Resistance Parasitics
CHAPTER 33.1 Parasitic Inductance and Capacitance of SMD Resistors Surface mount resistors come in sizes 0402, 0603, 0805 and higher. In its standard mounting condition, with face up, the resistor acts as a tiny inductor. The value of the inductance is larger if the loop area between the upper metallization layer and the ground plane is large. To minimize this loop area and the inductance, RF engineers sometimes prefer to mount the resistor with upside down.
Figure 3.2 – A real resistor with Parasitic inductance and parasitic capacitance
Mounted on the PCB in the normal position the resistor also has small capacitance formed by the upper metallic layer of the resistor and the ground plane. The exact value of the capacitance will depend upon the separation of the ground plane from the metallization layer as also the size of the two smd pads , the dielectric permittivity, Er, of the resistor substrate (typically alumina with Er = 10), the dielectric permittivity of the PCB material between top layer and the ground layer. The larger pad sizes and smaller ground layer to top layer separation will lead to higher parasitic capacitance. Mounting the resistor upside down will reduce the series inductance and increase the shunt capacitance. Parasitic Inductance and the Parasitic Capacitance of 0402 size resistors can be ignored for most designs up to 1 GHz. In some cases, you may need to take into account the parasitic values of the resistance. Some examples, if you are conducting simulation for GHz signals with end terminations. In such cases a resistor is modeled as an inductance in series and a capacitor in parallel, as shown in figure above.
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