Issue link: https://iconnect007.uberflip.com/i/1515637
74 DESIGN007 MAGAZINE I FEBRUARY 2024 The concept of embedded/ integrated devices and/or their function has deep roots in the printed circuit industry. function) into the actual electronic intercon- nection substrate (typically a printed circuit). e concept of embedded/integrated devices and/or their function has deep roots in the printed circuit industry. Seventy-five years ago, the U.S. government published "Printed Circuit Techniques" to promulgate knowl- edge gained in electronics research during World War II. It was also meant to jumpstart the post-war electronics industry in the U.S., as it described methods for making resistors in situ and trimming them to value in place. Such methods have been employed ever since, though more oen on the inner layers of mul- tilayer boards. e concept of distributed capacitance evolved with the introduction of multilayer PCBs in the 1960s and '70s when it was realized that the stacking of multiple layers of circuits, includ- ing ground and power, provided an opportunity to incorporate distributed capacitance intrinsi- cally embedded and made pos- sible by the built-in parallelism of adjacent layers which naturally contributed to the capacitance between them. Material suppli- ers recognized the opportunity and began to explore the use of alterna- tive materials for use in multilayer boards, including the use of high-permittivity dielec- tric materials to improve and better control distributed capacitance. Designers soon began considering capacitance a key factor in signal integrity and power distribution. e terms "buried capacitance" and "embedded capaci- tance" are much a part of the current printed circuit industry lexicon. Embedded discrete components also have a long history, depending on how rigid one wants to define embedment. One of the first volume applications was "chip on board" (COB) products that ushered in the video game console industry in the mid-1970s. ese structures could arguably embrace both ends of the spectrum described thus far. ey were programmed chips attached to PCBs, wire- bonded to make interconnections, and encap- sulated with epoxy. e method obviated the need for the packaging and subsequent solder assembly of the chip. e assembly was then given a housing identifying the game, and edge card connections were used to plug the mod- ule into the game player system. Common Approaches Turning attention to the embedment of other passive functions to provide discrete components, including discrete capacitors (as distributed capacitance as discussed ear- lier), resistors, and inductors, the most com- mon approach has been the attachment of the component to etched lands on inner layers of the multilayer PCB using solder. is approach requires cavities be made in the prepreg and/or the inner layer materials used in constructing the board. Yet unstated but presumed to be understood, the purpose of embedding discrete devices has typically been to free up space on the outer surfaces of the printed circuit. Beyond embedment of com- ponents, several techniques have been developed for forming the components in situ on inner layers. is was seen as an opportunity in the 1970s, as special laminates having bimetallic foils of copper and more resistive metals or coatings were developed to produce chosen resistor values by selectively etching the different foils to create the num- ber of squares required in the resistive layer. However, the printing of a resistive material between ends of copper traces is a long-time practice. e resistor value can be "roughed in" by choosing the right value for the ink and the value "fine-tuned" by laser trimming the material to the desired value.