Electricity: magnetically operated switches – magnets – and electr – Electromagnetically actuated switches – Vacuum or hermetically sealed type
Reexamination Certificate
2003-03-07
2004-01-27
Nguyen, Tuyen T. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Electromagnetically actuated switches
Vacuum or hermetically sealed type
Reexamination Certificate
active
06683518
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates generally to switching devices. More specifically, the present invention relates to improved packaging and circuit integration for electromagnetic devices, such as reed switches and electromagnetic devices, such as reed relays, for switching high frequency signals. These relays are intended for applications in industries such as Automated Testing Equipment (ATE), where test signals having frequency ranges from DC to 12 GHz must be switched with minimum power loss and minimum pulse distortion.
Electromagnetic relays have been known in the electronics industry for many years. Such electromagnetic relays include the reed relay which incorporates a reed switch. A reed switch is a magnetically activated device that typically includes two flat contact tongues which are merged in a hermetically sealed glass tube filled with a protective inert gas or vacuum. The switch is operated by an externally generated magnetic field, either from a coil or a permanent magnet. When the external magnetic field is enabled, the overlapping contact tongue ends attract each other and ultimately come into contact to close the switch. When the magnetic field is removed, the contact tongues demagnetize and spring back to return to their rest positions, thus opening the switch.
Reed switches, actuated by a magnetic coil, are typically housed within a bobbin or spool-like member. A coil of wire is wrapped about the outside of the bobbin and connected to a source of electric current. The current flowing through the coil creates the desired magnetic field to actuate the reed switch within the bobbin housing. Some applications of reed devices require the switch to carry signals with frequencies in excess of 500 MHz. For these applications, a ground shield conductor, commonly made of copper or brass is disposed about the body of the reed switch. The ground shield conductor is commonly in a cylindrical configuration. The shield conductor resides between the reed switch and the bobbin housing to form a co-axial high frequency transmission system. This co-axial system includes the outer shield conductor and the switch lead signal conductor co-axially through the center of the reed switch. The ground shield conductor is employed to contain the signal through the switch conductor in order to maintain the desired impedance of the signal path.
Currently available reed devices are then incorporated into a given circuit environment by users. For application at higher frequencies, a reed switch device must be ideally configured to match as closely as possible the desired impedance requirements of the circuit in which it is installed.
Within a circuit environment, a co-axial arrangement is preferred throughout the entire environment to maintain circuit integrity and the desired matched impedance. As stated above, the body of a reed switch includes the necessary co-axial environment. In addition, the signal trace on the user's circuit board commonly includes a “wave guide” where two ground leads reside on opposing sides of the signal lead and in the same plane or a “strip line” where a ground plane resides below the plane of the signal conductor. These techniques properly employed provide a two-dimensional, controlled impedance environment which is acceptable for maintaining the desired impedance for proper circuit function.
However, the reed switch device must be physically packaged and electrically interconnected to a circuit board carrying a given circuit configuration. It is common to terminate the shield and signal terminals to a lead frame architecture and enclose the entire assembly in a dielectric material like plastic for manufacturing and packaging ease. The external portion of the leads may be formed in a gull-wing or “J” shape for surface mount capability. The signal leads or terminals exit out of the reed switch body and into the air in order to make the electrical interconnection to the circuit board. This transition of the signal leads from plastic dielectric to air creates an undesirable discontinuity of the protective co-axial environment found within the body of the switch itself. Such discontinuity creates inaccuracy and uncertainty in the impedance of the reed switch device. As a result, circuit designers must compensate for this problem by specifically designing their circuits to accommodate and anticipate the inherent problems associated with the discontinuity of the protective co-axial environment and the degradation of the rated impedance of the reed switch device.
For example, the circuit may be tuned to compensate for the discontinuity by adding parasitic inductance and capacitance. This method of discontinuity compensation is not preferred because it complicates and slows the design process and can degrade the integrity of the circuit. There is a demand to reduce the need to tune the circuit as described above. The prior art uses a structure of carefully designed vias, which are expensive and difficult to manufacture, to control the impedance from the relay to the board transition.
There have been many attempts in the prior art to solve the aforementioned problems associated with the packaging and the incorporation of reed switch devices into a circuit. For example, prior art reed switch devices typically include a printed circuit board substrate onto which the reed switch itself is installed. Circuit board traces are deposited on the surface of the printed circuit board to provide a wave guide to extend the co-axial environment of the relay from the reed switch itself down to the main circuit board into which the device package is installed. However, there are problems associated with the use of a printed circuit board as a substrate within an overmolded device package as well as manufacturing limitations.
Since it is commonly desired that the reed switch package be as small as possible, the use of a very thin printed circuit board is required. While a thin printed circuit board substrate has good RF transmission characteristics, it is less than ideal mechanically. The epoxy/fiberglass material of a typical printed circuit board is thin and fragile, and is subject to distortion or cracking under the heat and pressure stresses of the encapsulation process. Distortion of the leads can lead to misalignment of the solder balls when they are fastened to the product after molding. If the misalignment is severe, one or more relay balls can miss the solder pads on the user's circuit board when the relay is fastened, causing electrical discontinuities that require expensive rework.
The substrate solder pads are also fragile and are, therefore, easily damaged when the relay solder balls are reflow soldered to the substrate. A further disadvantage of solder pads is that they are flat; because of this, the solder balls can wander on them during attachment, causing further misalignment. After the relay is molded, solder balls are fixed to pads provided on the exposed external portions of the substrate traces. The solder balls melt when the relay is applied to the user's circuit board, providing the electrical connections to the reed switch, coaxial shield and coil. Since the circuit board substrate has a fibrous edge profile that is exposed at the exterior of the relay, it also provides a potential path for ingress of moisture during circuit board cleaning processes. Water ingress is highly undesirable, since it can lower the relay's insulation resistance. Also, the printed circuit board is relatively expensive compared to the total component cost for the entire product. Therefore, it is desired for this part to be removed from the construction.
In the prior art, there have been attempts to eliminate the use of printed circuit board substrates in electronic device packages. Many molded electronic packages use an internal metal leadframe skeleton to support internal components and transmit electrical signals in and out of the package. The leadframe supports the internal components during assembly, and is cut away after the product is m
Barlow, Joseph & Holmes, Ltd.
Kearney-National Inc.
Nguyen Tuyen T.
LandOfFree
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