Low profile EMI shield with heat spreading plate

Details Low profile EMI shield with heat spreading plate Low profile EMI shield with heat spreading plate

1998-10-30

2002-02-12

Picard, Leo P. (Department: 2835)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C361S437000, C361S690000, C361S700000, C361S818000, C174S034000

Reexamination Certificate

active

06347035

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to Electromagnetic Interference (EMI) shielding structures. More specifically, this invention relates to a compact EMI shield structure that also incorporates a heat sink.
BACKGROUND OF THE INVENTION
Compact, portable computers are becoming increasingly popular among college students, businesspeople, writers, and others who require portable word-processing, e-mail, and computer graphics capabilities. In particular, notebook-sized computers, commonly known as “notebook computers,” are becoming increasingly popular because their small size and low weight make them portable and convenient to use.
Generally, a notebook computer comprises two main sections coupled together by hinges in a clamshell configuration. The first section contains a liquid crystal display (LCD) for displaying information. The LCD screen is disposed in a LCD bezel frame. Typically, the bezel frame is less than about one centimeter in thickness. The second section of the notebook computer comprises a compact computer base section with a keypad area for entering data. The computer base of a notebook computer has a low profile in that its vertical thickness is as thin as possible to minimize the bulk of the notebook computer in its folded configuration. Typically the computer base section of a notebook computer is less than about four centimeters in height and is preferably about two centimeters in height. Typically the computer base section has a width and length comparable to a notebook pad (e.g., preferably less than about 8½ inches by 11 inches). Generally it is desirable to reduce the width and the length of the computer base section as much as possible consistent with a keyboard design that is comfortable for the user to input data with for extended periods of time. Commonly, conventional QWERTY keyboards are used in notebook computers, with the QWERTY keyboard substantially filling the upper surface of the computer base section. However, alternate computer keyboard designs that achieve the function of a QWERTY keyboard in a more space efficient configuration are known to those of ordinary skill in the art.
Notebook computer designers face the challenge of increasing the functionality of a notebook computer while maintaining a low-profile housing. Typically, the centermost portion of the computer base is densely packed with electronic circuits that must fit into a volume that has a vertical height of between two-to-three centimeters. For example, the main motherboard is preferably a double-sided motherboard with electronic chips on both sides of a printed circuit board. In addition to the motherboard, the centermost portion of the computer base also contains support and mounting elements, electrical interconnection elements, and electrical isolation elements. Moreover, the frame of the computer base and the keyboard assembly also consumes part of the vertical height of the computer base.
The central processing unit (CPU) of a high performance notebook computer operates at a high clock rate. Heat is generated at every switching event. Consequently, a high clock rate causes the CPU to generate heat at a rapid rate. This heat must be dissipated to maintain the CPU at an acceptable operating temperature. One solution to dissipating the heat from high performance chips that are used in a conventional desk-top computer is to couple a high performance heat sink to the CPU. However, high performance heat sinks typically have a substantial thickness associated with the finned heat sink and the fan used to blow air over the fins of the heat sink. As an illustrative example, a compact heat sink and fan that is about one centimeter in height would consume a substantial fraction (e.g., about one-third) of the height of the computer base section of a notebook computer that is three centimeters in height. Although some miniaturization of a conventional heat sink and fan is possible, conventional extruded (finned) heat sinks are typically between about five-to-ten millimeters in height while conventional fans are typically a substantial fraction of a centimeter in height. Consequently, use of conventional heat dissipation structures which include a finned heat sink and fan may be inconsistent with a low profile notebook computer base that has sufficient vertical height for a double-sided motherboard and other mounting, support, and electronic interconnection elements.
Electromagnetic interference (EMI) is also a problem in high performance notebook computers. The high clock rate of the CPU of a high performance notebook computer is associated with high-frequency signal components. These high-frequency signal components may generate electromagnetic waves which propagate to other portions of the notebook computer or to neighboring electronic circuits and produce deleterious electromagnetic interference (EMI). An additional EMI shield comprising a conductive enclosure substantially surrounding the CPU is required to reduce the EMI to acceptable levels. According to well-known principles of electromagnetic theory, a conductive enclosure shields, or blocks, the propagation of electromagnetic radiation from an enclosed source. An EMI shield enclosure is commonly shaped as a six-sided box, although it may have other shapes that substantially enclose the EMI source. Some conventional notebook computers utilize five pieces of sheet metal in the computer base section with an additional separate metal cover to form a six-sided EMI shield substantially surrounding the motherboard. However, using sheet metal to form a six-sided EMI shield significantly increases the size and weight of a notebook computer. Alternatively, a substantially six-sided EMI shield enclosure may be formed in a notebook computer by coating the inner walls of the computer base housing with a conductive coating. However, since electronic components must be tightly spaced within a notebook computer, proactive measures must be taken to prevent shorting of electronic circuits to the EMI shield, such as adding additional insulating spacer elements to electronically isolate electronic circuit elements from the conductive walls of the EMI shield. Application of a conductive coating may also have fabrication disadvantages, such as problems associated with applying a high-quality conformal coating that properly adheres to housing surfaces.
One technique to reduce the weight of an EMI shield is to form a bag-like enclosure out of a flexible conductive film instead of sheet metal. Unfortunately, a flexible EMI bag-like enclosure comprised of insulating and conducting layers is largely inconsistent with the use of conventional heat sinks. For example, U.S. Pat. No. 5,436,803 teaches the use of a flexible electrically insulating bag with additional metal fibers embedded in the insulated material of the bag. Similarly, U.S. Pat. No. 5,597,979 teaches the use of a bag-like EMI shield comprising a conductive material either embedded in or laminated on one side of an insulating sheet. A flexible bag-like enclosure with an insulated interior surface provides the advantage that the bag-like enclosure may be slipped around an assembled printed circuit board. A neck or partially open end of a flexible bag-like enclosure may also facilitate making/changing electrical connections to the printed circuit board enclosed by the bag. However, bag-like enclosures are inconsistent with the use of conventional heat sinks. This is primarily because a bag-like EMI enclosure placed around a circuit board would block the flow of air across an interior heat sink. Thus, a bag-like EMI enclosure surrounding an internal circuit board assembly and heat sink is inconsistent with effective cooling of the electronics assembly by heat exchange to the atmosphere.
In addition, a conventional heat sink cannot be effectively thermally coupled to an electronics assembly contained within a flexible EMI enclosure. It is well known in the art of materials science that an electrically insulating layer tends to be a good thermal insulator. Consequentl

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