Industry demands within the passive components sector constitute a rather tall order: a wider range of values, higher-frequency performance characteristics, tighter tolerances, and greater levels of stability, all within small packages.
Passive components can be integrated into the same silicon substrate housing memory, logic, and microprocessors. Micro-electromechanical systems (MEMS) have enabled high-performance and integratable components, like RF circuits, switches, sensors, and oscillators. Efforts are also under way to embed passive components into pc-board layers for greater cost-effectiveness (Fig. 1).
CHIP-FORM COMPONENTS Resistors, capacitors, and inductors are becoming more available in chip form in 0402 and 0201 sizes, as well as in ball-grid-array packages, easing their integration into silicon structures (Fig. 2). They're also being made available in arrays that combine all three passive-component types.
Arrays might appear to cost more than the individual components they replace. But the fact that they require fewer pick-and-place assembly operations makes their use more attractive as a lower-cost production alternative versus individual discrete components.
For applications like cell phones, the tinier resistor die are more attractive, because they provide badly needed miniaturization form factors. Yet overvoltage protection is mandatory, making the larger-size resistor die more appealing in medical applications. Standard 1206, 2010, and 2512 sizes, which can handle up to 10 kV, are the choice here.
Many passive thin-film resistors and capacitors can be integrated on silicon and glass substrates or on active die. Inductors also can be integrated on polyimide tape substrates. Thick-film high-precision resistor pastes are arriving for high-resolution and high-frequency military and other applications that require a combination of a low temperature coefficient of resistance with tighter tolerances.
New materials, refined manufacturing processes, and advanced packaging in the resistors are helping passive resistor technology meet the demands for squeezing more components into tighter spaces—and still reach performance requirements. These materials include chromium silicon oxide and tantalum nitride formulations on glass, quartz, and silicon substrates.
One method that cuts down on stray capacitance effects integrates resistors on a single substrate with BGA terminals. This is particularly true for high-frequency applications, where there's a need to eliminate resistor leads and their corresponding wire bonds. Companies offer resistor arrangements in which the resistors are placed between the solder balls, putting the resistors very close to the circuits they're terminating.
Capacitors are advancing thanks to ceramic materials and multilayer ceramics. They're enabling capacitor components with very high efficiency levels per unit volume/mass, high-temperature capability, and high-voltage capability.
For high-speed digital electronics, ultra-thin (less than 25 µm) power ground cores for distributed embedded capacitor layers on pc boards have been created from epoxy dielectrics loaded with barium-titanate particles sandwiched between two copper layers. This new material outperforms conventional embedded capacitors and discrete surface-mounted components.
MEMS technology has indeed made great strides toward the development of high-performance inductor components. Q factors well over 10,000 at 1.5 GHz have already been shown feasible by researchers at the University of Michigan. The Q factor is a crucial performance attribute for RF circuit front ends. Design techniques being investigated for high-performance MEMS inductors include high-resistivity silicon or glass substrates, upon which the structure is grown to minimize inductor RF losses.
All-silicon oscillators are appearing in tiny packages from other firms. Dallas Semiconductor has introduced what it claims is the smallest 3-V all-silicon center-spread clock oscillator, the DS1089L, with an output of 130 kHz to 66.6 MHz. It's available in an eight-pin microSOP package. Also, Linear Technology has introduced a resistor-set 17- to 170-MHz silicon oscillator in an SOT-23 package.
CMOS image sensors are now satisfying a wide range of digital imaging applications from consumer digital cameras to sophisticated medical imaging systems. The latest CMOS image sensors with 5-Mpixel resolution are rivaling the image quality of costlier charge-coupled devices (CCDs). They're expected to measurably reduce the cost of high-resolution digital still cameras.
CAPTION 1. By embedding passive components into pc boards, pc-board manufacturers can make the boards smaller and less costly while improving performance. Passive components can be embedded within inner layers of multilayer pc boards. (Source: Mentor Graphics)