GB2347783A

GB2347783A - A spark discharge device for ESD protection

Info

Publication number: GB2347783A
Application number: GB9905074A
Authority: GB
Inventors: Kevin Trevor Talbot; Jeremy John Greenwood
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1999-03-06
Filing date: 1999-03-06
Publication date: 2000-09-13
Also published as: GB9905074D0

Abstract

A spark discharge device 10 is disclosed which is suitable for electrostatic discharge protection in electrical circuits. The device 10 comprises a pair of opposing electrodes 12 for connection in use to a protected circuit, the electrodes 12 defining an air gap A therebetween. The device 10 further comprises a casing 16 substantially enclosing the air gap A, so as to substantially seal the air gap A from the external environment. In this manner, the device 10 is suitable for application in particular to printed circuit boards which require conformal coating. Instead of air the casing, and thereby the gap, may be filled with another gas such as one of the noble gases.

Description

A Spark Discharge Device This invention relates to spark discharge devices and in particular to a spark discharge device suitable for providing electrostatic discharge protection in an electrical circuit.

Many electrical circuits include components which are sensitive to overvoltage conditions and it is known to provide over-voltage protection using a spark air gap formed by two pointed electrodes, such that the discharge of excess potential is promoted across the air gap. It is a problem with some known spark gap arrangements that the ingress of moisture can reduce the efficiency of the over-volt protection.

It is known to provide environmental protection for printed circuit boards (PCBs) by applying a conformal coating and in US 4,586,105 a spark gap is covered by a tape so that a conformal coating can be applied.

It is an object of this invention to provide an improved spark discharge device suitable for electrostatic discharge protection in an electrical circuit.

According to the invention there is provided a spark discharge device suitable for inclusion in an electrical circuit so as to provide electrostatic discharge protection, the device comprising at least two electrodes spaced apart so as to define a gas filled gap, wherein the device further comprises an integral casing substantially enclosing the gap such that the gap is sealed from the external environment, whereby the device is suitable for conformal coating.

The gas may comprise air or may comprise a noble gas.

The casing may comprise a dielectric material, and the electrodes may comprise a metal layer material. The electrodes may comprise substantially pointed and opposing members. The device may comprise a surface mount component or a leaded component.

The invention also provides a method of producing an electrical circuit board, including the steps of : a) attaching to the circuit board an electrostatic discharge device according to the invention; and b) conformally coating at least a portion of the circuit board which includes said device.

The invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is a sectional plan view of a component according to an embodiment of the invention; and Figure 2 is a sectional side view of the component of Figure 1.

Referring to the figures, a spark discharge device 10 comprises a pair of electrodes 12 which are spaced apart such that they define a gas filled gap A therebetween. The electrodes 12 oppose each other and in the region of opposition are substantially pointed so as to promote in use electrostatic discharge across the gap A. The gap in this embodiment is filled with air and thus comprises an air gap A.

The electrodes 12 comprise a metal layer material, similar to that used in some known capacitors but of thicker section. The thickness of the section will depend on the level of protection sought and can be optimised using known experimental techniques so as, for example, to minimise at the electrodes the effects of spark induced erosion for the rated voltage.

The electrodes 12 are electrically connected to electrical contacts 14 to allow connection to an external circuit (not shown). The contacts 14 in the embodiment shown are for a device 10 suitable for use as a surface mounted component on a printed circuit board, although they 14 could also be formed such that the component 10 can be used as a leaded component.

The spark discharge device 10 further comprises an integral casing 16 which substantially surrounds the electrodes 12 and the air gap A, such that the air gap A is substantially sealed from the external environment. The casing 16 comprises a dielectric material.

By sealing the air gap A from the external environment through the provision of the integral casing 16, the spark discharge device 10 becomes suitable for use in the protection of electrical circuits without the need for the provision of additional coverings prior to the direct application of a conformal coating. This means that an electrical circuit, such as a printed circuit board (PCB), can be manufactured by attaching to the circuit board (not shown) the spark discharge device 10 and then directly conformally coating at least the portion of the circuit board which includes the device 10, without having to take any special precautions to prevent the ingress of conformal coating into the gap.

Furthermore, by reducing the likelihood of moisture/humidity ingress, the breakdown voltage can be rated more predictably. The invention can be applied, for example, in a car radio which may be expected to operate in an environment where it can be subjected to extremes of moisture/humidity.

Because the gap is sealed in from the environment, it is not limited to an air gap. In another embodiment the casing, and consequently the gap, is substantially filled with a gas which comprises an inert gas such as one of the noble gases. In this manner, the breakdown voltage of the gap under electrostatic discharge can be reduced and the device may benefit from an even more stable rated breakdown voltage.

Claims

CLAIMS 1. A spark discharge device suitable for inclusion in an electrical circuit so as to provide electrostatic discharge protection, the device comprising at least two electrodes spaced apart so as to define a gas filled gap, wherein the device further comprises an integral casing substantially enclosing the gap such that the gap is sealed from the external environment, whereby the device is suitable for conformal coating.
2. A device according to Claim 1, wherein the gas comprises air.
3. A device according to Claim 1, wherein the gas comprises a noble gas.
4. A device according to any preceding claim, wherein the casing comprises a dielectric material.
5. A device according to any preceding claim, the electrodes comprising a metal layer material.
6. A device according to any preceding claim, the electrodes comprising substantially pointed and opposing members.
7. A device according to any preceding claim, the device comprising a surface mount component.
8. A device according to any one of Claims 1 to 6, the device comprising a leaded component.
9. A device substantially as described herein with reference to the accompanying drawings.
10. A method of producing an electrical circuit board, including the steps of : a) attaching to the circuit board an electrostatic discharge device according to any preceding claim; and b) conformally coating at least a portion of the circuit board which includes said device.