Magnetic Shield for Disk Drive Enclosure
Original Publication Date: 2001-Mar-01
Included in the Prior Art Database: 2003-Jun-19
Disclosed is a methodology for shielding the strong stray magnetic field created by the magnets used in the actuator of a hard disk drive. This strong magnetic field sometimes attracts nearby loose ferrous metallic objects towards the surface of the electronic card mounted on the disk drive enclosure, and cause electrical shorts, thereby disabling the disk drive operation. The magnetic shield reduces the stray magnetic field to a level where it does not easily attract nearby ferrous objects. With the ever increasing demand for higher performance from hard disk drives, the size and strength of magnets has been steadily increasing. At the same time, the need to keep the cost low has resulted in inadequate shielding of these magnets. A considerable amount of field leaks out of the magnet housing and disk enclosure. The strength of the magnetic field at the surface of disk enclosure can be as high as 400 Gauss. As detailed in the above paragraph, this magnetic field can indirectly cause drive failures. One methodology that has been used before to mitigate strong magnetic fields is to place a plastic cover on the pins of logic modules on the electronic card so as to prevent shorting of module pins in the event of any metallic object falling on the card, that has been attracted by the magnetic field. The main drawback of this methodology is that the shape and size of cover has to change to accommodate any change in module placement on the board. Also, once placed, the cover cannot be easily removed to troubleshoot the module. It also adds to the card assembly cost. The concept of using a magnetic shield to cut the magnetic field near the source is developed to overcome the drawbacks of using a plastic cover. When placed between the disk enclosure and the electronic card, the shield reduces the stray magnetic fields to an acceptable level. The magnetic field measured 10 mm above the disk drive form factor is about 66 Gauss. The goal is to reduce this field down to 25 Gauss. Initially, it was decided to use special high Mu metal sheet. However, because of very strong magnetic field in the proximity of disk enclosure (over 400 Gauss), the Mu metal sheet becomes saturated without providing adequate shielding. The presence of strong field also necessitates that two layers of shields be used instead of one to prevent saturation. After experimenting with several materials (transformer steel, spring steel, and ferrous Stainless Steel), it was decided to use ferrous stainless steel (403 Stainless Steel). The shielding property of 403 Stainless Steel in the presence of strong magnetic field is comparable to Transformer Steel with the added benefit of being rust-proof. A drawing showing all the components of magnetic shield is attached below. A piece of foam (item 3) is placed between the disk enclosure and the electronic card for the purpose of insulating the electrical connections on the card from the disk enclosure, as well as damping the acoustic noise generated by the disk drive. The disk enclosure surface has a 0.3 mm deep cavity. Hence, a 0.3 mm thick 403 Stainless Steel sheet is cut to fit the cavity (item 1). Another 0.1 mm thick Stainless Steel sheet (item 2) is cut to the size of foam so as to broadly cover the area of magnetic field. Due to limited clearance between disk enclosure and electronic card, 0.1 mm is the maximum allowable thickness for the second sheet. It is observed that with these two pieces placed between disk enclosure and electronic card, the magnetic field at the measurement point is reduced to about 25 Gauss. For simplifying the assembly of magnetic shield, the two pieces of Stainless Steel are glued together using a pressure sensitive adhesive. These pieces are then glued to the foam (item 3) to form a single piece, which can be placed above the disk enclosure surface in a single operation.