GB2113455A - High capacitance laminated bus - Google Patents

Abstract

In a laminated bus bar comprised of a pair of elongated flat conductors (1, 3) separated by a layer (5) of insulating material with incorporated capacitors (6), the electrodes (10, 11 Figure 4 not shown) of the capacitors (12) are nickel or nickel alloy materials. <IMAGE>

Description

SPECIFICATION High capacitance laminated bus This invention relates to multilayer or laminated bus bars which include embedded or incorporated condensers. These bus bars are usually assembled such that an insulating material and conductors are overlapped or laminated to form the bus bar and wherein the capacitors are incorporated directly in the bus bar. More particularly, this invention relates to an improved high capacitance laminated bus bar with incorporated capacitors wherein the electrodes of the capacitors are nickel or mainly nickel alloy to overcome a previous problem of the formation of leakage paths resulting from silver migration.

With the trend toward large scale piling of l/O in addition to the high density packaging of electronic parts on circuit boards, the power supply bus bars and signal supply bus bars are required to have low inductance and low characteristic impedance and high distributive capacitance in order to avoid the undesirable effects of high frequency noise.

Laminated bus bars in which conductor layers are interleaved and spaced with an insulating film of relatively high permittivity are known for use as power supply and signal supply components, these components commonly being referred to as multilayer or laminated bus bars. However, the dielectric materials normally used as the interleaved insulating spacer have limits as to dielectric constant. The use of very high dielectric materials has been proposed, but they pose problems not only as to cost, but also practical problems as to the level of and limitations on dielectric constant.

In U.S. Patent 4,266,091 is disclosed a laminated bus bar with incorporated capacitors which resulted in a significantly improved power system. This patent involved the direct incorporation in the bus bar of capacitors which preferably consisted of ceramic chips having silver electrodes. The capacitors are located in windows or cut-outs in the.

insulating sheet between the bus bar conductors and are electrically and/or mechanically connected to the bus bars. While bus bars constructed in accordance with this prior patent offer significant advantages, a problem of silver migration can be encountered which will cause leakage paths which impairthe effectiveness of the embedded capacitors. These leakage paths, which can be viewed as effectively reducing the insulation resistance of the individual capacitive elements, can impairthe performance of the bus bar system.

The purpose of the present invention is to overcome the silver migration problem by providing an improved high capacitance laminated bus.

In accordance with the present invention, there is provided a multi-conductor bus bar assembly comprising a first bus conductor, said first conductor being formed from an elongated flat strip of conductive material, at least a second bus conductor, said second conductor being formed from an elongated flat strip of conductive material, said first and second bus conductors being spaced apart and electrically insulated from each other, and at least one capacitor incorporated between said bus conductors, said capacitor comprising a flat body of dielectric material with opposed face surfaces, and a conductive electrode of nickel or primarily nickel alloy on each of said opposed face surfaces, said electrodes being electrically connected to opposed inwardly facing surfaces of said bus conductors.

Referring now to the drawings, wherein like elements are numbered alike in the several Figures: Figure 1 is a schematic view of a laminated bus bar with embedded or incorporated capacitors in accordance with U.S. Patent 4,266,091, Figure 2 shows the equivalent electrical circuit diagram of the bus bar of Figure 1, Figure 3 is a perspective view of the construction of a capacitor of the prior patent 4,266,091, and Figure 4 is a cross-sectional view of a bus bar capacitor in accordance with the present invention.

Referring first to Figures 1,2 and 3, a structure is shown of a laminated bus bar with embedded or incorporated capacitors to form an improved power or signal distribution system in accordance with the prior patent. As shown in Figure 1,two bus conductors 1 and 3 have integral terminals 2 and 4, respectively, positioned at required intervals along the conductor elements. These conductors 1 and 3 are arranged with the required offset of their respective integral terminals 2 and 4, and with the interposition of an insulating film 5 between the conductors 1 and 3. The insulating film will be provided with openings or windows to receive and house the capacitors 6, and the entire structure is then laminated into a complete assembly covered with an outer insulating film (not shown).The capacitors 6 are comprised of thin ceramic plates 7, characterized by a high dielectric constant, and conductive outer electrode layers 8 and 9 of silver. In the assembly of the unit, with the capacitors located in the windows of the insulating strip 5, one of the electrodes, e.g., electrode 8 will contact the inwardly facing surface of conductor 1, and the other electrode, e.g. 9, will contact the inwardly facing surface of conductor 3.

As disclosed in the prior patent, a conductive adhesive may be employed to form a conductive bond between the electrodes 8 and 9 and the inwardly facing surfaces of conductors 1 and 3 to be contacted by the electrodes; and the film 5 may be bonded by nonconductive adhesive to the conductors 1 and 3.

The laminated bus bar thus obtained in accordance with the prior patent has an extremely high total capacitance which, as illustrated in Figure 2, includes the capacitance of the capacitors 6 as well as the distributed capacitance 12 of the insulating film 5. It is, however, to be understood that the showing of Figure 2 is not intended to illustrate or depict relative capacitance values of the capacitors 6 and the insulating film 5. In fact, the capacitance of the capacitors 6 is much greater, perhaps several orders of magnitude greater, than tne capacitance of the insulating layer 5. High capacitance bus bars in accordance with this prior construction are particularly suitable for use with high density circuit boards and presents significant advantages over the use of external capacitors.

While significant advantages are achieved with the prior construction, a problem, which may be referred to silver migration, has been encountered in some situations. Silver in the electrodes 8 and 9 may, with the application of an electrical ioad in a humid environment, shift or migrate. This results in the formation of leakage paths around the edges of the capacitive chip (which may be viewed as effectively reducing the insulation resistance of the chip) resulting in significant reduction in effectiveness of the capacitive elements.Attempts have been made to overcome or reduce the silver migration problem by mixing palladium with the silver, or by impregnation with parafin or polybutadiene to protect against the humidity problem; or it has been suggested to create a dielectric margin around the capacitive chips by stopping the silver electrodes short of the edges of the ceramic centers. However, none of these approaches fundamentally change the fact that the use of silver or silver alloy electrodes may, depending on the effectiveness of the antihumidity treatment and other conditions, result in occurrence of the silver migration problem; and the formation of leakage paths with resultant loss in effectiveness of the capacitive elements is likely to occur.By way of example, in one test condition at 40 centigrade and 90" relative humidity and with a load of 25 volts DC, the resistance across a condenser 6 declined in a period of 24 hours from 104 M ohms to less than 1 M ohm.

In accordance with the present invention, which is illustrated in Figure 4, the opposed face surfaces of a flat sheet of high dielectric material 7 (preferably a ceramic chip) are provided with electrodes 10 and 11 which are either nickel or a metal alloy comprising mainly nickel. Capacitive chips constructed with the nickel alloy electrodes do not encounter the migration problem of the electrode metal as previously was the case with silver electrodes. Thus, leakage paths do not occur in the capacitive elements constructed in accordance with the present invention, and there is no reduction in the effective insulation resistance of such chips.Thus, by constructing laminated bus bars in accordance with the structure of Figure 1 incorporating capacitive chips 12 having the construction of Figure 4, the performance and reliability of high capacitance laminated buses with incorporated or embedded capacitors is markedly increased without any significant increase in cost.

The method of forming the condensors 12 of Figure 4 by forming nickel or high nickel alloy electrodes on the ceramic chip totally eliminates the elaborate, troublesome and expensive processes which had to be employed previously as counterme asures to the silver migration problem. Thus, the antihumidity treatments and the need for forming insulating dielectric margins on the capacitors can be eliminated. It is particularly important to note that since the nickel electrodes of the present invention need only be applied evenly on both surfaces of ceramic elements 7, there is no limit to the shape in which the capacitive chips can be made. The problems of the prior art often dictated that the ceramic capacitors be carefully and precisely formed in squares or rectangles with sharply defined edges or margins. That limitation is not encountered with the nickel electrodes of the present invention, with the result that there is a significant increase in freedom of design as weli as a significant increase in the performance and reliability of high capacitance laminated bus bars incorporating the improved capacitors of the present invention.

Claims (8)

1. A multi-conductor bus bar assembly comprising a first bus conductor, said first conductor being formed from an elongated flat strip of conductive material, at least a second bus conductor, said second conductor being formed from an elongated flat strip of conductive material, said first and second bus conductors being spaced apart and electrically insulated from each other, and at least one capacitor incorporated between said bus conductors, said capacitor comprising a flat body of dielectric material with opposed face surfaces, and a conductive electrode of nickel or primarily nickel alloy on each of said opposed face surfaces, said electrodes being electrically connected to opposed inwardly facing surfaces of said bus conductors.

2. A bus bar assembly as claimed in claim 1, wherein said dielectric material is a ceramic material.

3. A bus bar assembly as claimed in claims 1 or 2, including a layer of insulating material between said bus conductors, said inhibiting material having an opening to receive said capacitor.

4. A bus bar assembly as claimed in claim 1, including means for mechanically containing said bus bar assembly in a completed assembly.

5. A multi-conductor bus bar assembly comprising a first bus conductor, said first conductor being formed from an elongated flat strip of conductive material, at least a second bus conductor, said second conductor being formed from an elongated flat strip of conductive material, said first and second bus conductors being spaced apart and electrically insulated from each other, and a plurality of capacitors incorporated between said bus conductors, said capacitors each comprising a flat body of dielectric material with opposed face surfaces, and a conductive electrode of nickel or primarily nickel alloy on each of said opposed face surfaces, said electrodes being electrically connected to opposed inwardly facing surfaces of said bus conductors.

6. A bus bar assembly as claimed in claim 5, wherein said dielectric material is a ceramic material.

7. A bus bar assembly as claimed in claims 5 or 6, including a layer of insulating material between said bus conductors, said insulating material having a plurality of openings to receive said capacitors.

8. A multi-conductor substantiaily as hereinbefore described and as illustrated in the accompanying drawings.