Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2000-03-22
2004-02-03
Whitehead, Jr., Carl (Department: 2813)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06687097
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for protecting electrical circuits and the like from electrostatic discharges. More particularly, the present invention relates to a variably conductive polymer shunt for the protecting the read/write elements of a magnetic recording head.
BACKGROUND
Many computer systems store data on a hard disk drive. These drives generally consist of several magnetic recording disks (“hard disks”) mounted to a drive spindle. The hard disks and the drive spindle are collectively referred to as a “disk stack assembly.”
Hard drives store data by magnetizing portions of the disks in a pattern that represents the data. Conventional hard disk drives use several small ceramic blocks, commonly called “sliders,” to magnetize surfaces of the disks. Each slider contains a “write head” that flies over the surface of the disk and magnetizes the portion of the disk immediately below it. Each slider also contains a “read head” designed to retrieve the data stored on the hard disks. These read heads produce electrical signals whenever the slider passes over a magnetic transition on the disk. These electrical signals can be used to reconstruct the stored data.
Both sides of a hard disk are generally used to store data or other information necessary for the operation of the disk drive. Thus, every disk in the stack of disks will have at least two sliders. Each slider is typically attached to a suspension assembly and to some type of electrical connector or lead assembly that provides an electrical signal pathway to and from the slider. These components, together with a suspension assembly, are collectively referred to as a head gimbal assembly (“HGA”), a head suspension assembly, or a hard file “head.” The HGAs are, in turn, attached to a comb-like structure, known as an “E-block.” The E-block and the HGAs are collectively known as a head stack assembly (“HSA”).
The “areal density” (i.e., bits/inch×tracks/inch) of magnetic transitions on the surface of the magnetic recording disks has been increasing at a compound rate of about 60% per year. The technology requirements to read and write magnetic transitions at these densities has evolved from classic inductive devices, through magneto-resistive (“MR”) devices, to today's giant magneto-resistive (“GMR”) and “spin valve” devices. The hard disks have similarly evolved from aluminum platters with a “magnetic paint” coating, through highly polished electroless nickel-phosphorus (“NiP”) on aluminum disks with a sputtered magnetic layer and overcoat, to “fire polished” glass with ion exchanged “case hardening” and sputtered magnetic layers—all to support the ever increasing density of transitions demanded by customers.
As the technology has advanced, disk drive manufacturers have been forced to reduce the physical size of the functional components to achieve the desired recording densities. This reduction in size, particularly of the MR and GMR devices, has progressively led to the incidences of electrostatic discharge (“ESD”) damage at lower and lower threshold voltages. That is, ESD generally refers to a rapid, undesired flow of static electricity from a charged body to an uncharged body. The energy carried by an ESD event can be very damaging to the fine (i.e., small cross-section) multi-layered structures in MR and GMR head elements. By virtue of their relatively large size, the early MR devices were relatively resistant to damage by ESD events that had a voltage less than 50-100 volts. As the storage density increased, however, manufacturers needed to reduce the width and thickness of the functional (electrical and magnetic) components of the MR and GMR stripes. One consequence of this move towards smaller functional components is that the components became increasingly vulnerable to ESD damage. Accordingly, modern devices frequently display ESD damage at as little as 10-20 volts and electrical over stress (“EOS”) effects upon the dissipation of as little as 0.3 nano-Joules.
This problem is compounded because, although damage from higher voltage ESD events can be detected through visual inspection, damage from lower voltage ESD and EOS events are difficult to detect. Frequently, this type of damage is only detectable after the MR/GMR head has been built into a hard file and put through final testing, which is a very expensive place to detect a manufacturing problem. In addition, EOS-damage is difficult to prevent through normal or even extraordinary ESD control measures.
There are a number of alternative MR head and ESD protection schemes which have been disclosed or patented. One scheme involves using solder to bridge a gap between the MR pads on the slider. The solder would be put down at wafer level, and removed at actuator level, so ESD damage would be eliminated for most of the manufacturing process. A laser would be used to melt the solder to open the gap, which would be done as late as possible in the manufacturing process. Some drawbacks to this method are that solder is different to use in manufacturing and that the laser removal process is not 100% reliable.
Another scheme involves connecting the MR leads together somewhere along the head gimbal assembly (“HGA”) and only opening the leads when necessary for testing and for building (e.g., into an actuator). Unfortunately, the protection afforded by this scheme suffers because that the MR element is only protected for a limited portion of the manufacturing process. Thus, they do not sufficiently decrease the likelihood that a MR head will be damaged by ESD and EOS.
U.S. Pat. No. 5,748,412 to Murdock et al. (“Murdock”) discloses an apparatus for protecting a magneto-resistive sensor element from electrostatic discharge. The apparatus consists of a diode assembly that exhibits a nonlinear voltage-current relationship. This diode assembly is formed using laser thermal deposition and laser induced diffusion of the necessary dopants. Despite this complex and expensive manufacturing process, the diode assembly can only protect the sensor element from discharges below 20 volts. As a result, Murdock requires other methods to protect against higher voltage ESD. Another disadvantage of Murdock is that the diodes only protect a single magneto-resistive element. Thus, a multiple-head drive using this apparatus will require several dozen pairs of diodes.
U.S. Pat. No. 5,796,570 to Mekdhanasarn et al. (“Mekdhanasarn”) discloses the use of a variably conductive polymer to protect large scale electric circuits, such as printed circuit boards (“PCBs”), from high voltage ESD events. However, Mekdhanasarn does not teach the use of a variably conductive polymer to protect small-scale circuits, like those in an HGA, from low voltage ESD and EOS events. ESD protection methods used to protect large, printed circuit board sized, circuits from high voltage ESD events are different from those used to protect to small, HGA sized, circuits from low voltage ESD and EOS events. In addition, Mekdhanasarn does not teach the use of a variably conductive material made from materials known to be innocuous to hard disk drives.
Another partial solution is disclosed in copending U.S. application Ser. No. 09/054,090, filed Apr. 2, 1998 by Ahmann et al. (“Ahmann”), which is herein incorporated by reference. This application discloses a method of protecting MR/GMR devices from ESD and EOS damage through the use of a variably conductive path between the MR/GMR electrical connection pads and ground. In one embodiment, the tops or bottoms of the MR element electrical connection pads are elongated to nearly touch each other. A thin layer of a variably conductive polymer is then placed over the gap between the pads, followed by a protective overlayer. This application of a variably conductive polymer material provides ESD and EOS protection for MR and GMR type head elements from the fabrication process at the wafer or substrate level to, and including, life of the file in the final disk drive enclosure.
Although the teachings in Ahmann would be a desirable application of
Ahmann Robert D.
Anderson Nathaniel C.
Dolan Jennifer M
Dorsey & Whitney LLP
Jr. Carl Whitehead
Pemstar, Inc.
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