Surface mounted grounding clip for shielded enclosures

Details Surface mounted grounding clip for shielded enclosures Surface mounted grounding clip for shielded enclosures

2000-10-31

2002-09-24

Martin, David (Department: 2841)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C361S737000, C361S787000, C361S772000, C439S092000, C439S095000

Reexamination Certificate

active

06456504

ABSTRACT:

BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to electrical connections. More particularly, the present invention relates to printed circuit boards and electrical grounding thereof. In particular, the present invention relates to a cover-to-cover grounding contact clip in an electronic peripheral card for the control and management of electromagnetic interference and electrostatic discharge.
2. The Relevant Technology
The proliferation of personal computers has caused a large demand for expansion boards to enhance computer functionality. With older personal computers, expansion boards were mounted internally and therefore required the computer cover to be removed before installation thereof. The advent of electronic devices such as laptop and notebook personal computers, handheld computers, and other devices such as personal digital assistants (PDAs) necessitated the development of a portable expansion board. Such a portable expansion board has been referred to as a peripheral card with one standard configuration being a PCMCIA card. PCMCIA cards are insertable in external slots of laptops, notebooks, sub-notebooks, and other hand-held devices such as PDAs. The small size of these electronic devices and their portability facilitates the interchange of functionality by installing and removing different peripheral cards into the external slots of the computer or other electronic device. Typical peripheral cards include modems, wireless communicators, and memory expansion inserts.
Because these peripheral cards may be removed from the electronic device, both their installation and their operation within the electronic device may trigger electrostatic discharge (ESD) or also electromagnetic interference (EMI) during operation. An electrostatic charge may build-up on any conductive surface of a peripheral card and as such, ESD may cause damage to both the electronic components within the peripheral card as well as the electronic device into which the peripheral card is inserted. An ESD therefore must be properly managed for such peripheral cards. An effective discharge path to a ground source, such as a computer chassis, is needed from any major surface area of the card.
Another desirable design goal is that the peripheral card be adequately shielded against radiating or receiving EMI. The shield must have a sufficiently low ohmic resistivity such that no anticipated electromagnetic energy can penetrate it. The shield therefore must have the function of both adequate thickness and adequate ohmic resistance. Currently, peripheral cards such as PCMCIA cards are constructed with a relatively thin two-sheet, metal shield that can provide adequate shielding at all anticipated frequencies. The shield also needs to provide a conductive surface area in order to allow for a substantially continuous current flow through the shield surface to ground. If a discontinuity arises between the shield and the leads to ground, a slot antenna is created and EMI can penetrate the peripheral card or it can be generated by the peripheral card such that it interferes with the larger electronic device.
Several packaging methods have been developed for the construction of peripheral cards that assist to provide ESD and EMI protection. When an insufficient electrical conduction path to ground exists, many problems may arise during operation of the peripheral card and it may also cause problems during operation of the electronic device with which the peripheral card is connected. To overcome EMI problems, it is therefore preferable to increase the ground-to-signal ratio in a PCB to improve the signal flow therethrough which enhances signal transmission performance of the peripheral card. Additionally, where a PCB does not have proper connection to ground, the PCB can suffer from not only EMI interference problems, but also ESD problems which may arise due to the live electronic components on the peripheral card which may arc to the card's metallic shield.
ESD may also occur when two device surfaces exhibit a sufficient difference in electrical potential resulting from varying electrical charge on each surface. Such a condition forms an electrostatic capacitor between the two devices which emits electrostatic electrons that may also interfere with the performance of the related devices and other peripherals. When these charged surfaces abruptly encounter each other, such as when an electrical card and a related card connector are coupled together, an electrostatic discharge may result thereby causing damage to the components within the electrical card.
Several arrangements have been made in the art to provide adequate PCB-to-ground paths for peripheral cards. One example is a compression spring that is constrained in a pocket of the package frame for extendable portions of a peripheral card. The compression spring must be fitted onto the frame pocket by hand, it is small, and because it is a separate external piece, it may be easily lost.
Another example is a clip that is soldered to one side of the peripheral card's PCB. The soldered clip has the problem that it requires hand-soldering and handling and that a multiple number of clips may be required to accomplish a cover-to-cover contact for the peripheral card.
Another article that has been used to deal with both EMI and ESD is a clip that is crimped to the plastic frame of a peripheral card such as a PCMCIA card and that makes electrical contact with both the upper metallic sheet and the lower metallic sheet. Additionally, a portion of the clip has a tab that extends therefrom and makes electrical contact with an electrically conductive pad on the PCB. The crimped clip, because of its extremely thin nature, can become damaged due to conventional handling during assembly. Additionally, the tab, because it extends beyond the frame, may require intensive labor to install and to crimp. Additionally, the clip cannot be reworked and the tab may not make adequate contact with the pad on the PCB. Additionally, there are multiple steps of manufacture and assembly which are wasted if a subsequent process causes damage.
Another device that has been used to manage EMI and ESD is a formed tab that is integral to the metallic shield. As with the clip on the PCB and tab in the crimped clip, the formed tab can be easily damaged during assembly and may not make adequate contact with the PCB. It is also flimsy and potentially unreliable because its resilience may be damaged.
Another article that has been used for EMI and ESD management is a gasket material that is compressed between one of the metallic shields and the PCB. The gasket material, however, may not remain intact or in its proper orientation. As such, it may cause irreparable damage to electronic components contained in the peripheral card and it requires a great deal of handling to assemble.
As with all of these prior art attempts for EMI and ESD protection, where proper contact is not made, the peripheral card can become susceptible to ESD or act as an antenna and/or create unwanted electromagnetic emissions. Additionally, if a metal shielded connector with contact tabs is used as part of the peripheral card, and the contact tabs do not make proper contact, the peripheral card can become an antenna.
Another configuration that contains both EMI and ESD is the placement of a ground trace around the outer perimeter of a PCMCIA card, followed by the overmolding of the PCB with a first dielectric material, and a second conductive material with the placement of a conductive adhesive and a metallic sheet upon the adhesive material. The packaging of a PCMCIA card after this fashion allows for the dissipation of both EMI and ESD. This configuration has several drawbacks including (i) a substantially permanent sealing off of the PCB surface, (ii) the overmolding's blanket effect that prevents cooling of the PCB components, (iii) poor electrical conductivity through the electrically conductive adhesive and (iv) a very large portion of the card's surface area oc

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