operating condition. The sensitivities a, b, and c are determined from controlled experiments (described in Section II.A) as represented in Fig. 8 and Fig. 9. Similarly, the details of f(S) are defined through experiments on a sample HDD population under controlled conditions, for example to include the effects of operating in write mode versus read mode. Each parameter and sensitivity in (1) is associated with a distribution of values, because the response of each head-disk interface is slightly different. For example, the distribution of temperature sensitivity of our six-headed HDD has mean value -0.035 nm/C and standard deviation 0.014 nm/C, assuming a normal distribution (Fig. 9). To determine the population response to a given input change in environment or operating condition, the initial clearance distributions and distributions of clearance changes are combined by the Monte Carlo method. The new clearance distribution thus calculated represents the HDD population operating at the new, fixed condition, or distribution of conditions (Fig. 13). TABLE 2 HERE FIG. 14 HERE Table 2 and Fig. 14 show the effect of selected changes to environment on the population clearance distribution. Each change both broadens the distribution and shifts its mean to lower values. B. Failure Prediction As described in section 1.2, it stands to reason that an HDI operating at negative clearance will eventually fail. Indeed, the HDI may fail if it should operate at small, positive clearance values, because of intermittent disturbances in the HDI that cause contact when clearance is small. Such

FIG. 16 HERE Considering the tests conducted at 25C, we evaluate three candidate values of the clearance limit, i.e. the minimum clearance value for reliable operation. Compared to the experimental data plotted in Fig. 15, it appears the best fit is obtained if we assume model HDDs fail at a clearance limit 0.4 nm. That is, the model HDD can survive the Operation Altitude Test with up to 0.4 nm interference. For tests conducted at 60C, the best fit is obtained when the model failure criterion is selected as: z ≤ -1 nm, whereas at 0C, failure occurs when z ≤ 0.5 nm (Fig. 16). For tests conducted at successively lower temperatures, the best-fit failure criterion is found to match successively higher clearance values. Apparently, the HDD product requires greater clearance for reliable operation at lower temperatures. We conclude from these results that the minimum clearance required for successful HDD operation is temperature dependent. This is the first report of any such phenomenon. To understand this temperature dependence, we examine the details of the HDD failure mode (Table 3): At high temperatures, failures are due to uncorrectable errors, which are events where an error is detected and confirmed on each re-try. At lower temperatures, more failures are due to correctable errors, which are events where an error is detected, but can be recovered upon re-try. Intermittent contacts at 0.5 nm clearance are apparently more severe at lower temperatures, possibly due to stronger surface attractive forces at lower temperatures [26],[27]. At higher temperatures, such intermittent contacts may occur at 0.5 nm clearance, but are not severe enough to cause errors. TABLE 3 HERE

FIG. 17 HERE Having established the failure criteria for these three operating temperatures, failure criteria at other temperatures can be inferred from the curve in Fig. 17. For further confirmation of the model’s capacity to predict the population reliability, we consider results from CSS Tests of the kind

AA-2 Asia Pacific Magnetic Recording Conference described in Section 3.1, and summarize the results in Table 4. At 0ºC temperature, the model shows that 0.65% of HDI’s are operating below 0.5 nm clearance. Assuming any HDI operating with this clearance margin will fail, we calculate that HDD’s containing six heads will fail at the rate 3.8%, which matches closely the HDD sample failure rate 1/29, or 3.5%. At 70ºC temperature, 80% RH, the model shows 3.0% of HDI’s are operating below -1.3 nm clearance, the clearance limit at 70ºC extrapolated from the experiments at lower temperatures. The implied failure rate 16.7% for six-headed HDDs matches the actual HDD failure rate 2/10 or 20%. TABLE 4 HERE The success of the model predicting failure for HDDs operating at such variable environmental and operating conditions confirms the central thesis that clearance is the primary factor determining HDI reliability. This success also justifies using this model as the central design tool for reliable HDD products. V. CONCLUSIONS A reliability model for the HDD, until recently beyond the capabilities of applied science, is now within reach. The first steps toward producing a truly predictive model are described in the current work. The critical elements put forward are: 1.) a theoretical framework necessary to predict the impact of some slider, disk and environmental conditions on the resulting clearance; and 2.) an experimental methodology that is sensitive enough to measure the nanoscopic separations characteristic of the head disk interface. This clearance measurement methodology, along with HDD reliability test results, provide the data by which different aspects of the theoretical model can either be verified, modified, or discarded. While development of the reliability model is still in its infancy, the understanding derived to date allows us to target design parameters for greatest impact on HDD reliability. Specifically, our work suggests that sustained effort needs to be devoted to: a) alternative ABS designs with less dependence on the pressure drops associated high altitude or humid conditions, b) improved design tolerances on incoming components, e.g. flying height variance on heads, and c) selection of materials that minimize the impact of the intermolecular attractive forces, thus minimizing the frequency and severity of head-disk contacts. An important outcome of this work is the novel capability to quantitatively evaluate the impact of various design alternatives on the resulting clearance, and hence the HDD failure rates. Having such a capability will reduce the development time, and hence cost, of bringing new products to market while ensuring satisfactory reliability. TABLE 5 HERE

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[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

[25] [26]

E. Grochowsky, Data Storage Industry Roadmap, http://www.hitachigst.com/hdd/hddpdf/tech/hdd_technology2003.pdf 2003 A. Khurshudov and V. Raman, “Roughness effects on head-disk interface durability and reliability,” Tribology International, vol. 38 (67), pp. 646-651, 2005. R. J. Waltman and G. W. Tyndall, “Lubricant and overcoat systems for rigid magnetic recording media,” J. Magn. Soc. Japan, vol. 26, no. 3, pp. 97-107, 2002. R. J. Waltman, A. Khurshudov, G. Tyndall, “Autophobic dewetting of perfluoropolyether films on amorphous-nitrogenated carbon surfaces,” Trib. Lett. 12(3), pp. 163-169, 2002. A. Khurshudov, R.J. Waltman, “The contribution of thin PFPE lubricants to slider–disk spacing,” Trib. Lett. 11 (3–4) 143– 149, 2001. R.J. Waltman and A. Khurshudov, “The contribution of thin PFPE lubricants to slider-disk spacing. 2. Effect of film thickness and lubricant end groups,” Trib. Lett. 13 (3), pp. 197-202, 2002. L. Zhou, M. Beck, H. H. Gatzen, K. Kato and F. E. Talke, “Tribology of textured sliders in near contact situations,” IEEE Trans. Magn., 38, 5, pp. 2159–2161, 2002. K. Schouterden, M. Suk, V. Raman, “Influence of slider air-bearing design on disk effective take-off height,” MIPE'03 conference digest, Yokohama, JAST, 2003. D. W. Meyer, et al, “Slider with temperature responsive transducer positioning,” U. S. Patent 5,991,113, November 23, 1999. Y. Kokaku and M. Kitoh, “Influence of exposure to an atmosphere of high relative humidity on tribological properties of diamondlike carbon films,” J. Vac. Sci. Technol. A, vol. 7 (3), 1989. R. G. Walmsley, B. Natarajun, and J. Brandt, “Temperature and humidity effects in lubricant loss and recovery,” proceedings of the IEEE International Conference on Magnetics, 1993. B. D. Strom, S. Zhang, S. C. Lee, A. Khurshudov, and G. W. Tyndall, “Effects of Humid Air on Air Bearing Flying Height,” IEEE Trans. Magn., submitted for publication. S. Zhang, B. D. Strom, S. C. Lee, and G. W. Tyndall, “Calculating Air Bearing Pressure and Flying Height in a Humid Environment,” proceedings of APMRC 2006, IEEE Trans. Magn., submitted for publication. CRC Handbook of Chemistry and Physics, 59th ed. D-232 (1979). W. K. Shi, L. Y. Zhu, D. B. Bogy, “Use of readback signal modulation to measure head/disk spacing variations in magnetic disk files,” IEEE Trans. Magn., vol. MAG-23, no. 1, pp. 233-240, 1987. K. B. Klaassen and J. C. K. van Peppen, “Slider-disk clearance measurements in magnetic disk drives using the readback transducer,” IEEE Trans. Instrum. Meas., vol. 43, pp. 121–126, 1994. A. Khurshudov and P. Ivett, “Head-disk contact detection in the harddisk drives,” Wear, vol. 255 (7-12), pp. 1314-1322, 2003. G. R. Briggs and P. G. Herkart, “Unshielded capacitor probe technique for determining disk file ceramic slider flying characteristics,” IEEE Trans. Magn., Vol. MAG-7, pp. 428-431, 1971. M. Suk, T. Ishii, and D.B. Bogy, “Evaluation of capacitance displacement sensors used for slider-disk spacing measurements in magnetic disk drives,” IEEE Trans. Magn., vol. 28, no. 5, 1992. R. I. Wallace Jr., “The reproduction of magnetically recorded signals,” Bell Tech. J., pp. 1145–1173, 1951. D. P. Danson, “Pseudo-contact recording,” IDEMA Insight, vol. 9, 1996. W. C. Cain, “Modeling of various magnetoresistive head designs for contact recording,” IEEE Trans. Magn., vol. 31, pp. 2645–2647, 1995. H. Hamilton, R. Anderson, and K. Goodson, “Contact perpendicular recording on rigid media,” IEEE Trans. Magn., vol. 27, pp. 4921–4926, 1991. Y. Li and A. R. Kumaran, “The determination of flash temperature in intermittent magnetic head/disk contacts using magnetoresistive heads: Part 1—model and laser simulation,” Trans. ASME J. Tribol., vol. 115, pp. 170–184, 1993. J. K. Spong, M. M. Dovek, and G. Vurens, “Mechanically-induced readback errors in contact recording,” IEEE Trans. Magn., vol. 30, pp. 4152–4154, 1994. J. N. Israelachvilli, Intermolecular and Surface Forces, 2nd ed., Academic, New York, 1991.

AA-2 Asia Pacific Magnetic Recording Conference [27] A. W. Adamson and A. P. Gast, Physical Chemistry of Surfaces, 6th Ed., John Wiley and Sons, New York 1997.

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(a)

100%

100

Slider

Cumulative Failure Rate (%)

Suspension

8

ABS

Disk media

1 head-disk interface 2 head-disk interfaces 4 head-disk interfaces 6 head-disk interfaces

80

60

50% 40

20

0

slide r z0o - 4σ Z

Slider

Air flow

++

+

COC layer

Zzo0

Fig. 3. Cumulative failure rates predicted for HDDs having 1-6 HDIs as a function of mean clearance z0, assuming the flying height distribution is normal with standard deviation σ.

Head-disk clearance

Lubricant

Zzo0 - 2σ

Clearance ClearanceChange, change ∆zZ

Specified MTTF ( hours / 1000 )

(b)

Magnetic layer

Under layers + substrate

Fig. 1. Schematic diagrams of the head-disk interface. a.) The slider contains the read/write heads and is positioned on the disk by its suspension. The slider also includes the air bearing surface (ABS) facing the disk. b.) The slider in side view, flying over a cross-section of the disk surface (roughness exaggerated). Recording transducers write to and read from the disk magnetic layer, while carbon overcoat (COC), lubricant layers, and surface roughness limit how close to the disk the recording elements can fly.

1400 MTTF

1000

600

200 0 1985

1990

1995

2000

Date of Product Introduction Fig. 4. HDD manufacturer’s MTTF Specifications [1]

A. Nominal conditions

C. Severe interference

B. Minor interference

Probability Density Function

7.5

12.5

Mean FH Initial Clearance (z0)

5.0 2.5

DISK GLIDE AVALANCHE (GHA) DISK

12.5 10.0

10.0

∆z

7.5

Clearance (z)

5.0 2.5

Clearance (nm)

10.0

Clearance (nm)

Clearance (nm)

12.5

DISK GLIDE AVALANCHE (GHA) DISK

∆z

7.5 5.0

z= 0 nm

2.5

DISK

Fig. 2. Schematic representation of A.) the initial clearance distribution in a well-designed interface. The mean clearance of the entire slider population is denoted as z0 and is defined as: z0 = FΗ - GHA. B.), C.) Distributions with higher probability of head-disk interference. The hashed regions represent the portion of the population in contact with the disk asperities.

2005

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25'C

25'C

At each step of altitude condition, do sequential/random read/write

1.0

16Kft … 10Kft

Cumulative Failure Rate

0.8

8Kft Start

Stop

0.6 0'C

0ft

Time

-1Kft

0.4

Fig. 10. Operational altitude test profile

0.2 RH=80 %

0.0 70 ºC

50

60

70

80 RH (%)

90

100 RH=50 %

RH=50 %

25 ºC

25 ºC

Fig. 5. Cumulative failure rate as it depends on humidity, at 70C temperature. Start

Stop

50k CSS Time

Fig. 11. 50k CSS test under 70ºC/80%RH condition

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Computer 7 Temperature sensitivity = -0.035nm/ºC PATA or SATA

Clearance (nm) Clearance (nm)

6 TCP/IP

Oscilloscope

Testing drive In chamber

5 4

3

2 RS232

25

35

45

55

65

75

85

Drive Drivetemperature temperature(C)(ºC)

Fig. 9. Clearance sensitivity to temperature measured on 18 HDI’s showing average sensitivity b = - 0.03 nm/C

Altitude & Temperature controller

Fig. 6. Diagram of altitude clearance tester

Increase or decrease chamber temperature & humidity

Start Move to specified cylinder Pump down to 20,000 ft

Write specified data at specified cylinder Measure the amplitude of read-back signal for each head Use it as a reference data at specified cylinder

At specified chamber pressure Measure the amplitude of read-back signal for each head Contact is detected by either saturation or loss of read-back signals

.999

Mean=5.4nm Std.=0.6nm

.99

Pump back to ambient

Pump back to ambient

Probability

Move to another cylinder

.95

Move to another cylinder to test

Yes No

Stop

.80 .50 .20 .05

Data analysis

.01

Fig. 7. Sequence of events in altitude clearance test

.001 4

Clearance (nm)

0

5

6

Clearance (nm) Clearance Fig. 12. Anderson-Darling normal probability plot of initial clearance distribution of the target product, measured while operating in read mode at 25C, 20% RH. Insert shows clearance mean (5.4 nm) and standard deviation (0.6 nm).

2 4 6

Touch-down Altitude sensitivity = 0.13nm/kPa

8 100

90

80

70

60

Pressure (kPa)

Constant value or other distributions

Fig. 8. Pressure (altitude) sensitivity of clearance showing average sensitivity a = 0.13 nm/kPa and total clearance 5-6 nm.

Flying clearance ( nm) = z0 + a∆T + b∆P + c∆Pw [+ f ( s)]

Probability Probability

Pw =

% RH * Po 100 17.5T

P0 = 0.00611e 241+T

2

4

6

8

0

0.02

0.04

0.06

0.080.05

0.1

0.15

0.2

o

60 50 40 30 20 10 0 1.18

Frequency

Frequency

Incoming clearance (nm) sensitivity (nm/kPa) sensitivity (nm/ C) Altitude Init. Clearance (nm) Temperature Temp. sens. (nm/ºC) Alt. sens. (nm/kPa)

2.02

2.86

3.71

4.55

60 50 40 30 20 10 0 -2.00

-0.94

Equations for Humidity effect

0.12

1.18

2.24

nm

nm

T=60ºC, RH=30%, Altitude=101.3kPa No failure for 1-Ch

T=70ºC, RH=80%, Altitude=101.3kPa 3.4% failure rate for 1-Ch

Fig. 13. Graphical representation of the model structure, showing the distribution of sensitivities combined through (6) to predict clearance distributions at any operating condition.

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0.06 Temp change only

0.04

Altitude change only

0.03

120%

Temp & Altitude change

100%

0.02 0.01 0.00 -3.96 4

-1.56

3.24 4

0 0.84

8

5.64

(a)

Failure rate (%)

Probability

Ambient

0.05

80% 60%

Experiment

Clearance (nm)

20%

Fig. 14. Effect of temperature and altitude changes on flying clearance distributions, as predicted by the model. “Ambient” is initial clearance distribution at 25C, sea level; temperature and altitude changes are to 70C, 10,000 ft. For each condition, (Mean, Standard Deviation) are: Ambient (5.41, 0.62), Temp (3.83, 0.91), Altitude (1.31, 0.92), Temp & Altitude (-0.27, 1.13). The population comprised 2,000 samples.

0% 8

14

16

0.5nm

100% Failure rate (%)

Failure rate (%)

12

120%

0.0nm -0.4nm -1.0nm

80%

10

Altitude (kft)

120% 100%

-1.0nm

40%

Experiment

80% 60% 40%

(b)

E xperiment

20% 60%

0% 40%

8

10

12

14

16

Altitude (kft)

20% 0%

8

10

12

14

16

Altitude (kft)

Fig. 16. Operational Test Failure data and best-fit model results for temperatures (a) 60C, clearance limit -1.0 nm and (b) 0C, clearance limit 0.5 nm. More clearance margin may be required at lower temperatures to avoid failure.

Fig. 15. Model failure predictions for three chosen values of clearance limit: 0, 0.4, and 1.0 nm, plotted as a function of altitude. Also shown are the Operational Altitude Test failure data. The temperature for both model and test was 25ºC.

Failure criterion (nm)

0.7 Failure criterion

0.3

Linear fit

-0.1 -0.5 -0.9 -1.3 0

10

20

30

40

50

60

70

Temperature Temperature(ºC) (C)

Fig. 17. Clearance limit for reliable operation as a function of temperature, determined from Operational Altitude Test results. Greater clearance is needed for reliable operation at lower temperatures.

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TABLE 1 APPROXIMATE PRESSURES (IN ATM) OF GASEOUS COMPONENTS, A) INSIDE THE DRIVE INTERIOR, B) FOLLOWING COMPRESSION UNDER THE REAR PAD OF FLYING SLIDER, AND C) AFTER COALESCENCE OF THE WATER VAPOR DUE TO SUPERSATURATION

Condition

PNitrogen

POxygen

PWater

ΣPj

Inside drive (50%RH, T=50ºC)

0.751

0.188

0.06

1

10× compression

7.51

1.88

0.60

10

After water condensation

7.51

1.88

0.12

9.51

TABLE 2 EFFECT OF SELECTED PARAMETERS ON FLYING CLEARANCE STATISTICS, FURTHER ILLUSTRATED IN FIG. Effect Mean Standard deviation Ambient 5.41 0.62 T=70ºC 3.83 0.91 Altitude=10kft (P=70kPa) 1.31 0.92 T=70ºC, Altitude=10kft -0.27 1.13

TABLE 3 OPERATIONAL ALTITUDE EXPERIMENT DATA UNDER DIFFERENT TEMPERATURE CONDITIONS

OpAltitude test # of tested drives Failure criterion (nm) ECC error A-list error Time out error Uncorrectable error

0ºC 21

25ºC 6

60ºC 6

0.5

-0.4

-1.0

42.9% 52.4% 4.8% 52.4%

66.7 16.7% 0.2% 16.7%

100.0%

100.0%

TABLE 4 EXTENDED 50K CSS EXPERIMENT DATA AND MODEL PREDICTIONS UNDER DIFFERENT TEMPERATURE AND HUMIDITY

50k CSS test # of tested HDDs Failure criterion (nm) Experiment failure rate (%) Failure rate of model prediction (%)

0ºC 29 0.5 3.5

70ºC/80%RH 10 -1.3 20.0

3.8

16.7

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TABLE 5 NOMENCLATURE

a b c f(S) h d

λ

z z0 A P P0

Temperature sensitivity Altitude sensitivity Humidity sensitivity Clearance change with HDD operating mode Head-media spacing Constant value in (4) Wavelength of the recorded data pattern Clearance Initial clearance Signal amplitude Gas partial pressure Saturation vapor pressure

ABS ATA COC CSS GHA HDD HDI IMF MTTF RH

Air bearing surface Advanced Technology Attachment Carbon overcoat Contact start stop Glide height avalanche Hard disk drive Head-disk interface Intermolecular forces Mean time to failure Relative humidity

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