Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Reexamination Certificate
1999-02-23
2001-02-20
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
C361S056000, C361S111000, C361S118000
Reexamination Certificate
active
06191928
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to surface-mountable devices (SMDs) for the protection of electrical circuits. More particularly, this invention relates to surface-mountable devices for protection against electrical overstress associated with electrostatic discharge, indirect lightning discharges, human and structural discharges, and electromagnetic pulse discharges within electrical circuits (hereafter collectively referred to as ESD).
BACKGROUND OF INVENTION
Printed circuit (PC) boards have found increasing application in electrical and electronic equipment of all kinds. The electrical circuits formed on these PC boards, like larger scale, conventional electrical circuits, need protection against electrical overvoltage. This protection is typically provided by commonly known electrostatic discharge devices that are physically secured to the PC board.
Examples of such a devices include silicon diodes and metal oxide varistor (MOV) devices. However, there are several problems with these devices. First, there are numerous aging problems associated with these types of devices, as is well known. Second, these types of devices can experience catastrophic failures, also as is well known. Third, these types of devices may burn or fail during a short mode situation. Numerous other disadvantages come to mind when using these devices during the manufacture of a PC board.
It has been found in the past that certain types of materials can provide protection against fast transient overvoltage pulses within electronic circuitry. These materials at least include those types of materials found in U.S. Pat. Nos. 4,097,834, 4,726,991, 4,977,357, and 5,262,754. However, the time and costs associated with incorporating and effectively using these materials in microelectronic circuitry is and has been significant. The present invention is provided to alleviate and solve these and other problems.
SUMMARY OF THE INVENTION
The present invention is a thin film, electrostatic discharge surface mounted device (ESD/SMD). According one aspect of the present invention there is an electrically insulating substrate having a first surface. First and second electrodes are disposed on the substrate surface. The electrodes are spaced apart from one another to form a gap. A portion of the substrate is removed to form a cavity in the gap region. A voltage variable material is disposed in the cavity and connects the first electrode to the second electrode.
According to various embodiments of the present invention, the electrodes and their profiles are selectively shaped to improve the electrical characteristics of the device. In general, the electrode profiles are rounded to eliminate edges and the build-up of electrical field concentrations associated electrode edges. In one embodiment, the electrodes are regrown to form a rounded profile and a greater thickness in the active electrode area, i.e., the surface area of the electrode in direct contact with the voltage variable material. In another embodiment the electrodes have a curvilinear periphery in the gap region. Preferably, the path distance of the curvilinear electrode peripheries is longer than rest of the electrode peripheries in order to increase the volume of the voltage variable material disposed between the electrodes. The electrode profiles can be sloped or stepped to form a containment region to increase the active electrode area. The electrode thickness can also be increased to form the containment region. By shaping the electrodes and their profiles according to the present invention devices having improved electrical characteristics can be achieved.
Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings.
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Elliott Andrew W. S.
Hyatt Hugh M.
Minervini Anthony
Neuhalfen Andrew J.
Rector Louis
Bell Boyd & Lloyd LLC
Jackson Stephen W.
Littelfuse Inc.
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