Electrical resistors – Mechanically variable – Resistance value varied by removing or adding material
Patent
1998-12-15
2000-11-07
Easthom, Karl
Electrical resistors
Mechanically variable
Resistance value varied by removing or adding material
338309, 338313, 338254, 29620, H01C 1000
Patent
active
061442879
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a chip resistor including a chip-type insulating substrate and at least one resistor film formed thereon, and to a method of making such a chip resistor.
BACKGROUND ART
A conventional chip resistor of this kind includes an insulating substrate, a pair of main electrodes formed on a surface of the insulating substrate and spaced from each other, a resistor film which is formed on the surface of the insulating substrate to bridge between the main electrodes and provided with a trimming groove for resistor adjustment, a protective coating formed to cover the resistor film, and a metal plating formed in electrical conduction with each of the main electrodes, as disclosed in Japanese Patent Application Laid-open Nos. 56-148804 and 4-102302 for example. The protective coating includes an undercoat layer of glass formed on the resistor film, a middle-coat layer of glass formed on the undercoat layer and an overcoat layer formed on the middle-coat layer. The trimming groove may be formed by irradiating the resistor film with e.g. a laser beam from above the undercoat layer. The metal plating is formed by solder-plating treatment after performing nickel-plating for the chip resistor as a whole.
The purpose of forming the middle-coat layer of glass on the chip resistor having the above arrangement is to seal the resistor film hermetically against the exterior air with the use of the glass-made middle-coat layer, and to fill up the trimming groove of the resistor film. However, it has been found that if the middle-coat layer is not properly formed, pinholes tend to be formed in the trimming groove, thereby breaking the sealing condition as described below.
Specifically, the middle-coat layer is formed by applying a glass paste by screen-printing and baking it at a substantially high temperature. However, at the time of applying a glass paste by screen-printing for the middle-coat layer, the glass paste often traps bubbles in the trimming groove formed in the resistor film and the undercoat layer. The bubbles will expand upon baking the glass paste, leading to generation of pinholes communicating with the exterior air. As a result, an amount of plating liquid flows into the pinholes during metal plating treatment performed subsequently, and metal plating pieces will grow therein.
If the pinholes are small, the growth of the metal plating pieces within the pinholes does not cause any particular problem. However, if the pinholes are large, the grown metal plating pieces in the pinholes will bridge between the side surfaces at the trimming groove, thereby causing the overall resistance of the resistor film to deviate from a predetermined value. Thus, conventionally, there has been a problem of high occurrence of rejective devices.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a chip resistor overcoming the above problem.
It is another object of the present invention to provide a method of making such a chip resistor.
According to a first aspect of the present invention, there is provided a chip resistor including an insulating substrate, a pair of main electrodes formed on a surface of the insulating substrate and spaced from each other, a resistor film formed on the surface of the insulating substrate to bridge between the main electrodes, the resistor film being provided with a trimming groove for resistor adjustment, a protective coating formed to cover the resistor film, and a metal plating formed in electrical conduction with each of the main electrodes, wherein the trimming groove has a width which is no less than 80 .mu.m but smaller than an interval between the main electrodes.
The technical advantages obtainable from the above arrangement will be described in detail with reference to the embodiments described hereinafter.
Preferably, the width of the trimming groove is 90-150 .mu.m. Advantageously, the trimming groove is inverted-trapezoidal in cross section. In this instance, the trimming groove preferably has a pair of in
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patent: 5510594 (1996-04-01), Mori et al.
patent: 5966067 (1992-08-01), Hashimoto
International Search Report for PCT/JP97/02219, Oct. 7, 1997.
Easthom Karl
Rohm & Co., Ltd.
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