Abstract:
In accordance with various embodiments, a data storage device comprises a sealed housing which encloses a data storage medium. The sealed housing comprises a substantially planar first housing plate with respective length, width and thickness dimensions and a circumferentially extending first peripheral edge, a substantially planar second housing plate with a circumferentially extending second peripheral edge, and a circumferentially extending first sealing member contactingly disposed between the respective first and second peripheral edges. A substantially planar third housing plate extends adjacent the first planar housing plate with respective length and width dimensions substantially corresponding to the length and width dimensions of the first housing plate, and a circumferentially extending third peripheral edge. A circumferentially extending second seal member is contactingly disposed between the second and third peripheral edges to form a hermetic seal for the low density gas.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/185,797 filed Jun. 27, 2002, now U.S. Pat. No. 7,218,473, and claims priority of U.S. provisional application Ser. No. 60/366,972, filed Mar. 22, 2002, entitled Method and Apparatus for Testing and Sealing a Helium Filled Disk Drive. 
    
    
     BACKGROUND OF THE INVENTION 
     A disc drive typically includes a base deck to which various components of the disc drive are mounted. A top cover cooperates with the base to form a housing that defines an internal, sealed environment for the disc drive. The components include a spindle motor, which rotates one or more discs at a constant high speed. Information is written to and read from tracks on the discs through the use of an actuator assembly. The actuator assembly includes actuator arms, which extend towards the discs, with one or more suspensions or flexures extending from each of the actuator arms. Mounted at the distal end of each of the flexures is a read/write head, which includes an air bearing slider enabling the head to fly in close proximity above the corresponding surface of the associated disc. 
     Disc drives are constructed in a clean room environment to prevent contaminants from entering the drive prior to the final assembly of the drive. Thus, the atmosphere within the assembled disc drive is typically that of the clean room, i.e., the filtered room air that is trapped within the drive once the cover is sealed to the base. While the seal between the base and the cover is sufficient to keep contaminants from entering the drive, it is possible for air and other gases to seep past (or permeate through) the seal and either enter or exit the drive. However, such small gas leaks are not an issue since most drives include a filtered port to equalize the air pressure within the drive to that of the ambient air pressure in order to prevent large stresses from being applied to the drive (such as during air shipment of the disc drive where the ambient air pressure is relatively low). 
     While air filled disc drives are currently prevalent, it is known that filling disc drives with low-density gases other than air (i.e., a gas such as helium having a lower density than air at similar pressures) can enhance drive performance. For example, helium (or another low density gas) can reduce the aerodynamic drag experienced by the spinning discs within the drive, thereby reducing the power requirements for the spindle motor. A helium filled drive thus uses substantially less power than a comparable disc drive that operates in an air environment. Additionally, the reduction in drag forces within the helium filled drive also reduces the amount of aerodynamic turbulence that is experienced by the drive components such as the actuator arms, the suspensions and the heads. These reductions in spindle motor power and “air” turbulence allow drives filled with low density gases to be operated at higher speeds than conventional air filled drives while maintaining the same tolerances (e.g., the same percentage of read/write errors). Additionally, helium filled drives may allow for higher storage capacities (i.e., higher recording densities) due to the fact that there is less turbulence within the drive and the heads may fly more closely to the disc surface. 
     Despite the advantages of helium filled drives, such drives have not been commercially successful. This is mainly due to problems associated with the helium (or other low density gas) leaking from the drives over time. Unlike air filled disc drives, helium filled drives do not include a filtered port to equalize the pressure within the drive to the ambient pressure. However, while prior art helium drives are completely sealed, it is still possible for the helium gas to leak out past the conventional rubber gasket seals used to seal the top cover to the drive base. Such leakage is not surprising given the relatively smaller size (lower atomic weight) of the helium atoms in comparison to the constituent gases found in air (i.e., nitrogen and oxygen). That is, the rubber gasket seals on prior art drives allow the relatively smaller helium atoms to diffuse through the rubber membrane. Indeed, such prior art gaskets do not provide hermetic seals with respect to air (i.e., the gaskets are also permeable to the larger atoms of nitrogen and oxygen in air) since it is air that typically displaces the helium gas that leaks from the drive. 
     As noted above, the prior art gasket seals are only intended to keep relatively large contaminants such as dust or smoke from the interior of the drive. Such gasket seals are preferred to other, more permanent methods of sealing a drive for two main reasons. First, the seals do not outgas and thus do not contribute to the contamination of the interior of the drive. Secondly, the seals may be reused if necessary during the assembly of the disc drive, such as when an assembled drive fails to pass certification testing and must be “reworked.” Reworking a drive typically entails removing the top cover from the base and replacing a defective disc or read/write head while the drive is still in the clean room environment. The reworked drive is then reassembled using the same rubber gasket positioned between the base and the top cover. Unfortunately, while such gasket seals are convenient, they simply do not provide a sufficient hermetic seal to maintain the required concentration of helium (or other low density gas) within the disc drive over the service life of the drive. 
     As helium leaks out of a disc drive and is replaced by air, the drive is subjected to undesirable operational effects possibly leading to failure of the drive. For example, the increased concentration of air may increase the turbulent forces on the drive heads due to the increased drag forces within the drive, and may further cause the heads to fly at too great a distance above the discs, thereby increasing the instances of read/write errors. The risk of unexpected failure due to inadequate amounts of helium is a considerable drawback to helium filled disc drives, particularly since the data stored within the disc drive can be irretrievably lost if the disc drive fails. 
     Accordingly there is a need for an improved disc drive that can effectively prevent helium (or another low density gas) from leaking out of the drive once the drive is finally assembled. The present invention provides a solution to this and other problems, and offers other advantages over the prior art. 
     SUMMARY 
     In accordance with various embodiments, a data storage device comprises a sealed substantially planar first housing plate with respective length, width and thickness dimensions and a circumferentially extending first peripheral edge, a substantially planar second housing plate with a circumferentially extending second peripheral edge, and a circumferentially extending first sealing member contactingly disposed between the respective first and second peripheral edges. 
     A substantially planar third housing plate extends adjacent the first planar housing plate with respective length and width dimensions substantially corresponding to the length and width dimensions of the first housing plate, and a circumferentially extending third peripheral edge. A circumferentially extending second seal member is contactingly disposed between the second and third peripheral edges to form a hermetic seal for the low density gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a disc drive incorporating a preferred embodiment of the present invention including a dual cover system for sealing the disc drive, and showing the primary internal components of the disc drive. 
         FIG. 2  is an exploded view of the disc drive shown in  FIG. 1  illustrating a first embodiment of the dual cover system of the present invention and further illustrating a fill port for filling the drive with a low density gas. 
         FIG. 3  is an enlarged section view taken substantially along the line  3 - 3  in  FIG. 1 . 
         FIG. 4  is an exploded view similar to  FIG. 2  illustrating a second embodiment of the dual cover system of the present invention. 
         FIG. 5  is an enlarged section view taken substantially along the line  5 - 5  in  FIG. 4 . 
         FIG. 6  is an exploded view similar to  FIGS. 2 and 4  illustrating a third embodiment of the dual cover system of the present invention and further illustrating a side-mounted fill port for filling the drive with a low density gas. 
         FIG. 7  is an enlarged section view taken substantially along the line  7 - 7  in  FIG. 6 . 
         FIG. 8  is an elevated side view of the disc drive shown in  FIG. 6  illustrating an outer cover of the dual cover system sealing the side-mounted fill port. 
         FIG. 9  is a flow chart of a general method of filling a disc drive with a low density gas and sealing the disc drive according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A disc drive  100  constructed in accordance with a preferred embodiment of the present invention is shown in  FIG. 1 . The disc drive  100  includes a base  102  to which various components of the disc drive  100  are mounted. A structural cover  104 , shown partially cut away, cooperates with the base  102  to form a housing that defines an internal environment for the disc drive in a conventional manner. The drive components include a spindle motor  106 , which rotates one or more discs  108  at a constant high speed. Information is written to and read from tracks on the discs  108  through the use of an actuator assembly  110 , which rotates during a seek operation about a bearing shaft assembly  112  positioned adjacent the discs  108 . The actuator assembly  110  includes a plurality of actuator arms  114  which extend towards the discs  108 , with one or more flexures  116  extending from each of the actuator arms  114 . Mounted at the distal end of each of the flexures  116  is a head  118 , which includes a slider enabling the head  118  to fly in close proximity above the corresponding surface of the associated disc  108 . 
     During a seek operation, the track position of the heads  118  is controlled through the use of a voice coil motor  124 , which typically includes a coil  126  attached to the actuator assembly  110 , as well as one or more permanent magnets  128 , which establish a magnetic field in which the coil  126  is immersed. The controlled application of current to the coil  126  causes magnetic interaction between the permanent magnets  128  and the coil  126  so that the coil  126  moves in accordance with the well-known Lorentz relationship. As the coil  126  moves, the actuator assembly  110  pivots about the bearing shaft assembly  112 , and the heads  118  are caused to move across the surfaces of the discs  108 . 
     The spindle motor  106  is typically de-energized when the disc drive  100  is not in use for extended periods of time. The heads  118  are moved over a park zone  120  near the inner diameter of the discs  108  when the drive motor is de-energized. The heads  118  are secured over the part zone  120  through the use of an actuator latch arrangement, which prevents inadvertent rotation of the actuator assembly  110  when the heads are parked. 
     A flex assembly  130  provides the requisite electrical connection paths for the actuator assembly  110  while allowing pivotal movement of the actuator assembly  110  during operation. The flex assembly includes a printed circuit board  132  to which head wires (not shown) reconnected; the head wires being routed along the actuator arms  114  and the flexures  116  to the heads  118 . The printed circuit board  132  typically includes circuitry for controlling the write currents applied to the heads  118  during a write operation and a preamplifier for amplifying read signals generated by the heads  118  during a read operation. The flex assembly terminates at a flex bracket  134  for communication through the base deck  102  to a disc drive printed circuit board  136  ( FIG. 2 ) mounted to the bottom side of the disc drive  100 . 
       FIGS. 1 and 2  further illustrate a second, sealing cover  150  secured over top of the first structural cover  104 . The use of the second sealing cover  150  in combination with the structural cover  104  provides the requisite degree of sealing necessary to prevent the helium within the drive  100  from leaking out during the service lifetime of the drive. While  FIGS. 1-3  show a first embodiment  150  of the sealing cover, alternative embodiments are described in  FIGS. 4-8 . Additionally,  FIG. 9  describes a preferred method for assembling and testing the drive  100  utilizing the dual cover system. 
       FIG. 2  is an exploded view of the disc drive  100  and the two covers  104  and  150 . It is noted that the base  102  includes a raised contact surface or shoulder  160  (best shown in the cross section of  FIG. 3 ) that extends around a perimeter of the base deck  102  to provide a mating surface for a perimeter of the structural cover  104 . The cover  104  is preferably sculpted to match the shape of the contact surface  160 , while screw holes  162  in the cover  104  match corresponding holes  164  in the raised shoulder  160  so that a plurality of screws  166  can be used to secure the cover  104  to the shoulder  160 . 
     Prior to screwing the cover  104  to the base deck  102 , a seal  170  is preferably placed on one of the raised shoulder  160  of the base  102  or the underside perimeter of the cover  104 . In the preferred embodiment shown in  FIG. 2 , a continuous rubber gasket seal  170  is positioned around the perimeter of the raised shoulder  160  so that the seal  170  engages the bottom surface of the cover  104  to seal the internal environment of the disc drive  100  against contaminants. The seal  170  is preferably arranged so that the screw holes  164  are positioned outside the perimeter of the seal  170  to prevent gas or contaminant leakage past the threaded screws  166 . 
       FIG. 2  further illustrates that the structural cover  104  preferably provides structural support for the spindle motor  106  and the bearing shaft assembly  112  of the disc drive  100 . Specifically, the cover  104  includes a countersunk openings  172  and  174  that correspond to the spindle motor  106  and the bearing shaft assembly  112 , respectively. The importance of countersinking the holes  172  and  174  (in order to maintain a substantially flat top surface of the cover  104 ) is described below. However, the inclusion of the holes  172  and  174  allows the structural cover  104  to stabilize the spindle motor  106  and the actuator assembly  110  in a known manner. Additionally, the screws (not shown) holding the spindle motor  106  and the bearing shaft assembly  112  to the cover  104  include a pre-attached o-ring to provide a substantially gas tight seal with the corresponding holes  172  and  174  of the structural cover  104 . 
       FIG. 2  also illustrates one preferred location for a fill port  176  within the structural cover  140 . The fill port  176  retains one end of a gas valve  180 , such as a conventional Schrader valve, while the opposite end of the valve  180  extends into the internal environment of the disc drive  100 . Because the valve  180  extends downward from the cover  104 , the fill port  176  is preferably located over a portion of the base deck  102  that does not contain either the discs  108  or the actuator assembly  110 . As noted below, the fill port  176  need not be located in the cover  104  provided that the fill port  176  is located in a position that can be hermetically sealed. The fill port  176  is also preferably countersunk within the cover  104  so that a chuck (not shown) can mate with the valve  180  to fill the internal environment of the drive  100  with the helium. That is, once the cover  104  has been secured to the base deck  102  with the screws  166 , and once additional screws (not shown) have been secured to the spindle motor  106  and the bearing shaft assembly  112  through the holes  172  and  174 , respectively, a source of helium (or another low density gas) is preferably connected to the valve  180  to fill the interior of the drive with the gas. The gas supply system preferably provides a method of evacuating the drive before filling the drive with helium gas. 
     As described in greater detail below with respect to  FIG. 9 , the helium (or other low density) gas is preferably supplied until a concentration of greater than 99 percent helium is attained and the pressure of the gas within the drive is approximately one atmosphere (at sea level). Once filled with the low density gas, the disc drive  100  is subjected to a certification process that typically lasts 24 hours or less. This process is designed to simulate real-world disc drive activity at actual operating temperatures. For example, the certification process may include conducting numerous random seek operations at an elevated temperature (simulating temperatures found within a computer), followed by numerous read/write operations. If a number of read/write errors detected during the certification process exceeds a predetermined maximum number, or if testing confirms that a particular head  118  or disc  108  is defective, then the drive  100  is typically “reworked” in an effort to correct the errors. 
     Reworking a drive entails removing the structural cover  104  from the base deck  102  and then repairing or replacing the defective component. Once repairs are complete, the cover  104  is repositioned on the raised shoulder  160  to engage the seal  170 , and the screws  166  are replaced to re-seal the drive  100 . The seal  170  is preferably formed from a rubber polymer which can be reused after a rework procedure. However, if the seal  170  is damaged as the drive  100  is reworked, the seal  170  may be replaced with a new seal. Additionally, the seal  170  preferably does not contribute any outgassed components to the interior of the drive  100  either during the rework process or after the cover  104  is secured to the base  102 . This is in contrast to prior art tape seals where the adhesive coating would likely outgas compounds within the drive interior. Furthermore, tape seals must often be cut to open the drive if rework of the drive is required following certification testing. Cutting a tape seal in this manner increases the danger that small pieces of the seal may contaminate the drive interior during the rework process. Additionally, a new tape seal would be required to seal the drive  100  once the drive had been successfully reworked. 
     Thus, the gasket seal  170  represents an improvement over prior tape seals in that the gasket seal  170  is “clean” and can be reused if the drive must be reworked (is it estimated that 5-20 percent of drives on a typical manufacturing line must be reworked to some extent). Furthermore, it is preferred that the seal  170  be relatively impervious to the passage of helium (or other low density gas) there through, at least with respect to the short time period (e.g., 24 hours) required to conduct certification testing on the drive. That is, the seal  170  should be sufficiently impervious to helium so that there will be only an insignificant amount of leakage during the approximate one day certification testing period. A preferred gasket seal is manufactured by the Dyneon Corp. of North Oakdale Minn. and has a leak rate of less than 1×10 −6  cubic centimeters per second (“cc/sec”) for a disc drive having an internal volume of approximately 100 cubic centimeters. Indeed, the seal  170  preferably has a leak rate of less than 1×10 −7  cc/sec of helium in one preferred embodiment. Similarly, investigation has shown that the Schrader valve  180  used to fill the drive with helium typically has a leak rate of approximately 1×10 −8  cc/sec of helium, or approximately one to two orders of magnitude less than the seal  170 . 
     Thus, although the seal  170  represents the largest potential source for leakage of helium from the drive  100 , the small amount of leakage described above will not reduce the concentration or pressure of the helium gas to any appreciable extent over the course of a 24 hour testing period. However, the leak rate from the seal  170  is sufficient to impact the operation of the drive  100  if the leak is allowed to continue past the testing phase. For example, assuming the seal  170  leaks helium at a rate of 1×10 −7  cc/sec, it would take approximately 115 days for a single cubic centimeter of helium to leak from the drive  100 . Thus, if the concentration of the helium within the drive must remain above 99 percent, and assuming the concentration was 100 percent immediately after the drive was filled, the drive  100  would be operating outside of specifications within four months if the drive relied only on the seal  170 . 
     While it is possible to provide internal helium reservoirs or other systems for periodically refilling the drive  100  with helium, a better solution to the problem of leaking seals is to provide a true hermetic seal that prevents any appreciable leakage of helium from the drive  100 . The present invention provides such a hermetic seal through the use of a second sealing cover  150  as described above.  FIGS. 2 and 3  illustrate one embodiment  150  of the sealing cover that is installed on the drive  100  once the drive has passed certification testing.  FIG. 2  illustrates that the embodiment  150  of the sealing cover is a thin-walled metal cover having a flat top surface  184  and downward depending sides  186 . The cover  150  may be formed from aluminum or brass, provided that it is substantially impervious to helium or other low density gases. 
     In the preferred embodiment shown in  FIG. 3 , the cover  150  is attached to the drive  150  within a gap or groove  188  formed between a raised outer edge  190  of the base  102  and an outer surface  192  of the structural cover  104 . That is, the cover  104  is supported on the raised shoulder  160  of the base deck  102  so that the seal  170  is compressed between a lower surface of the cover  104  and the raised shoulder  160 . The cover  104  is preferably sized to form the gap or groove  188  that runs along the entire perimeter of the cover  104  between the outer surface  192  of the cover and the raised outer edge  190  of the base deck. A liquid adhesive  196  is then preferably injected to a predetermined depth within the groove  188  to allow for the later insertion of the sides  186  of the cover  150 . Once the groove  188  is filled all the way around with the adhesive  196 , the sealing cover  150  is applied over the top of the structural cover  104  so that the downward depending sides  186  of the cover extend into the adhesive-filled groove  188 , as shown in  FIG. 3 . The adhesive  196  surrounds the sides  186  of the sealing cover  150  and cures quickly to form a hermetic seal about the perimeter of the structural cover  104 . Indeed, as shown in  FIG. 3 , the adhesive  196  abuts the rubber seal  170  and thereby prevents any leakage of the helium gas through the rubber seal  170 . 
     In one preferred embodiment the adhesive is an epoxy mixture such as that manufactured by Loctite Corp. of Rocky Hill, Conn., under the name Hysol® E-20NS. This medium-viscosity epoxy cures at room temperature in a relatively short time (about 20 minutes) and thus can be used without any special heating or cooling equipment. In a preferred embodiment, the application of the epoxy adhesive  196  within the groove  188  is performed by an automated machine (not shown) that produces a uniform bead of adhesive to prevent any spillage (excess) or any gaps in coverage within the groove  188 . 
     An underside of the sealing cover  150  preferably includes a pressure sensitive adhesive (“PSA”) so that the cover  150  seals tightly against the sop surface of the structural cover  104 . As discussed above, the structural cover  104  preferably presents a substantially flat top surface, and any openings in the top cover (such as the screw holes  162  and the openings  172 ,  174  and  176 ) are preferably countersunk so that the screw heads do not extend above the top surface of the cover  104 . In this manner, the bottom surface of the sealing cover  150  may sit flush against the top surface of the structural cover  104  ( FIG. 3 ), and the PSA applied to the sealing cover  150  will help to keep the cover  150  in place while the adhesive  196  cures within the groove  188 . Alternatively, a similar epoxy adhesive could be applied to one of the top surface of the structural cover  104  or the bottom surface of the sealing cover  150  in place of the PSA. 
     Additionally, in order to allow air to escape as the sealing cover  150  is placed over top of the structural cover  104 , an air hole  197  may optionally be formed in the sealing cover as shown in  FIG. 2 . The air hole  197  allows any air trapped between the two covers  104  and  150  to escape as the cover  150  is applied, thereby preventing air bubbles from being trapped between the covers. Of course, the air hole  197  must itself be sealed to maintain the overall hermetic seal of the drive  100 , and thus a metallic patch seal  198  is preferably applied over top of the air hole  197 . The patch seal  198  is preferably formed from the same material as the cover  150  itself and the same epoxy adhesive may be used to secure the patch seal  198  over the hole  197 . Alternatively, the seal  198  may be coated with a PSA or an adhesive sticker may be used as the patch seal  198 . 
     As noted above, the sealing cover  150  may be formed from a number of materials that are impervious to helium, although it is preferred to use an aluminum or a brass material having a thickness of approximately 0.010 inches. In this manner, the cover  150  will only add approximately ten thousandths of an inch to the overall height of the drive  100 , which additional height will not likely impact the ability of the given drive to meet the required form factor dimensions. 
     Thus, the sealing cover  150  works in conjunction with the structural cover  104  to provide a hermetic seal only after the drive  100  has passed certification testing. In this manner, the more traditional rubber seal  170  of the structural cover  104  will hold the helium gas within the drive for a short period of time (i.e., will allow an insignificant amount of helium gas leakage) while allowing for a relatively simple rework process if the drive  100  does not pass certification testing. That is, by waiting until the drive has been certified before adding the more permanent sealing cover  150 , manufacturers will not be forced to break the hermetic seal to rework a disc drive. Furthermore, by providing both a structural cover  104  and a sealing cover  150 , the drive can be fully tested prior to the application of the sealing cover  150  since the cover  150  is not necessary for proper operation of the drive  100 . Additionally, by waiting until the structural cover  104  and the rubber seal  170  are in place before applying the adhesive  196 , there is no concern that the adhesive may outgas or otherwise contaminate the interior of the drive  100  since the inner seal  170  will prevent such contamination. Thus, the use of a “dirty” sealant (such as the epoxy adhesive  196 ) is possible with the present invention, while prior art (single cover) drives would not be able to use such “dirty” seals due to outgassing concerns. Furthermore, the sealing cover  150  not only prevents leakage through the rubber seal  170  but also prevents leakage through the other openings formed in the structural cover  104 , and particularly through the fill port  176  for the valve  180 . 
     Thus, the cover  150  creates a hermetic seal that will maintain desired concentrations of helium (or other low density gases) within the drive  100  over the operational lifespan of the drive. For instance, experiments have shown that disc drives constructed as described above will leak helium as such law rate that it would take over  70  years for the helium concentration to drop below a predetermined lower limit. 
     A second embodiment  200  of the sealing cover is shown in  FIGS. 4 and 5 , where the parts are referred to with the same reference numerals used above. Specifically, the drive  100  includes substantially the same base deck  102  and structural cover  104 , as well as the same rubber seal  170  extending around the perimeter of the base deck  102 . Furthermore, the perimeter of the base deck  102  includes a raised outer edge  190  that extends upward to the same level as the top surface of the structural cover  104 , as shown in  FIG. 5 . Thus, when the cover  104  is assembled to the base  102  as described above, a similar gap  188  is preferably maintained between the outer surface  192  of the cover  104  and the upper edge  190  of the base deck  102 . However, due to the specific design of the alternative sealing cover  200 , the gap or groove  188  need not be as large as that shown in  FIG. 3  with respect to the first embodiment  150  of the sealing cover. This is because the sealing cover  200  shown in  FIGS. 4 and 5  does not include an edge that sits within the gap  188  but rather comprises a substantially flat sheet that is soldered to a top surface  202  of the raised outer edge  190  of the base  102 . 
     Specifically, the cover  200  is preferably formed from a piece of tin-plated brass having a thickness of approximately 0.010 inches. While it is possible to plate only the edges of the brass cover (as only these edges will be soldered), it is more economically viable to stamp the cover  200  from a role or sheet of pretinned brass. Of course, one skilled in the art may substitute alternative metals for the brass sheet provided that the metals may be soldered to the base plate  102 . Additionally, in order to enhance the solder connection to the base plate  102 , the entire aluminum base deck  102  is preferably provided with an electroless nickel coat. 
     The preferred method for attaching the cover  200  to the top surface  202  of the edge  190  of the base deck is to pre-coat the top surface  202  with a solder paste (a tin-bismuth paste is preferred) and then place the cover  200  on top of the cover  104  as shown in  FIGS. 4 and 5 . As described above, the bottom surface of the cover  200  preferably includes a PSA which engages the top surface of the substantially flat structural cover  104  to maintain the sealing cover  200  in place during the soldering process. A hot shoe (not shown) is then run along the seam between the cover  200  and the raised outer edge  190  of the base deck to melt the solder paste and form the solder joint. The hot shoe preferably has sufficient thermal mass (i.e., a starting temperature of approximately 600 degrees Fahrenheit) to quickly raise the temperature of the joint above the melt point for the solder (approximately 280 degrees Fahrenheit). In this manner, the joint is formed quickly (i.e., the heat is applied for a relatively short period of time) so that the overall temperature of the base deck  102  is not appreciably raised. A short solder time is important to prevent the electronic circuitry within the drive  100  (and on the printed circuit board  132 ) from being damaged by the elevated temperatures. The ability to quickly form the solder joint (i.e., the ability to quickly raise the temperature of the joint) is aided by the fact that the raised outer edge  190  of the base deck  102  is preferably thin (approximately 0.05 inches) so that the top surface  202  of the edge  190  may be quickly heated. Indeed, experiments have shown that the joint temperature is raised to approximately 450 degrees Fahrenheit in approximately 0.5 seconds. Additionally, the relatively thin outer edge  190  helps to limit thermal conductivity from the edge  190  into the remainder of the base deck. This fact, plus the small air gap  188  between the outer edge  190  and the structural cover  104 , helps to insulate the solder joint from the other components of the disc drive  100 . 
     Thus, the alternative sealing cover  200  functions like the cover  150  shown in  FIGS. 2 and 3  to form a hermetic seal by sandwiching the structural cover  104  and the rubber seal  170  between the base deck  102  and the sealing cover  200 . As described above, the cover  200  preferably includes a air hole  197  and a separate small cover  198  to seal the air hole. Furthermore, while specifically preferred materials have been described above for the cover  200 , the base plate coating and the solder paste, it is understood that those skilled in the art may substitute alternative materials for one or all of these components. The present invention is considered to cover all such substitute components provided that the drive includes a second cover  200  sealed over top the first cover  104  to prevent helium gas from leaking through the first cover or the seal  170  between the first cover  104  and the base deck  102 . 
     While the adhesive embodiment shown in  FIGS. 2-3  is presently preferred by the inventors, the choice between the two embodiments of the sealing cover  150  and  200  may turn on whether there is a preference for soldering or using an epoxy adhesive on the drive assembly line. In either case, it is possible to apply the final cover (either  150  or  200 ) outside of the clean room environment since the drive  100  is sufficiently well sealed by the rubber seal  170  to prevent contaminants from entering the drive. 
     Both of the embodiments of the present invention shown in  FIGS. 1-5  disclose a fill port  176  positioned in the structural cover  104  so that the valve  180  extends downward into the interior volume of the drive. However, an alternative fill port  210  ( FIGS. 6-8 ) may be formed on one of the sides or ends of the disc drive  100  so that a Schrader valve  212  extends laterally as opposed to vertically into the interior of the drive, as shown in  FIG. 6 . In such an embodiment, an alternative embodiment  220  of the sealing cover preferably includes downwardly depending sides  222  that extend outside of the outer edge  190  of the base deck. In this manner, the sides  222  may extend down beyond the position of the fill port  210  to hermetically seal the fill port, as shown in  FIG. 8 . 
     A further reason to utilize the alternative sealing cover  220  may be that there is simply insufficient room within the interior of the drive  100  to accommodate the sealing covers  150  and  200 . That is, the cover  150  requires a groove  188  that is sufficiently wide to hold the downward depending edges  186  of the cover on either side of the discs  108 , while the alternative cover  200  requires a sufficiently wide top surface  202  of the outer edge  190  to provide a base for the solder joint. Thus, in both of these cases, the discs  108  must be of a sufficiently small diameter to provide the necessary clearance on either side of the discs (given the fixed form factor of the drive  100 ) to accommodate the sealing covers  150  and  200 . This is typically not a problem with high performance drives (which will most likely benefit from the performance increase of a helium environment) since these drives typically utilize smaller, faster rotating discs than those found in more mass market-oriented drives. For example, within a 3.5 inch form factor drive, the diameter of discs  108  that are typically found in a high performance drive rotating at 10,000 revolutions per minute (“RPM”) or higher is approximately 84 millimeters, while the diameter of discs  108  found in drives with a rotational speed of 5,400 or 7,200 RPM is approximately 95 millimeters. Thus, the larger diameter of the slower-speed discs may require too much room within the drive interior to accommodate either of the covers  150  or  200  described above. 
     Therefore, in those cases where an opening on one of the sides or ends of the drive needs to be hermetically sealed (such as the fill port  210 ), or where there is insufficient room to accommodate the covers shown in  FIGS. 1-5 , the alternative sealing cover  220  is preferably used. As with the covers  150  and  200 , the sealing cover  200  is preferably formed from an aluminum or brass sheet having a thickness of approximately 0.010 inches. Additionally, the downward depending sides  222  of the cover  220  are preferably formed with as great a length as possible to provide maximum sealing area against the sides and ends of the base deck  102 . Although it is understood that the sides  222  of the cover  220  should not be so long that they interfere with either an electrical connector at one end of the printed circuit board  132  or the mounting holes  224  formed in the side rails  226  of the base deck  102  ( FIG. 8 ). 
     The preferred method for securing the sealing cover  220  to the structural cover  104  and the sides and ends of the base deck  102  is to apply an adhesive epoxy as described above to both the underside of the cover  220  and to the interior surface of the downward depending sides  222  so that a continuous seal is formed about the perimeter of the cover  220  as the cover  220  is slid into place over the cover  104  and the base deck  102 . Alternatively, a PSA may be used in place of the epoxy on the bottom side of the cover  220 , and/or the liquid epoxy may be applied to the sides and ends of the base plate as opposed to the interior surface of the cover  220 . In either event, the tight dimensions of the cover  220  relative to the outer perimeter of the base plate ensure that the epoxy will be spread evenly as the cover  220  is lowered over top of the drive  100 . Furthermore, because the thickness of the cover is preferably only about 0.010 inches, the cover  220  will only increase the width and depth dimensions of the drive by approximately 0.020 inches and will only increase the height by 0.010 inches. Thus, it is likely that the addition of the cover  220  on the outer surface of the drive  100  will not violate the form factor envelope of the drive  100 . 
     Referring now to  FIG. 9 , a method of hermetically sealing a disc drive  100  that is filled with helium (or another low density gas) will be described. The method starts at  900  and a first operation  902  entails securing the structural cover  104  to the base plate  102  with the rubber seal  170 . This sealing step prevents the interior of the drive from being contaminated while the attachment of the structural cover  104  allows the drive  100  to function properly for testing purposes. In operation  904 , a source of helium is connected to the fill port ( 176  or  210 ) and helium is supplied to the drive (while air is first evacuated from the drive) until a predetermined helium concentration and pressure is achieved. Next, the drive  100  is tested in operation  906  to determine whether the drive meets operational specifications. This is known as certification testing. 
     Following step  906 , a determination is made at  908  as to whether the drive  100  requires a reworking procedure to cure any defects found during certification testing. If the determination  908  is positive, the method continues to operation  910  where the structural cover  104  is removed (thereby allowing the helium gas within the drive  100  to escape) and any needed repairs are made. Once the rework procedure is completed, the method then returns to operation  902  where the structural cover  104  is again attached to the base  102 . The drive  100  is then refilled with helium (step  904 ) and is once again tested in operation  906 . If the determination  908  is negative (i.e., if the drive  100  does not require reworking), the method continues to operation  912  where the sealing cover ( 150 ,  200  or  220 ) is permanently affixed over top of the structural cover  104 . The method then terminates at  914 . 
     One significant advantage to the dual cover technique of the present invention is that the first cover and the associated rubber seal will leak helium at a sufficiently low rate that the drive will not require a refill of the helium (or other low density) gas after the certification testing and prior to affixing the sealing cover to the drive. Of course, such a refill option is available if a helium filled drive was forced to endure a longer than average certification process (i.e., more than a few days). Alternatively, it is possible to account for the helium gas that will leak during the certification testing by slightly overfilling the drive with helium in step  904 . Thus, by initially pressurizing the drive above atmospheric pressure, any helium leakage that occurs during certification testing will not necessarily be replaced by air from outside the drive. Therefore, intentionally overfilling the drive to account for the initial leakage through the seal  170  allows manufacturers to achieve the desired concentration and pressure of the helium gas within the drive when the sealing cover ( 150 ,  200  or  220 ) is fixed to the drive at the end of the testing period. 
     A second advantage to the dual cover technique is that the two separate seals provide a measure of redundancy that will safeguard the operation of the disc drive should either seal fail. For example, if the rubber seal  170  were to fail after a number of years, the hermetic seal created by the sealing cover ( 150 ,  200  or  220 ) would continue to prevent any leakage of gas or contaminants into or out of the disc drive  100 . Alternatively, if the hermetic seal of the second cover were to break at any point (e.g. if the solder joint were to separate), the helium gas within the drive  100  would only leak out at a very slow rate as described above. That is, the redundant rubber seal  170  between the base plate  102  and the structural cover  104  will hold a minimal quantity or concentration of the helium gas for a sufficient period of time to enable circuitry within the drive (e.g., circuitry that monitors helium concentration) to detect the leak and warn the user that drive failure is imminent. Such a warning would allow the user to back up crucial data or completely copy or image the contents of the drive to a new drive before the leaking drive experiences a failure. 
     Described in another way, disc drive (such as  100 ) in accordance with an exemplary preferred embodiment of the present invention has a rotatable disc (such as  108 ) carried by a spindle motor (such as  106 ) and an actuator assembly (such as  110 ) having a read/write head (such as  118 ) adapted to operate in a low density gas environment. The disc drive includes a base plate (such as  102 ) supporting the spindle motor (such as  106 ) and the actuator assembly (such as  110 ) and a structural cover (such as  104 ) removably attached to the base plate (such as  102 ) to form an internal environment within the disc drive. The internal environment of the drive is filled with a low density gas such as helium, and a sealing cover (such as  150 ,  200  and  220 ) is permanently attached to the base plate (such as  102 ) and the structural cover (such as  104 ) to form a hermetic seal that maintains a predetermined concentration of the low density gas within the internal environment over a service lifetime of the disc drive (such as  100 ). In one embodiment, the structural cover (such as  104 ) is fastened to one of the spindle motor (such as  106 ) and the actuator assembly (such as  110 ) to permit operation of the drive. 
     The disc drive (such as  100 ) further includes a first seal (such as  170 ) secured between the base plate (such as  102 ) and the structural cover (such as  104 ) to prevent contaminants from entering the internal environment of the disc drive. The first seal (such as  170 ) is formed from a material such as rubber that allows leakage of the low density gas from the internal environment at a sufficiently low rate to allow operation of the disc drive (such as  100 ) for a predetermined period of time in the absence of the sealing cover (such as  150 ,  200  and  220 ). 
     In one embodiment, the sealing cover (such as  150  and  220 ) is attached to the base plate (such as  102 ) and the structural cover (such as  104 ) by an adhesive (such as  196 ). The base plate (such as  102 ) includes a raised outer edge (such as  190 ) and the sealing cover (such as  150 ) includes a downward depending edge (such as  186 ) that is adhesively bonded within a groove (such as  188 ) formed between an outer surface (such as  192 ) of the structural cover (such as  104 ) and the raised outer edge (such as  190 ) of the base plate (such as  102 ). Alternatively, the sealing cover (such as  220 ) includes a downward depending edge (such as  222 ) that is adhesively secured to an outer perimeter wall of the base plate (such as  102 ). In an alternative embodiment the base plate (such as  102 ) includes a raised outer edge (such as  190 ) and the sealing cover (such as  200 ) is soldered to a top surface (such as  202 ) of the raised outer edge (such as  190 ) of the base plate. 
     Another embodiment of the present invention may be described as a method of hermetically sealing a disc drive (such as  100 ) filled with a low density gas. The method includes a step (such as  902 ) of removably attaching a structural cover (such as  104 ) to a base plate (such as  102 ) to define an internal environment of the disc drive. A next step (such as  904 ) includes filling the internal environment of the disc drive (such as  100 ) with the low density gas. A final step (such as  912 ) includes permanently attaching a sealing cover (such as  150 ,  200  and  220 ) to the base plate (such as  102 ) and the structural cover (such as  104 ) to form a hermetic seal that maintains a predetermined concentration of the low density gas within the internal environment over a service lifetime of the disc drive. In one embodiment, the method may include the steps (such as  906  and  9190 ) of testing the disc drive prior to permanently attaching the sealing cover (such as  150 ,  200  and  220 ) and removing the structural cover (such as  104 ) and reworking the disc drive (such as  100 ) if the testing step (such as  906 ) discloses a failure in the disc drive. 
     Yet another embodiment of the present invention may be described as a disc drive (such as  100 ) having a base plate (such as  102 ) supporting an actuator assembly (such as  110 ) having a read/write head (such as  118 ) adapted to operate in a low density gas environment. The disc drive includes a structural cover (such as  104 ) removably attached to the base plate (such as  102 ) to form an internal environment within the disc drive that is filled with a low density gas such as helium. The disc drive further includes means (such as  150 ,  200  and  220 ) for hermetically sealing the low density gas within the internal environment over a service lifetime of the disc drive (such as  100 ). The disc drive also includes a first seal (such as  170 ) secured between the structural cover (such as  104 ) and the base plate (such as  102 ), wherein the first seal (such as  170 ) is formed from a material that allows leakage of the low density gas from the internal environment at a sufficiently low rate to allow operation of the disc drive (such as  100 ) for a predetermined period of time in the absence of the means (such as  150 ,  200  and  220 ) for hermetically sealing the low density gas. 
     Thus, the present invention provides an improvement over prior helium filled drives which rely on traditional sealing methods (such as a rubber gasket seal between the cover and the base plate) to attempt to maintain the helium (or other low density gas) within the drive interior. This is because such prior art sealing methods are unable to provide the type of hermetic seal required to prevent the leakage of gas, particularly when the gas has a relatively small atomic weight as in the case of helium. Indeed, such prior art seals were intended to keep environmental contaminants from entering the drive as opposed to keeping gas from leaking from the drive. The present invention solves this problem by providing a system of two separate covers: a first cover (similar to prior art covers) that provides a structural support for the drive but that also may be removed if necessary to rework the drive; and a second cover that may be affixed to the drive using “dirty” sealing techniques capable of providing a hermetic seal. Such dirty sealing techniques will not impact the cleanliness of the drive interior since the second cover is affixed outside of both the first cover and the first seal so that any fumes generated by the epoxy adhesive (or any debris generated by the soldering technique) will not invade the drive interior. 
     It will be clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While embodiments have been described for purposes of this disclosure, various changes and modifications may be made which are well within the scope of the present invention. For example, the configuration of the base plate and the structural/sealing covers may be altered as desired to accommodate different drive designs and different form factors. Additionally, the materials used for the sealing cover may be altered depending on the precise sealing technique used to achieve a hermetic seal. Numerous other changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the scope of the invention disclosed and as defined in the appended claims.