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22 DESIGN007 MAGAZINE I FEBRUARY 2024 Resistors, in particular, tend to dominate the surface area of the circuit board, oen restrict- ing the circuit paths between active compo- nents. So, transferring most of the resistor ele- ments onto subsurface layers of the circuit board allows the designer to optimize semiconductor placement, provide more area for circuit inter- connect, and ultimately achieve the most effi- cient interconnect between principal functions. Planning for Embedded Resistors e first step during the planning phase of the circuit board is to select and identify on the schematic the most logical components for embedding. ese will include components closely coupled to related active device(s) on the circuit board's outer surface(s). Next, consider the value range and tolerance limits specified for passive functions. Formed passive resistors, for example, will have a limited tol- erance range, while the discrete passive com- ponents designed for placement and solder attachment onto the outer surfaces of the cir- cuit board can furnish significantly more value choices and greater tolerance parameters. Embedding Formed Resistor Elements Formed resistor elements may be furnished either as a printed or deposited thick-film com- position or an imaged and chemically etched thin-film process. • ick-film resistor materials are formu- lated to furnish a wide range of primary values and are successfully used for a broad number of commercial applications. e resistor formulations are based on car- bon-filled polymer chemistry that enables screen printing or pattern deposition to form the elements directly onto termina- tion lands furnished on a designated circuit board layer. • in-film resistor elements are formed using thin copper foil sheets pre-coated with a resistive material. e resistor layer is pre-deposited onto the copper sheet material using vapor disposition or electro- plating. Both processes ensure uniformity of the resistor base value across the entire sheet. Initial planning for embedding formed resis- tors: 1. Identify resistors for embedding. 2. Establish R-value and target tolerance. 3. Determine the power rating requirement. 4. Define finished element geometry. 5. Select location (layer) and orientation. When identifying candidate resistors for embedding, the designer must consider the resistor value range, the allowable tolerance range, and the application. e thick-film resis- tor forming process is employed where toler- ances are less critical, primarily used in digital and analog circuit applications for terminat- ing resistors, current limiting, transistor bias- ing, and for pull-up/pull-down resistors where value tolerances that range between 5–10% will probably meet the operational criteria of the final product. On most printed circuit board designs, resis- tor value distribution will vary between 1 ohm at the low end and 10M ohms at the high- est. Selecting the most practical composition for the thick-film resistors, the circuit board 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 probably fall between 10 ohm and 10K ohm, as illustrated in Table 1. With that in mind, selecting the 10-ohm material as the base value will provide greater flexibility in expanding the resistor geometry to accommodate a wide range of finished resistance values. e geometry of the resistance material can be a simple rectangle, or a serpentine shape designed to maximize resistor element length while minimizing area. In each case, the resis- tance material must terminate or overlap with the copper lands furnished within the circuit pattern.