Investigation of Trim Notch and its Effect on Surface Mount Resistors in RF Circuits

Surface mount (SMD) resistors are often employed in microstrip circuitry at RF frequencies. Typical SMD resistors designed for general purpose circuitry are usually trimmed using plunge trim or "L" trim techniques. This involves removal of material in a narrow groove either perpendicular to the current flow (plunge trim) or in a pattern first perpendicular to current flow and then parallel to the current flow ("L" trim). These configurations can exhibit large variations in RF impedance depending upon the degree of trimming and are therefore not considered here. Instead, SMD resistors intended for use in RF applications are typically trimmed using the scan trim technique. In this process the material is ablated parallel to the flow of current starting at one edge of the film. This effectively narrows the resistive film without changing the uniform area perpendicular to the current flow. The effect of this trim technique on the RF performance of a typical SMD resistor is investigated here.