Though the concept may seem anathema to this magazine, it is nevertheless axiomatic that the best assembly is often no assembly. If a feature can be molded, machined or fabricated into a part, assemblers can save the cost of fastening, welding or bonding it.
This principle is relatively easy to follow if you're designing the plastic housing for a kitchen appliance. But, if the product is an electronics assembly, which has always been predicated on populating a printed circuit board (PCB) with various active and passive components, the task is more difficult. Today, that may be changing, thanks to advances in technology for embedded passives.
Resistors, capacitors and other passives account for as much as 95 percent of all the components in an electronics assembly, and they can occupy up to 40 percent of the surface area of a PCB. For example, the average cell phone has 445 passives, outnumbering integrated circuits (ICs) by a ratio of 25-to-1.
Embedding resistors and capacitors directly into the circuit board has a number of benefits, not the least of which is that it shifts responsibility for adding these components from the assembler to the board fabricator. Embedding passives can dramatically reduce the overall part count for a PCB assembly. Fewer parts to place means lower inventory and assembly costs, and more reliable assemblies, especially if the board has lots of tiny components, like 0201s. Fewer surface-mount passives also frees up space, enabling designers to reduce the size of the board or add more active components.
Embedding passives gives designers more flexibility for routing ICs with high I/O counts, and it enhances frequency response by reducing parasitic inductance. Embedded passives can be located directly below ICs. Shortening the distance between passive and active components results in better signal transmission and less cross talk, especially at high frequencies.
The high operating frequencies of today's electronic devices are challenging standard circuit designs. With only two surfaces available for component placement, signal routing can become long and complex. Because the inductance of a signal trace is directly proportional to its length and inversely proportional to its width, lengthy traces can lead to problems with capacitive and inductive decoupling.
Embedding passives solves this problem. Locating passives in the inner layers of the board shortens many traces. Relocating traces inside the board also decreases the number of traces arranged in close, parallel lines, which reduces cross talk.
Because of these advantages, embedded passives are increasingly being used in cell phones, pagers, personal digital assistants and other products with high package densities and high operating frequencies. Consumer electronics aren't the only devices that can benefit from embedded passives, though. Embedded passives are also being used in automotive, aerospace and military applications. For example, Delphi Delco Electronics Systems (Kokomo, IN) is experimenting with using embedded passives in engine control modules.