PCB007 Magazine

PCB-Jun2017

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12 The PCB Magazine • June 2017 by Vern Solberg Embedding Components, Part 1 FEATURE COLUMN: PCB DESIGNERS NOTEBOOK The printed circuit has traditionally served as the platform for mounting and interconnect- ing active and passive components on the out- er surfaces. Companies attempting to improve functionality and minimize space are now con- sidering embedding a broad range of these com- ponents within the circuit structure. Both un- cased active and passive component elements are candidates for embedding but the decision to embed components within the multilayer circuit structure must be made early in the de- sign process. Some components are easy candidates for integrating into the substrate while others may involve more complex processes and will be difficult to rationalize. Processes have evolved for embedding and interconnecting a range of common passive components: resistor, capaci- tor and inductor elements. Embedding formed resistor elements, however, is the most ma- ture and economical process to implement. A formed resistor will have two copper lands with resistance material applied between the lands. The shape of the resistance material can be a simple rectangle, or a shape designed to maxi- mize resistor element length while minimizing area. In each case, the resistance material must overlap the copper lands. Formed resistor elements may be furnished as a printed thick-film composition or by em- ploying a chemically etched thin-film process. • Thick-film resistor materials are formulated to furnish a wide range of primary values. The resistor formulations are based on carbon-filled epoxy chemistry that enables screen printing the elements directly onto pre-patterned termi- nations on a designated circuit board layer fol- lowed by curing the product at temperatures in the range of 150°C. This carbon-filled epoxy thick film (TF) chemistry has been successfully used for a broad number of commercial applica- tions. The process is generally employed where tolerances are less critical. Applications are var- ied and range from simple discrete potentiome- ters to pull-up and pull-down resistors.

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