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Design007-Apr2021

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66 DESIGN007 MAGAZINE I APRIL 2021 rarely the sole decision maker for embedding components. Because of the potential cost impact on the printed circuit board, the deci- sion to embed resistors is more likely an engi- neering and management level issue, typically justified by the restricted surface area reserved for mounting passive components and/or the potential for enhanced performance of the fin- ished product. In preparation for implementing embedded resistor technology the designer and/or pro- gram manager must first seek an experienced supplier company that can furnish practical guidance in selecting a process (thick-film or thin-film) that will meet both technical and budgetary (cost) goals established for the end product. Formed resistor elements may be furnished as a printed thick-film composition 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 have been successfully used for a broad number of commercial applica- tions. e resistor formulations are based on carbon-filled polymer chemistry that enables screen printing or deposition to form elements directly onto pre-patterned termination lands furnished on a desig- nated circuit board layer. • in-film resistors are formed using copper foil material that is pre-coated with resistive material. e resist layer is depos- ited onto the copper sheet material using vapor disposition that provides uniformity of the resistor base value across the entire sheet. in-film resistor materials are supplied in a variety of base values using Grade 3 copper foil. e copper sheets developed for the thin-film resistor form- ing process are available in thicknesses of 18 µm (0.5 oz) and 35 µm (1 oz). e information furnished in this install- ment of this series has been prepared to pro- vide guidance to the circuit board design pro- fessional considering the implementation for embedding "thick-film" resistors. When identifying candidate resistors for embedding, the designer must consider both resistor value range, the allowable tolerance bandwidth, and application. e thick-film resistor forming process is generally employed where tolerances are less critical, primarily used in digital and analog circuit applications for terminating resistors, current limiting, transistor biasing as well as for pull-up/pull- down resistors where precise value tolerances are not critical. Resistor Functionality Termination resistors are placed at the end of an electrical transmission line or when working with differential pair signals. Pull-up and pull-down resistors on, the other hand, are commonly used in logic circuit applications. For example, the function of the pull-down resistor is to hold the logic signal near to zero volts when no other active device is connected. e pull-up resistor's function is to ensure that the voltage between power and ground cannot be directly connected. Depending on the cir- cuit logic type, typical values selected for ter- mination, pull-up and pull-down resistors can vary in values that range from 500 ohm to 10K ohm and may tolerate value tolerance limits as high as ±20%. Resistor elements designated for "current limiting" are used for setting an upper limit to the amount of current that flows through a component while "transistor bias- ing" resistors are commonly used in combina- tion with transistors and semiconductor com- ponents. e overall performance of the thick film resistor materials is related to the optimized circuit design and fabrication process. e materials and process parameters of poly- mer thick-film resistor must be considered in order to successfully achieve the performance requirements of circuit designs. For example, the decision on what landsize-to-aspect-ratio to use for a particular resistor element depends

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