The present invention relates to a chip resistor having low resistance of no greater than e.g. 1Ω, and also relates to a method of making the same.
In a conventional chip resistor of the above-mentioned type, as disclosed in JP-A-2001-118701 for example, the resistor element is formed of an alloy into a rectangular solid, the alloy being composed of a base material metal, such as copper, having low resistance (hereinafter referred to as low-resistant metal) and a metal having high resistance (hereinafter referred to as high-resistant metal), such as nickel, which is greater than that of the base material metal. In the resistor element, the rectangular solid has ends provided with connection terminal electrodes to be connected to a printed circuit board or the like by soldering, for example.
The resistance between the connection terminal electrodes of such a chip resistor largely depends on the resistivity of the alloy making the resistor element. The resistivity of an alloy decreases as the percentage of low-resistant metal in the alloy becomes higher as compared to the high-resistant metal, whereas it increases as the percentage of high-resistant metal in the alloy becomes higher as compared to the low-resistant metal. In other words, the resistivity of the alloy decreases in proportion to the percentage of the low-resistant metal relative to the high-resistant material, while increasing in proportion to the percentage of the high-resistant metal relative to the low-resistant metal.
Thus, in a conventional chip resistor including a resistor element of a rectangular solid having predetermined length and width, the resistance between the connection terminal electrodes, or the resistivity of the chip resistor, is reduced by one or both of the following methods:
(1) Using an alloy containing an increased ratio of low-resistant metal relative to high-resistant metal.
(2) Increasing the thickness of the resistor element.
Generally, however, a metal material has a temperature coefficient of resistance, which describes the resistance change in relation to the temperature. It is known that the temperature coefficient of resistance is higher in a pure metal than in an alloy.
When the option (1) is taken to reduce the resistance of the chip resistor, the ratio of the low-resistant metal (base material metal) in the alloy making the resistor element is increased, whereby the alloy has increased purity of the low-resistant metal (base material metal). Unfavorably, this results in a higher temperature coefficient of resistance in the chip resistor.
When the option (2) is taken to reduce the resistance of the chip resistor, the thickness of the resistor element increases, whereby the weight of the chip resistor becomes greater, and it becomes difficult to bend the lengthwise-spaced ends of the resistor element into connection terminals. Additionally, it becomes significantly difficult to perform trimming adjustment by making a trimming groove in the resistor element for adjustment of the resistance.
Further, most of pure metals have positive temperature coefficient of resistance (directly proportional to temperature), whereas some alloys composed of pure metals have negative temperature coefficient of resistance (inversely proportional to temperature). When an alloy with such negative temperature coefficient of resistance is used to make a resistor element, unfavorably the negative temperature coefficient of resistance appears as a minus temperature coefficient of resistance of the chip resistor.
As another example of such a low-resistant chip resistor, JP-A-2002-57009 discloses a conventional structure, according to which a resistor element comprises a metal plate or rectangular chip formed of an alloy of low-resistant metal such as copper and high-resistant metal such as nickel. The lower surface of the resistor element has lengthwise-spaced ends to which connection terminals are attached, the terminals being made of a metal having a lower resistance than the alloy making the resistor element. The surfaces of the connection terminals are formed with metal-plated layers for facilitating soldering to e.g. a printed circuit board.
However, in the chip resistor of JP-A-2002-57009, metal connection terminals to be soldered to a printed circuit board are attached to the ends of the lower surface of the resistor element. Due to this structure, melted solder may swell up to the lower surface of the resistor element beyond the connection terminals, whereby the resistance can change. To avoid such change in resistance, spacing between the lower surface of the resistor element and a printed circuit board should be increased. Unfavorably, this structure increases the entire height and the weight of the chip resistor.