Open any cellphone, GPS receiver, camcorder, computer, or other consumer electronics system and what will you see? Less than you might expect, but still too much. A circuit board, maybe two, on which are mounted a few integrated circuits and dozens and dozens of tiny discrete devices—capacitors, resistors, and maybe a few inductors.
It is those so-called passive devices that, in fact, dominate the board’s real estate. Consider the Nokia 6161 cellphone, whose 40-cm2 circuit board contains just 15 ICs scattered among 232 capacitors, 149 resistors, and 24 inductors. The phone’s ICs contain millions of transistors, and yet it is those 405 passive components that mostly determine the area of the circuit board and the size of the cellphone.
Now imagine what designers could do if the passives were so small and flat that they could be inserted between layers of the circuit board itself, rather than taking up space on top of it. Phones could be thinner and sleeker than they are today, or they could contain more electronics, such as GPS receivers, or they could simply have much larger batteries and therefore longer talk time and bigger, brighter color screens. The same goes for almost every electronic device that you now stick in your pocket or carry in your briefcase, from PDAs to portable DVD players.
It gets better. These integrated passives would be a part of the circuit board itself, formed when the board was, so odds are good that their overall cost could eventually be less than what manufacturers pay today to buy and solder on discrete devices. Speaking of solder, eliminating it is another advantage of integration, because bad solder joints are one of the most common reasons electronic gear fails. Less solder also means less harm from lead waste.
The list of advantages goes on: putting the passives ”underground” leaves more room on the surface of the board for ICs, which means more design flexibility. And there are electrical benefits, too. Because current travels along a different path in integrated capacitors than in surface-mounted components, integrated capacitors can be made freer of the trace amounts of the inductance, called parasitic inductance, that plagues any capacitor and limits usefulness in high-frequency circuits. Finally, because the components are custom-made when the board is, the resistors, capacitors, and inductors can be sized to any desired value, rather than being chosen from a manufacturer’s list of available parts.
Advantages like these point to a potentially huge shift for the electronics industry. Over a trillion passive components were bonded to boards last year, according to the National Electronics Manufacturing Initiative’s (NEMI’s) road map. These devices are minuscule, and that makes putting them in place a chore. The smallest discrete passives today measure 0.50 mm by 0.25 mm; spread on a sheet of paper, they’d look like ground pepper. Such compact components are difficult to handle and attach, even for automated assembly equipment. And though the total cost of each part—including capital, assembly, and the prorated cost of the underlying board—is less than two cents on average, collectively the impact of integrated passives on system cost, reliability, and, most of all, size, could be enormous.
But for these passives to make a big dent in the US $18-billion-a-year market for discrete passive components, makers of circuit boards will have to reposition themselves as purveyors of passive electronic networks. It’s starting to happen, but slowly. Such manufacturers as Gould, Shipley, Ohmega, MacDermid, DuPont, Oak-Mitsui, 3M, and Sanmina all market products and processes for integrating resistors directly into printed-circuit boards, using at least four different technologies; and for integrating capacitors, using at least five. These sizable companies have all poured tens of millions of dollars in R&D funds into proving the concept. In the meantime, several other companies, including California Micro Devices Corp. (Milpitas, Calif.) and AVX Corp. (Myrtle Beach, S.C.), have been working on an alternative approach to integrating passives. They are selling arrays and networks of miniaturized passive devices in single IC-like packages.
In addition, a few equipment makers are starting to include integrated passives in their products. Motorola Inc. (Schaumburg, Ill.) is leading the pack with integrated resistors and capacitors in some of its newer cellphones, and several Japanese manufacturers are close to introducing products that take advantage of this approach.
In a sense, the situation with passive components today is a lot like that of active devices 40 years ago, when Intel, Fairchild, and others had just introduced ICs that combined active devices like transistors and diodes on a single substrate. But don’t expect Moore’s Law to apply to passives. These components cannot be scaled down into the submicron realm occupied by active devices. The reason, of course, is that passive components have to handle signals whose amplitude cannot be reduced arbitrarily—say, microwave signals going to a cellphone antenna or inputs for analog-to-digital conversion. Despite this fundamental limit, passive integration will make for much more miniaturization.
Integrated passives are not exactly new. They have been used for decades in the ceramic substrates that underlie circuits in military, microwave, and mainframe computer systems. But those represent a specialty within the electronics market. The vast majority of circuit boards today are made using FR4, the ubiquitous green epoxy insulator reinforced with glass fiber. FR4 boards are formed by sandwiching alternating layers of insulator with etched copper circuit traces and laminating them under heat and pressure. Drilled holes, or vias, plated with copper, connect conductor segments on different layers to form circuit interconnects.
A smaller but growing portion of the circuit board market has been going to ”flex,” which are laminated stacks of unreinforced polyimide (trademarked Kapton), polyester, or layers of other polymer film, each 25 to 125 µm thick, with copper traces on one or both sides. Because the polymer layers can be thinner, enabling smaller vias, flex allows more interconnects to be crammed into a given area than is possible with FR4. But flex costs more per square centimeter than FR4.