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APRIL 2021 I DESIGN007 MAGAZINE 67 on a number of factors. ese include target resistance values, electrical considerations, available resistivity values, trimming require- ments, and the distribution of resistances of all the resistors present on the same layer of the board. In general, on most printed circuit board designs, resistor value distribution will vary between 1W at the low end and 10MW at the highest. Selecting the most practical com- position for the thick-film resistors, the PCB designer should consider the most prominent base-value usage and select a material that facilitates the lower end of the value range. From a statistical standpoint, the greater num- ber of resistors in a digital or analog circuit will likely fall into a range between 10W and 10KW (Table 1). With that in mind, selecting the 10W material as the base value will provide greater flexibility in expanding the resistor geome- try to accommodate a wide range of finished resister values. Geometry Principles e geometry of the resistance material can be as simple as a square or rectangle, or for more complicated resistor values, a serpen- tine shape designed to maximize resistor ele- ment length while minimizing area. e values provided are based on the resistance measured between opposite edges of a square. For exam- ple, a single square of 1K material printed or deposited between two copper lands will pro- vide a 1K resistor element while a pattern that is twice the length, or two squares, furnishes a 2K resistor. e rectangular "bar" geometry (Figure 1) is most common for resistors with values close to the basic thick film composition selected while the serpentine geometry is employed when resistor values are significantly greater than the thick film materials base value. e "top-hat" shaped resistor geometry is commonly applied for elements that will likely require extensive laser trimming to reach their target value. Suppliers recommend that designers fur- nish resistor widths and lengths greater than 0.25 mm (0.010 in). Larger resistor dimensions will reduce the reliance on the print variations or accuracy of the copper etching processes. Regarding terminating the resistor elements, the land pattern geometry provided for the resistor termination should allow for a nominal 0.25–0.50 mm overlap of the thick-film resist material and consider allowances for printing process variables. As noted, the thick-film resistor ink formu- lations are based on a carbon-filled polymer chemistry. By adjusting the ratio of carbon con- tent within the polymer medium, the material can be formulated to furnish a wide range of primary values. Following printing or deposi- tion of the resist compound the circuit boards are transferred to an oven for curing at tem- peratures in a range between 150–250°C. Five Table 1: Typical percentage distribution by resistor value. Figure 1: Thick film resistor pattern variations.