Abstract:
Methods and devices are provided that allow heads for use in disk drives to be tested in a helium environment in a cost-effective way with little or no impact on test time and with minimal consumption of helium.

Description:
TECHNICAL FIELD 
     This invention relates to methods and devices for testing heads for use in sealed hard drive assemblies. 
     BACKGROUND 
     Hard drive assemblies (HDAs), otherwise known as disk drives, are commonly used for mass storage of computer programs and data. Magnetic recording media (“disks”) are coated with a magnetic material. Data is stored (“written”) on a disk by magnetizing spots on the coating with changes in polarity corresponding to binary bits. 
     Referring to FIG. 1, the data is written and read by spinning the disk  10  while a magnetic head  12 , mounted on a load beam  16 , “flies” over the upper surface  18  of the disk. The head (a hydrodynamic air bearing slider) flies as a result of a gliding action caused by compression of a layer of air that is dragged along by the spinning disk surface. The air layer is compressed between the upper surface  18  of the disk and the adjacent lower, air-bearing surface  20  of the head. 
     The head flies very close to the surface of the disk, without touching the disk surface. Referring again to FIG. 1, the distance H between the lower, air-bearing surface  20  of the head  12  and the upper surface  18  of disk  10  is referred to as the “flying height”. 
     Recently, proposals have been made to design sealed hard drive assemblies (HDAs) that enclose a pure helium atmosphere. It is necessary to test heads that are to be used in sealed HDAs using standard single head dynamic electrical testing (DET). Because the environment in which a head is used affects the way that it will perform, e.g., by changing the flying height of the head, testing should be done with the head in a helium atmosphere. 
     Heads can be tested in a helium atmosphere by putting a hermetic chamber around the DET tester and filling it with helium. However, almost all DET testers use air bearing spindles that exhaust air, and thus the tester tends to contaminate the helium atmosphere in the chamber. Running the spindle with helium instead of air requires a heavy consumption of helium. The exhaust from the spindle could be ducted outside of the chamber, but this would be mechanically difficult to retrofit onto most common spinstands. 
     The chamber approach also requires that parts to be tested be transferred into and out of the chamber. Most production DET loads the heads onto the tester manually, so some form of access into the chamber must be provided. Opening the chamber is undesirable due to the loss of helium (typically on the order of several cubic feet), increasing the production cost of the heads. Moreover, purging the air and refilling the chamber with helium is time consuming. Test time in mass production is very expensive, costing on the order of tens of thousands of dollars per second (i.e., one second of test time is equivalent to several tester-months per program), and thus lost time also significantly increases the cost of the heads. 
     SUMMARY 
     The invention provides methods and devices that allow heads to be tested in a helium environment in a cost-effective way with little or no impact on test time and with minimal consumption of helium. In some implementations, the helium environment adds only a penny or less to the cost of a sealed drive. 
     In one aspect, the invention features a method of testing a head to be used in a sealed disk drive, including directing a flow of a gas across the head while subjecting the head to electrical testing. 
     Some implementations may include one or more of the following features. The gas includes helium. The method further includes causing the gas to flow between the head and a disk that is being used in the electrical testing. The method further includes causing the gas to flow out of a manifold. The method includes causing the gas to flow at a flow rate of from about 40 to 60 ft 3 /hr. The method further includes causing the gas to flow across the head for a predetermined time substantially equal to the time required for dynamic electrical testing, e.g., less than 30 seconds, typically less than 15 seconds. The manifold includes an exit through which the gas flows, and the method further includes positioning the exit from about 0.005 to 0.010 inch above the surface of a disk that is being used in the electrical testing. The head and equipment used for electrical testing are exposed to the ambient atmosphere. The electrical testing comprises dynamic electrical testing. The head is surrounded by a shroud. 
     In another aspect, the invention features a device for testing a head to be used in a sealed disk drive to read/write data of a storage disk. The device includes a manifold having at least one opening for providing a flow of gas from a source of gas, the at least one opening being positioned to direct the flow of gas to a surface of a storage disk positioned adjacent to the head. 
     Other features and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a diagrammatic side view of a disk and magnetic head. 
     FIG. 1A is a diagrammatic perspective view of a helium injection manifold on a Guzik HGA holder. 
     FIG. 2 is a schematic view indicating the path of helium flow under a slider. 
     FIG. 3 is a graph including saturation curves in air and with helium injection. 
     FIG. 4 is a Spectrum Analyzer trace of residual low frequency (LF) after overwrite (OVW). 
     FIG. 5 is a histogram of the LF amplitude from the traces in FIG. 4, normalized for amplitude. 
     FIG. 6 is a diagrammatic view showing the helium injection manifold of FIG. 1 used with an optional shroud. 
     FIGS. 7-7C are perspective, top, and side views of a helium injection manifold according to an alternate embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1A shows a manifold  30  for providing a helium layer between the air bearing surface  20  of a head (or “slider”)  12  and the surface  18  of disk  10  (FIG. 1; the direction of disk movement is indicated in FIG. 1A by the arrow labeled “Disk Velocity”.) The manifold  30  is mounted on a Guzik Head Gimbal Assembly (HGA) holder  33 . The helium layer  34  (FIG. 2) is localized to the slider-disk boundary layer. The helium is delivered through square tubing  16 , e.g., brass tubing. Helium is fed from the lower left (the area labeled “Helium In”). 
     Helium exits the manifold  30  through apertures  32 . The area labeled “Helium Out”, when mounted on a dynamic electric tester, is generally about 0.005 to 0.010 inch above the disk surface. The apertures  32  direct the helium flow at the disk surface, forming a “curtain” that excludes the air coming in from the direction of “Disk Velocity” (the air that is generated by the air bearing surface of the slider). The motion of the disk then “drags” the helium under the slider, as illustrated in FIG.  2 . The apertures preferably have a diameter of from about 0.0465″ (#56 drill) to 0.0625″ ({fraction (1/16)}″). The size of the apertures, the spacing between the apertures and the number of apertures will determine the relationship between the pressure and flow rate that has to be supplied to the manifold, and the efficacy of the manifold to both supply a sufficient flow of helium to support the slider and also to block the incoming air from mixing with the helium under the slider. 
     Preferably, the flow rate of helium from the manifold is from about 40 to 60 ft 3 /hr. If the flow rate is too low, air may be admitted into the helium bearing under the slider, increasing the fly-height. If the flow rate is too high, the pressure of the helium under the slider may increase, increasing the fly-height. Too high a flow rate could also produce more turbulence around the slider resulting in increased Non-repeatable Run-out (NRRO) which is an inability of the slider to remain perfectly above the track under test. 
     EXAMPLES 
     Example 1 
     Saturation Curves Indicating FH Reduction 
     In order to verify that manifold  30  generates a helium layer between the slider and disk, read/write tests were performed to measure the flying height, to determine whether the flying height was reduced to the level that would occur in a helium atmosphere. 
     FIG. 3 shows several “Saturation Curves” plotting readback measurements that were taken as a function of write current. These measurements were taken using DET equipment. The parameters measured were Track Average Amplitude (TAA) measured at a High Frequency (HF) and at a Low Frequency (LF, one-sixth of HF), Pulse-width at the 50% amplitude point (PW50) and Overwrite(OVW). TAA is a well-accepted measurement for characterizing disk drives, used to measure the efficiency of the read channel, determine the signal range required in a drive design, and provide a reference for other test methods. TAA is a parametric test for providing an average peak-to-peak amplitude measurement of equally spaced transitions of a read channel. (See, e.g., IDEMA Standards Test Methods, Document No. T3-91.) These measurements were taken first at normal atmospheric conditions, and then again with helium flowing through the manifold 30 at a rate of approx. 50 ft 3 /hr. (approx 0.4 liters/sec.). A very clear change in parametrics was observed due to the helium flow. TAA and OVW both increased, while PW50 decreased. These changes are consistent with a reduction of flying height due to helium replacing air as the gas bearing under the slider. 
     Example 2 
     Estimation of Flying Height Change Using Wallace Spacing Loss Equation 
     The change in amplitude at HF can be used to estimate the change in flying height according to the equation:          A   2     =       A   1     ·     exp        (       -   2            π        [       d   2     -     d   1       ]       /   λ       )                     d   air     -     d   he       =         -   λ     ·     ln        (       A   air     /     A   he       )           2      π                              
     Where the A n  terms are the measured amplitudes and the d n  terms are the respective flying heights. This equation was used to determine the change in flying height that could be expected using manifold  30  to deliver helium, as compared to the flying height in air. The following table presents the tester setup that was used, the measurements that were taken, and the results of applying the above equation. The wavelength is calculated by dividing the media velocity by the frequency of the written pattern. The media velocity is calculated from the radius of the track and the angular velocity (rpm) of the disk. The other terms in the Test Condition table were derived as follows: test_radius=id+(max_trk-test_trk)*trk_pitch; linear_velocity=2*pi*test_radius*spindle_velocity/60; density=2*frequency/linear_velocity; wavelength=2/density. 
     
       
         
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Tester Setup 
               
               
                   
               
             
          
           
               
                   
                 id 
                 0.73 
                 in 
               
               
                   
                 trk pitch 
                 31 
                 μin 
               
               
                   
                 max trk (id) 
                 20268 
               
               
                   
                 spindle velocity 
                 10000 
                 rpm 
               
               
                   
                   
               
             
          
           
               
                 Test Condition 
               
               
                   
               
             
          
           
               
                   
                 test trk (0 = od) 
                 0 
                   
               
               
                   
                 test radius 
                 1.358308 
                 in 
               
               
                   
                 linear velocity 
                 1422.417 
                 ips 
               
               
                   
                 frequency 
                 175 
                 MHz 
               
               
                   
                 density 
                 246.1 
                 kfci 
               
               
                   
                 wavelength (λ) 
                 8.12810 
                 μin 
               
               
                   
                   
               
             
          
           
               
                 Amplitude Measurement 
               
               
                   
               
             
          
           
               
                   
                 A he   
                 270.2 
                 μV 
               
               
                   
                 A air   
                 200 
                 μV 
               
               
                   
                   
               
             
          
           
               
                 Fly-Height-Change 
               
               
                   
               
             
          
           
               
                   
                 d air  − d he   
                 0.39 
                 μin 
               
               
                   
                   
               
             
          
         
       
     
     This test was repeated with the HGA holder  33  in a helium filled tent in order to compare the change in flying height that would result from using manifold  30  with the change that would result from using the helium filled enclosure, and thereby determine the quality of the helium bearing provided by the manifold  30 . The enclosure was approximately two feet wide, three feet deep, and one foot high. The tent was filled with helium and the proportion of residual oxygen was monitored. The oxygen content was reduced to about 5% from its normal 20%. The residual 5% oxygen was mainly due to air introduced to the chamber by the air bearing spindle; a high flow of helium would be required to eliminate the air. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Amplitude Measurement 
               
             
          
           
               
                   
                 tent 
                   
                   
               
               
                   
                 (5% O 2 ) 
                 manifold 
               
               
                   
                   
               
             
          
           
               
                   
                 A he   
                 260.15 
                 267.38 
                 μV 
               
               
                   
                 A air   
                 190   
                 189.87 
                 μV 
               
               
                   
                   
               
             
          
           
               
                 Fly-Height-Change 
               
               
                   
               
             
          
           
               
                   
                 d air  − d he   
                 0.41 
                 0.44 
                 μin 
               
               
                   
                   
               
             
          
         
       
     
     The manifold performed as well as the tent and possibly slightly better at reducing the flying height. 
     Example 3 
     Helium Injection Nozzle: Effect on T.M.R. 
     Track Mis-Registration (TMR) with and without the helium flow was characterized by measuring residual LF signal after being overwritten by HF with the reader centered over edge of track. The track was first written with the LF pattern and then overwritten by HF in air. The residual LF was then measured with a spectrum analyzer configured to measure at only the fundamental frequency of the LF in air and then again with helium flow. The spectrum analyzer trace was triggered by the spindle index. The reader was positioned over the edge of the track as determined by a track profile of the residual LF. FIG. 4 plots the “once around the revolution” amplitudes for the case of air, helium, and the difference between the two. 
     The difference in amplitude is due to the lower fly-height in helium. The modulation is due to Non-Repeatable Run-out (NRRO). The NRRO is very similar for both helium and air. Table 3 analyzes the statistics of both traces. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 He 
                 Air 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Mean 
                 0.004293 
                 0.003612 
               
               
                   
                 Standard Error 
                 0.000029 
                 0.000028 
               
               
                   
                 Median 
                 0.004266 
                 0.003548 
               
               
                   
                 Mode 
                 0.004198 
                 0.003475 
               
               
                   
                 Standard Deviation 
                 0.000504 
                 0.000486 
               
               
                   
                 Sample Variance 
                 2.54E − 07 
                 2.36E − 07 
               
               
                   
                 Kurtosis 
                 3.296043 
                 1.237789 
               
               
                   
                 Skewness 
                 −0.455148   
                 −0.026398   
               
               
                   
                 Range 
                 0.004310 
                 0.003564 
               
               
                   
                 Minimum 
                 0.001300 
                 0.001300 
               
               
                   
                 Maximum 
                 0.005610 
                 0.004864 
               
               
                   
                 Sum 
                 1.292291 
                 1.087269 
               
               
                   
                 Count 
                 301      
                 301      
               
               
                   
                 Confidence Level (95.0%) 
                 5.71E − 05 
                 5.51E − 05 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 5 is a histogram of the LF amplitude from the traces in FIG. 4, normalized for amplitude. The two are very close, indicating that the helium flow is having very little impact on TMR performance. 
     Example 4 
     Helium Cost for Prototype Manifold 
     The flow rate of helium in the previous experiments was approximately 50 cubic feet per hour (approx. 0.4 liters per second). In mass production typical DET test time is less than 30 seconds of actual on-disk time, and in fact most tests can be performed in less time than that. Assuming the helium only needs to flow for 15 seconds per head, the volume of helium required is about 0.2 ft 3  (5.7 liters) per head. This is summarized in Table 4. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Helium Consumption Estimate 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 helium flow rate 
                 48.13 
                 cu. ft./hr 
                 0.3785 
                 liters/sec 
               
               
                 test-time per head 
                   
                   
                 15 
                 sec 
               
               
                 helium per head 
                 0.2005 
                 cu-ft. 
                 5.678 
                 liters 
               
               
                   
               
             
          
         
       
     
     The next table (Table 5) breaks down the cost of this volume of helium based on 2 types of purchase; small quantities delivered in 4 foot tall steel bottles, and large volume delivered in a truck sized tank. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Cost Estimate (@ 2 cu-ft per head) 
               
             
          
           
               
                   
                 container 
                 bottle 
                 truck 
               
               
                   
                   
               
             
          
           
               
                   
                 volume 
                 291 
                 45000 
                 cu-ft 
               
               
                   
                 @ 
                 $34.36 
                 $7.50 
                 per 100 cu-ft 
               
               
                   
                 cost per container 
                 $100.00 
                 $3,375.00 
               
               
                   
                 heads/container 
                 1,451.37 
                 224,438.90 
               
               
                   
                 cost per head 
                 $0.069 
                 $0.015 
               
               
                   
                   
               
             
          
         
       
     
     It may be possible to reduce this cost even further. The easiest approach would be to reduce the time that the helium is flowing. There are only a few parameters in DET testing that are critically dependent on flying height. Using the manifold  30 , the helium flow can be isolated to just those tests that are dependent on flying height, reducing consumption to just a few seconds. 
     The flow rate may be reduced by trapping the helium around the HGA by means of a small “shroud” placed around the HGA, as illustrated in FIG.  6 . For example, one suitable shroud design is approximately 1 inch wide, 1 inch long and about 0.156″ thick, and has a polygonal shape similar to “home plate” used in baseball. 
     Other embodiments are within the scope of the following claims. 
     For example, an alternate manifold configuration is shown in FIGS. 7-7C. Manifold assembly  100  includes a body  101  having an inlet  102  constructed to receive a fitting for delivery of gas to the manifold. Inlet  102  may be threaded to allow the fitting to be screwed onto the manifold assembly. Inlet  102  is in fluid communication with an angled bore  104  which is constructed to deliver gas from the inlet  102  to the U-shaped manifold  106 . Bore  104  extends at an angle. The U-shaped manifold  106  includes a plurality of apertures  108 , through which gas is delivered to the disk surface. Angled bore  104  is preferably formed by machining a trough in the body  101 , and covering the trough with a cover that is secured in place, e.g., with adhesive. This two-part construction allows the manifold assembly to be relatively small and have a low profile, allowing the manifold assembly to fit in the fixturing that holds the HGA on different testers, both DET and fly-height testers. 
     Moreover, while helium has been used as the gas in the detailed description above, other gases may also be used in the methods and devices described herein.