1. Field of the Invention
The present invention relates to a material for a Bi—In—Sn thermal fuse element, and also to an alloy type thermal fuse.
An alloy type thermal fuse is widely used as a thermo-protector for an electrical appliance or a circuit element, for example, a semiconductor device, a capacitor, or a resistor.
Such an alloy type thermal fuse has a configuration in which an alloy of a predetermined melting point is used as a fuse element, the fuse element is bonded between a pair of lead conductors, a flux is applied to the fuse element, and the flux-applied fuse element is sealed by an insulator.
The alloy type thermal fuse has the following operation mechanism.
The alloy type thermal fuse is disposed so as to thermally contact an electrical appliance or a circuit element which is to be protected. When the electrical appliance or the circuit element is caused to generate heat by any abnormality, the fuse element alloy of the thermal fuse is melted by the generated heat, and the molten alloy is divided and spheroidized because of the wettability with respect to the lead conductors or electrodes under the coexistence with the activated flux that has already melted. The power supply is finally interrupted as a result of advancement of the spheroid division. The temperature of the appliance is lowered by the power supply interruption, and the divided molten alloys are solidified, whereby the non-return cut-off operation is completed.
2. Description of the Prior Art
Conventionally, a technique in which an alloy composition having a narrow solid-liquid coexisting region between the solidus and liquidus temperatures, and ideally a eutectic composition is used as such a fuse element is usually employed, so that the fuse element is fused off at approximately the liquidus temperature (in a eutectic composition, the solidus temperature is equal to the liquidus temperature). In a fuse element having an alloy composition in which there is a solid-liquid coexisting region, namely, there is the possibility that the fuse element is fused off at an uncertain temperature in the solid-liquid coexisting region. When an alloy composition has a wide solid-liquid coexisting region, the uncertain temperature width in which a fuse element is fused off in the solid-liquid coexisting region becomes large, and the operating temperature is largely dispersed. In order to reduce the dispersion, therefore, the technique in which an alloy composition having a narrow solid-liquid coexisting region, and ideally a eutectic composition is used is usually employed.
Because of increased awareness of environment conservation, the trend to prohibit the use of materials harmful to a living body is recently growing as a requirement on an alloy type thermal fuse. Also an element for such a thermal fuse is requested not to contain a harmful material.
As an alloy composition for such a thermal fuse element, known is a Bi—In—Sn system. Conventionally, known are alloy compositions such as that of 47 to 49% Sn, 51 to 53% In, and the balance Bi (Japanese Patent Application Laying-Open No. 56-114237), that of 42 to 44% Sn, 51 to 53% In, and 4 to 6% Bi (Japanese Patent Application Laying-Open No. 59-8229), that of 44 to 48% Sn, 48 to 52% In, and 2 to 6% Bi (Japanese Patent Application Laying-Open No. 3-236130), that of 0.3 to 1.5% Sn, 51 to 54% In, and the balance Bi (Japanese Patent Application Laying-Open No. 6-325670), that of 33 to 43% Sn, 0.5 to 10% In, the balance Bi (Japanese Patent Application Laying-Open No. 2001-266723), that of 40 to 46% Sn, 7 to 12% Bi, the balance In (Japanese Patent Application Laying-Open No. 2001-266724), that of 2.5 to 10% Sn, 25 to 35% Bi, the balance In (Japanese Patent Application Laying-Open No. 2001-291459), and that of 1 to 15% Sn, 20 to 33% Bi, and the balance In (Japanese Patent Application Laying-Open No. 2001-325867).
When the liquidus phase diagram of a ternary Bi—In—Sn alloy is obtained, there are a binary eutectic point of 52In-48Sn and a ternary eutectic point of 21Sn-48In-31Bi, and the binary eutectic curve which elongates from the binary eutectic point toward the ternary eutectic point passes approximately through a frame of 24 to 47 Sn, 50 to 47 In, and 0 to 28 Bi.
As well known, when a heat energy is applied to an alloy at a constant rate, the heat energy is spent only in raising the temperature of the alloy as far as the solidus or liquidus state is maintained. When the alloy starts to melt, however, the temperature is raised while part of the energy is spent in the phase change. When the liquidification is then completed, the heat energy is spent only in temperature rise while the phase state is unchanged. The temperature rise/heat energy state can be obtained by a differential scanning calorimetry analysis [in which a reference specimen (unchanged) and a measurement specimen are housed in an N2 gas-filled vessel, an electric power is supplied to a heater of the vessel to heat the samples at a constant rate, and a variation of the heat energy input amount due to a state change of the measurement specimen is detected by a differential thermocouple, and which is called a DSC].
Results of the DSC measurement are varied depending on the alloy composition. The inventor measured and eagerly studied DSCs of Bi—In—Sn alloys of various compositions, and unexpectedly found the following phenomenon. When an alloy composition in a specific region which is separated from the binary eutectic curve is used as fuse elements, the fuse elements can be concentrically fused off in the vicinity of the maximum endothermic peak, and excellent overload characteristic and dielectric breakdown characteristic are obtained.
By contrast, also the followings were known. In the case where a composition which is along or in the vicinity of the binary eutectic curve is used as fuse elements, even when the fuse elements can be concentrically fused off at concentrated temperatures by the usual technique, satisfactory overload characteristic and dielectric breakdown characteristic are hardly obtained.
The overload characteristic means external stability in which, even when a thermal fuse operates in an raised ambient temperature under the state where a current and a voltage of a specified degree are applied to the thermal fuse, the fuse is not damaged or does not generate an arc, a flame, or the like, thereby preventing a dangerous condition from occurring. The dielectric breakdown characteristic means insulation stability in which, even at a specified high voltage, a thermal fuse that has operated does not cause dielectric breakdown and the insulation can be maintained.
A method of evaluating the overload characteristic and the dielectric breakdown characteristic is specified in IEC (International Electrotechnical Commission) Standard 60691 which is a typical standard, as follows. When, while a rated voltage×1.1 and a rated current×1.5 are applied to a thermal fuse, the temperature is raised at a rate of 2±1 K/min, to cause the thermal fuse to operate, the fuse does not generate an arc, a flame, or the like, thereby preventing a dangerous condition from occurring. After the thermal fuse operates, even when a voltage of the rated voltage×2+1,000 V is applied for 1 min. between a metal foil wrapped around the body of the fuse and lead conductors, and, even when a voltage of the rated voltage×2 is applied for 1 min. between the lead conductors, discharge or dielectric breakdown does not occur.