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FEBRUARY 2024 I DESIGN007 MAGAZINE 23 Calculating Formed Resistor Geometry Both thick- and thin-film resistor materials are formulated to furnish a wide range of pri- mary values. Although resistor element geom- etry can be developed by the designer math- ematically, soware tools are also available that can automatically calculate the embedded resistor geometry based on inputs for ohms, power, and tolerance. e calculated values are based on the resistance measured between the opposite edges of a square. For example, a sin- gle square of 1KW material printed or depos- ited between two copper lands will provide a 1KW resistor element, while a pattern of the same 1KW material that is twice the length or two squares furnishes a 2KW resistor, and two- and one-half squares (Figure 2) will provide a 2.5KW. Suppliers recommend that designers fur- nish resistor widths and lengths that are no less than 0.25 mm (0.010"). Increasing the resistor elements' dimensions, on the other hand, will reduce print variations and help in achieving the final resistor target values and tolerance accurac y. Additionally, when ter minat ing the thick-film resistor ele- ments, the land pattern geometry provided on the surface for resistor termi- nation should allow for a nominal 0.25–0.50 mm overlap of the deposited resistor material. Basic do's for embed- ding resistor elements: • Select resistor values that can be devel- oped using the same basic value paste or film material. • Maintain separation between elements to enable testing and, when required, laser trimming before lamination. • Locate all the embedded resistor elements onto a single subsurface layer of the multi- layer printed circuit board. • Select material that will match the primary value range and tolerance requirement of the embedded resistor elements. Don't consider the following: • Embedding resistors with limited toler- ance and that may be subject to value adjustment. • Distributing resistor elements onto mul- tiple layers of the multilayer circuit board. • Attempting to provide excessively wide resistor value variations. • Embedding resistors that may require post-lamination process access for trimming. Most soware developers are currently fur- nishing soware tools to automatically calcu- late the embedded resistor elements' geometry within the multilayer circuit board. Material suppliers recommend using Mentor, Alle- gro, Intergraph, and PAD Power PCB with an Excel program to aid in developing the more complex resistor element geometries. While several of the discrete resistor elements will Figure 2: Basic "bar" thick-film resistor element design.