Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
1999-06-14
2001-03-27
Kincaid, Kristine (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S308100, C361S310000, C361S311000
Reexamination Certificate
active
06208501
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to capacitors and in particular concerns surface mount capacitors that can be connected directly to printed metallic traces.
Small dielectric chip capacitors are frequently used in microwave and millimeter wave circuits and are also employed as a DC block for passing a signal. There are several simple designs for passing signal from one printed circuit trace to another, but to date, these have been characterized by significant insertion losses at certain frequencies, and have been difficult to attach automatically to a printed circuit board. One approach has been to use a standing dielectric chip with metallized surfaces, with metal leads attached to the respective surfaces. The capacitor has to be held in place while the leads are soldered to the respective traces. Because the leads have a finite inductance, this approach typically has significant signal resonances, especially for frequencies above a few gigahertz. Also, the leads can break or fail. Another approach is a flat horizontal capacitor with the metallization on its lower side having a gap. A device of this type is the Gap-Cap, manufactured by Dielectric Laboratories, Inc. These devices can also create unwanted resonances at frequencies above a few gigahertz. In addition, these capacitors are all quite small, typically about 20 to 25 mils (0.5 to 0.6 millimeters). At these sizes, the capacitors are difficult to handle, and must be installed using a microscope. There is a definite top and bottom, and it is crucial to install them in the proper orientation, to avoid shorting the circuit. Also, in the case of the capacitor, the requirement to attach metal leads can make their manufacturing costs prohibitive.
The small size of the capacitor is necessary to match the metal film traces as nearly as possible. While it is possible to employ wider bonding pads to accommodate capacitors of a larger physical size, the bonding pads also have to be matched to a transmission line, and any change in width from the bond pad to the transmission line can also result in undesirable insertion loss.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a standing ceramic capacitor that avoids the drawbacks of the prior art, and in particular which easily match the size of the transmission line trace of a microwave or millimeter wave printed circuit, and which have a substantially flat, low-loss transmission characteristic.
It is a further object to provide a standing axial-leaded surface mount capacitor that can be placed and soldered in circuit without regard for orientation about its long axis.
It is another object to provide a standing axial-leaded surface mount capacitor that can be fabricated economically and simply.
In accordance with an aspect of the present invention, a standing axial-leaded surface mount capacitor is formed of a dielectric chip having a rectangular profile and with metallized front and back surfaces, and first and second blocks which have profiles to match the profile of said dielectric chip and which are conductive (at least on outer surfaces) to serve as leads to attaching to metallic surface traces of a printed circuit. In one preferred embodiment, the end blocks are ceramic cubes with metallic surfaces on its faces. The metallized cubes give the device rotational symmetry about its long axis, so that the capacitor will have identical performance in any orientation. This feature makes it possible to automatically pick and place the device onto a circuit board. The metal faces can be of Ni—Au or other suitable metallization to permit solder attachment. Preferably, the dielectric chip has a square profile of the same dimension as the ceramic cubes. The profile of the cubes and the chip can be 20 to 25 mils to match the width of a typical printed circuit trace. The capacitors can be of any typical value, i.e., from 0.05 pf to several hundred pf, and favorably between 5 and 50 pf.
The standing axial-leaded surface mount capacitor of this invention can be fabricated by creating dielectric chips of rectangular profile, preferably square, which are provided with metallic front and back faces. End blocks are created with a profile to match the profile of the ceramic chip, and the end blocks are given metallic outer surfaces. In a preferred method, a ceramic sheet is diced into cubes, and the cubes are plated so that all six faces of each are metallized. Then, two of the end blocks are positioned in a fixture with one of the dielectric chips sandwiched between them. The blocks and chip are aligned in registry so that respective faces of the blocks are in contact with the front and back surfaces of the dielectric chip. Then, with these three components held in place in the fixture, the blocks are fused to the metallic surfaces of the dielectric chip such that the end blocks now serve as the conductive leads to permit the capacitor to be mounted onto metallic traces of a printed circuit. Preferably, a small weight is placed on the upper surfaces of the blocks and chip in the fixture, and then the fixture is place into an oven. The metal on the end blocks can be a conductive material, such as gold, and the fixture is heated through the eutectic temperature of the attaching material to fuse the end blocks to the dielectric chip.
The dielectric chip can be formed by dicing a ceramic dielectric sheet into squares, with the front and back surfaces of the ceramic sheet having been coated with metal, e.g., by sputter coating a metal such as Au—Sn or Ti, W, Ni, and Au.
REFERENCES:
patent: 3633079 (1972-01-01), Watson
patent: 3732469 (1973-05-01), Watson
patent: 4346429 (1982-08-01), DeMatos
patent: 4870439 (1989-09-01), Chance et al.
patent: 6094335 (2000-07-01), Early
Arnold Eric D.
Bates David A.
Ingalls Mark W.
Semans Bruce F.
Dielectric Laboratories, Inc.
Kincaid Kristine
Molldrem, Jr. Bernhard P.
Thomas Eric
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