Issue link: https://iconnect007.uberflip.com/i/1381013
74 DESIGN007 MAGAZINE I JUNE 2021 Base Materials for Thin-Film Resistor Forming To begin, a resistive alloy coating is depos- ited onto copper foil to create the base material for embedded resistor applications. e resis- tive alloy is electrodeposited or deposited onto the copper foil using a roll-to-roll sputtering process. Resistive alloy thickness determines the overall resistance value (ohms/sq.) of the coated copper foil. Alloys commonly applied for embedded resistor applications include nickel phosphorous (NiP), chromium silicon monoxide (CrSiO), nickel chromium alumi- num silicon (NCAS), and nickel chromium (NiCr). e sheet resistance of nickel-chrome alloy film containing 20% chromium, as an example, will furnish the designer with a resis- tance range as high as 3 K-ohms. Resistor Element Planning Typical of the discrete resistor element, the formed resistor will span the area between two copper lands. e shape of the resistance mate- rial between the copper lands can be a simple square, a series of squares to form a rectangle, or a shape designed to maximize resistor ele- ment length while minimizing area. Initial planning: 1. Identify resistors for embedding. 2. Establish R-value and target tolerance. 3. Determine power rating requirement. 4. Define finished element geometry. 5. Select location (layer) and orientation. e power dissipation is the rate at which resist energy is lost in elements. e power capability for embedded resistors will depend on the physical size of the resistor elements, temperature rating of the surrounding sub- strate materials, and the board stack-up. In the end it boils down to how the heat generated is managed. Typical power dissipation for most thin-film resistor designs operating at an ambi- ent of less than 70°C is approximately 1/10 to 1/8 watt. Typical of the thick-film composites, the base values of the thin-film resist-coated cop- per foil sheet materials are based on a single square geometry. While terminating resistor values are predominantly 50 ohms, and pull-up resistors fall in a range between 1K ohm and 10K ohm, these base values can be extended to furnish significantly higher resistor elements. Implementation: 1. Establish land pattern (termination) geometry. 2. Define overall element dimensions. 3. Select optimum element position. 4. Plan most efficient circuit interface. 5. Provide features for test probe access. As noted, the "square" geometry represents the basic ohm value of the resistive material. e designer can increase the resistance value by simply extending the length of the resis- tor pattern with additional squares or partial square segments (Figure 2). e resist-coated copper foil will become an integral part of the multilayer circuit board construction that, when processed, will fur- nish both formed resistor elements and pro- vide general interconnect functions. e formed NiP resistor element examples (Figure 3) represent a subsurface interconnect layer prepared for lamination within a multilayer PCB. Aer chemically removing copper and defin- ing the resistor image, the now exposed nickel- phosphorous resistive material exhibits a matte grey finish. To enable efficient utilization of the primary base value of the coated foils, the element Figure 2: Basic "bar" resistor element design.