GB2340988A

GB2340988A - Method and apparatus for electrostatic discharge protection in hard disk drives

Info

Publication number: GB2340988A
Application number: GB9912902A
Authority: GB
Inventors: Al Wallash
Original assignee: Quantum Corp
Current assignee: Quantum Corp
Priority date: 1998-06-05
Filing date: 1999-06-03
Publication date: 2000-03-01
Also published as: GB9912902D0; DE19925697A1; JP2000011337A

Abstract

A method for protecting a Hard Disk Drive's ("HDD") read/write transducers from Electro-Static Discharges ("ESD") is described. In known HDDs, a plurality of read/write transducers are stacked atop one another with their attached actuator arms to form a Head Stack Assembly ("HSA"). To protect the transducers of the HSA from ESD, the input/output lines of the flexible cable and its connector, which couple the HSA to the HDD's electronics, are coated with a polymer material having non-linear electrical resistance properties. Under normal conditions the polymer material has a high electrical resistance and does not affect the operation of the HDD. When the material's voltage threshold is exceeded, it changes to a low electrical resistance state. Thus, when a ESD strikes the cable/connector, the material changes state and shunts the generated current to ground, protecting the transducers.

Description

2340988

Method and Apparatus for Electrostatic Discharge Protection In Hard Disk Drives Background of the Invention

The present invention is in the field of Hard Disk Drive ("HDD") mass storage devices. The particular subject matter of the present invention is electrostatic discharge ("ESD") suppression in HDD sub-assemblies.

The use of Magneto-Resistive ("MR") and Giant Magneto-Resistive ("GMR") recording transducers in HDDs is known. Both MR and GMR transducers are a type of thin film resistor, approximately 100-200 A thick. This small volume of metal can be seriously affected byjoule heating. Small transient currents can not only damage these transducers but can indeed melt and destroy them. Damage to the magnetic properties of the transducers occurs well before actual melting. A current transient of as little as 20-30 milliamps for I nanosecond, amounting to approximately I nanojoule of energy, is sufficient to ruin a transducer of either type.

MR transducers incorporated into Head Stack Assemblies ("HSA"s) can be damaged and rendered unreliable if an ESD event occurs at the end of the flexible connector which couples the HSA to the HDD's electronics. During assembly of the HSA, this flexible connector is not grounded and is exposed to unintended contact. If touched by an ungrounded technician or assembly worker, an ESD event may occur. An ungrounded person or metal object can easily accumulate a static electric charge with a voltage as large as 30,OOOV. As any voltage greater than 1,000V will damage the transducer, it is clear that ESD precautions must be taken to eliminate ESD discharges through the flexible connector.

A possible method to protect the HSA from such ESD events involves shorting all of the flexible connector's input/output leads together to a ground potential. Any ESD transient would then flow directly to ground and leave the transducer unaffected. Although this solution increases the damage threshold for accidental ESD events, the shorting must be removed during testing and before final assembly. 'Mis adds additional processing steps and leaves the transducers unprotected during testing and after final assembly.

Placing diodes across the transducers themselves to protect them firom ESD transients is another known method for protecting the transducers. However, the addition of these components to the MR transducer increases the complexity of the transducer and can affect its performance. Additionally, the diodes themselves can be damaged by an ESD event.

A method or apparatus which can protect the MR/GMR transducers in a HSA from ESD transients during assembly, testing and operation without adding undesirable complexity to the HSA would be desirable.

I I J, Summary of the Invention

In a preferred embodiment of the present invention, a material which exhibits a high electrical resistance under normal conditions but changes to a low electrical resistance during ESD events is placed over the flexible connector near its connector end. All flexible input/output leads are coupled through this material to a ground plane. The threshold voltage for the change in electrical resistance characteristics is approximately 100 V. When an ESD transient exceeds this voltage threshold, the material conducts, shunting the ESD transient current to ground. This prevents the ESD current from reaching the MR and GMR Umnsducers and protects them from damage.

Ile material is easily applied and does not need to be removed. It provides continuous ESD protection during assembly, testing, and indeed during the product's entire useful I ife. Further ESD protection can be obtained by placing the material on other connectors within the HDD.

The preferred embodiment of the present invention will now be described in detail with reference to the figures listed below.

Brief Description of the Drawings

Fig. I is a perspective drawing illustrating a HSA with a flexible cable and connectoqWor Art); and Figs. 2a and 2b are, respectively, a top plan view and a cross-section of an HSA flexible connector incorporating the first embodiment of the present invention.

Detailed Description of the Preferred Embodiment

Fig. I illustrates a known HSA 10 comprised of five read/vnite transducers 12 mounted on five actuator arms 14. Flexible cable 16 with connector 18 provides input/output data and address paths to and from the HSA assembly.

The exposed nature of flexible cable 16 and connector 18 make it apparent that accidental contact with electrically charged objects and persons can easily occur. Should a person or object having a charge voltage level greater than 1000 V touch the connector or flexible cable, a destructive ESD transient current might flow to transducers 12. As ungrounded persons and metal objects can easily achieve voltage levels greater than 5,000 V, such transients occur frequently and result in damage to the transducers in known HSAs.

The first embodiment of the present invention decreases the likelihood of damage to the HSA from these transients by applying a material 21 with a non-linear electrical resistance characteristic to the junction between connector 18 and cable 16. Figs. 2a and 2b show, respectively, a top plan view and a cross-section of this junction. Input/output leads 17 and 2 ground lead 19 are coupled through a layer of material 2 1. Under normal operating conditions, material 21 has a high electrical resistance. Above its threshold voltage of 100 V, material 21 exhibits a low electrical resistance. In operation, an ESD transient event triggered by contact with a charged person or object will usually exceed material 2 I's threshold voltage, causing it to conduct electricity with little resistance, and thus allowing the transient current to flow through it into the ground line.

One example of such a material with non- linear electrical resistance is SURGY., produced by Oryx Technology. The material has a threshold voltage of 100 V and an electrical resistance of - 10 0 in its conductive state.

Although it had previously been suggested to use SURGX or a similar material to shunt ESD transients, the previous suggestions sought to place the material in close proximity to the read/write transducers themselves. In this location, the material cannot function in the desired manner. A voltage level of a little as 3 V at the transducer can result in damage to or destruction of the transducer. Unfortunately, SURGX does not begin to conduct electricity fi-ecly until its threshold voltage of 100 V is reached. Clearly, the previous suggested application could not function as desired.

Placing material 21 at the connector/flexible cable junction, which is significantly further away from the transducers than the previously suggested location is counterintuitive. Although generating an ESD through the flexible cable was known, known attempts to correct the problem focused on the transducers themselves, at their connection to the actuator arm. Placing material 21 at this location would not protect the transducer, as the voltage level where these materials change state is higher that the level of voltage that would result in damage. Indeed, all known attempts to solve the ESD problem located their proposed solutions adjacent to the transducer head. In the present invention, moving material 21 to the connector allows it to serve its purpose despite the relatively high voltage level necessary for it to change state. The present invention has the additional advantage that no materials are added to the transducer itself or its supporting arm, where the materials might affect arm resonance and transducer flying height, critical operating parameters of these devices.

Experiments with the present invention indicate that the ESD failure level of transducers protected with it has been raised from I OOOV to 200OV. As the material does not need to be removed at any time during assembly or operation, it provides protection to the transducers for their entire life cycle.

Readily envisioned variations of the present invention include placing material 21 on the connectors that couple the HIDD to the host computer's address and data bus, and on all other 3 --) d connectors which are internal to the HDD. Placing material 21 in these additional locations would result in a series of shunts to ground, providing multiple levels of ESD protection.

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Claims

What Is Claimed Is:

1. A method for protecting a read/write transducer from electrostatic discharges, the transducer being mounted on an arm and coupled to read/write circuitry through an electrical conduit carrying at least data, the method comprising placing a material with non-linear electrical resistance characteristics on the electrical conduit, the material having a first high resistance state and a second low resistance state, an electrostatic discharge placing the material in its second state, the material then providing a low resistance circuit path from the electrical conduit to a ground potential, thereby shunting the electrostatic discharge from the transducer.

2. A head stack assembly with electrostatic discharge protection comprising: a plurality of read/write transducers mounted respectively on a plurality of read/write arms, the arms being stacked atop one another, a signal bus coupled to each transducer, the signal bus having at least a first ground plane and the signal bus being capable of transmitting data to each of the transducers to write the data and capable of transmitting data that the transducers have read; at least a first connector for coupling the head stack assembly to read/write electronics, the at least first connector being coupled to the signal bus; and a first material having at least two states of electrical resistance, a first high resistance state and a second low resistance state, the first material being placed on the signal bus adjacent to the at least first connector, the material changing from its first state to its second state and providing a low resistance electrical path between the signal bus and the ground plane when an electrostatic discharge occurs.

3. The head stack assembly of claim 2 wherein the head stack assembly is coupled to a plurality of signal and power buses and each signal and power bus has at least one ground plane and at least one connector, the first material being placed on the buses adjacent to each of their respective connectors, and the first material providing a low resistance path from each bus to its ground plane when the respective buses experience an electrostatic discharge event.

4. In an electrical device having circuitry capable of being damaged by electrostatic discharge events, a system for protecting the device from the electrostatic discharge events, the system comprising a material with non-linear electrical resistance properties, the material being placed on each electrical connection between the device and externally generated signals and externally generated power, the material providing a low resistance pathway to a ground potential from each of these connections when the device experiences an electrostatic discharge on any of these connections.

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5. The system of claim 4 wherein the device is a hard disk drive having a plurality of read/write transducers, each transducer being protected by the material from electrostatic discharge by placement on the connection between the electrical connections between the transducers and the hard disk drive electronics.

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