Some power resistors of Microhm improve the heat radiation by adding heat sink. For chip resistors, the most of the generated heat conducted from the electrode to Cu foil pattern, and finally dissipated to the air or chassis. Temperature rise of the resistor is also controlled by improving thermal conductivity of PWB; by thick Cu foil pattern, forming solid pattern on the back side of PWB or solid pattern inside the layer if it is a multilayer substrate.
Resistors have small parasitic inductance and parasitic capacitance. Thus the higher the frequency becomes, the more the parasitic components are affected, which leads the resistors have reactance components.
We have to pay attention for using resistors in high frequency range because impedance of resistors are affected depending on frequency. The impedance change of resistor depends on resistance. When the resistance is low, parasitic inductance becomes dominant in the high frequency range, which makes higher impedance . When the resistance is high, the parasitic capacitance becomes dominant in high frequency range, which makes smaller impedance. When the resistance is extremely small e.g. under milliohm, surface effect enables to lead the phenomenon that parasitic inductance drops and resistance of resistive body itself increases.
