May 8, 1997

LILLIPUTIAN PASSIVES
herald hybridlike pc-board density

BILL TRAVIS, SENIOR TECHNICAL EDITOR

In microscopic form factors, components, arrays, and networks--both resistive and reactive--allow unprecedented board density.

The latest generation of minuscule passive components lets you achieve unprecedented circuit density using plain-old FR-4 pc boards. Formerly, you could achieve such density only by using expensive, high-tooling-cost hybrid circuits. But now, resistors, capacitors, inductors, and even transformers are available in almost-microscopic sizes. Moreover, you can further increase density by using multiple independent components, arrays, and networks in tiny, IC-size packages.

In individual passive components, you have a rich array of tiny devices to choose from. Most of the components follow standard EIA sizes, expressed in mils/10 length by mils/10 width. For example, an 0805 component measures 80×50 mils. The 0603 size has become a de facto standard, and smaller devices are on the horizon. Several 0402 components, including inductors, have recently emerged. Murata Electronics offers the smallest of all--0201.

Tinyness is not without its problems, however (References 1 and 2). Minuscule components bring slower manufacturing throughput time and lower yields. One thorny problem is "tombstoning," the tendency of a component to upend itself during solder reflow. Tiny devices also present other handling problems, such as sticking to the carrier tape or refusing to adhere to the pc board. They also pose problems in placement registration and invite solder bridges and short circuits.

What's new in resistors?

Most major manufacturers offer individual resistors in all the standard EIA sizes, including the smallest form factors. Many of these products use thick-film cermet processing, which yields stable, low-temperature-coefficient resistors. The devices come with solderable terminations for surface mounting. IRC has taken a different tack with its WBC series of silicon-based resistor chips.

The WBC resistors use tantalum-nitride thin-film technology, which allows tolerances as tight as ±0.05%, temperature coefficients of resistance as low as ±25 ppm/°C, and tracking as close as ±5 ppm/°C. The resistors are available as single components on a 20×20-mil chip or as two-resistor elements on a 30×30-mil die. The singles offer two wire-bondable pads; the duals have six pads (two extra to avoid the need for jumpers). Values range from 10 ohms to 100 kilohms. The resistors cost $0.90 (1000).

IRC uses the same thin-film technology to produce its line of TanNSil divider networks in a SOT-23 package. Absolute resistor tolerances range from 0.1 to 10%, with ratio tolerances as tight as ±0.05% and tracking to ±2 ppm/°C. Power rating per resistor is 0.25W. The price is $0.30 (100,000).

A series of surface-mount, chip-resistor networks from Methode uses thick-film processing on alumina substrates. The leadless networks come in seven- and nine-resistor isolated configurations. Tolerances range from ±1 to ±20%, and power dissipation is 0.25W maximum. Prices are $0.01 to $0.05 (1000) per resistor, depending on tolerance. In addition to the above simple networks, more elaborate ones are available as well.

Adding a C to the R

RC networks come in a number of configurations and form factors. One product from Rohm, for example, integrates one resistor and one capacitor in a 1206-size surface-mount package. The RCC18 targets line-termination applications and offers a resistance range of 100 to 300 ohms and capacitances of 22 to 100 pF. Figure 1 shows the result of driving an unterminated line. You could match the line impedance at the load by connecting resistors to V_CC and ground. However, this connection imposes a dc load on the line. An ac load to ground, consisting of an RCC18 series RC network, provides the same transient response but presents no dc load to the line. The RCC18 costs $35 per 1000 (minimum order 1 million pieces).

Upping the scale of integration, IRC's TaNCap series of RC networks combines tantalum-nitride resistors and thin-film capacitors on silicon substrates. The RC networks come in several configurations, including one that combines a pullup resistor, a filter capacitor, and a terminating resistor for as many as nine digital lines in an IEEE-1284 printer-interface system. Resistor values range from 10 ohms to 100 kilohms; capacitor values cover 10 to 300 pF. Surface-mount packages include 20- and 24-pin QSOPs and 20-pin SOICs and TSSOPs. Prices are $0.85 to $1.25 (10,000).

Passive components are continuously shrinking. Micro sizes create macro problems in manufacturing. Reactive components in tiny form factors are keeping pace with resistors. Passive-component arrays or networks in minuscule packages further reduce pc-board area.

Capacitors continue to shrink

Multilayer capacitors (MLCs), of-fered in various formulations--NP0, X7R (and its voltage-stable variant BX), Y5V, and Z5U--are now readily available in the 0402 form factor. The cited formulas are in increasing order of temperature stability and in decreasing order of available capacitance. In Vitramon's 0402 family, for example, the NP0 dielectric offers values from 15 to 200 pF, the X7R has values from 120 to 10,000 pF, and the BX dielectric yields values from 120 to 3900 pF. These MLCs traditionally use tape-and-reel packaging; Kemet now also offers cassette packaging. The Vitramon devices typically cost $0.12 (1000). Vitramon, like AVX, switches the length and width dimensions--for example, 0612 instead of 1206--on certain models to cut inductance for high-speed applications.

Multiple-device chips of-fer a way to further reduce board real estate. An 0612 chip from AVX integrates four MLCs, thus saving approximately 50% in surface area compared with mounting four discrete 0603 capacitors. Moreover, the multiple-capacitor chip is easier to handle than the smaller 0603 or 0402 sizes. The $0.06 to $0.13 (100,000) W3A series is available in NP0 dielectric, with values from 10 to 470 pF; in X7R formulation, with values from 470 pF to 0.1 µF; and in Y5V dielectric, with values covering 0.01 to 0.33 µF. Again, these three formulations are in increasing order of temperature stability and tolerance tightness: ±10%; ±20%; and +80, ­20%, respectively.

In a major development, AVX has announced the first 0603 tantalum chip capacitor. The devices use a tantalum-wafer substrate and a flat plate at the cathode end instead of the traditional lead-frame construction technique. With the older technology, the active tantalum element accounts for just 30% of the volume of the lead-frame device; with the AVX process, this figure increases to 70%. This more effective tantalum utilization leads to the industry's highest capacitance-voltage (CV) product: CV=10 (for example, 1 µF at 10V; 3.3 µF at 3V). AVX has also upped the volumetric efficiency of the tantalum material in its more traditional, EIA-size molded-case capacitors to yield higher CV ratings.

Micro magnetics

Superminiature inductors are also making their appearance 180° away from capacitors on the reactance scale. Coilcraft, for example, asserts that its 0603HS series contains the world's smallest wirewound inductors. Inductance ranges from 1.8 to 120 nH, with Q greater than 100 at 1.7 GHz. Coilcraft asserts that the wirewound construction yields significantly higher Q and lower series resistance compared with units using nonwirewound technology. The Coilcraft units cost $0.24 (10,000). Dale offers 0603-size inductors in an inductance range of 1.5 to 100 nH at $0.17 (3000). Dale also offers 0805 inductors of 3.9 to 1000 nH.

Now for something really small. Toko has an 0402-size inductor that uses multilayer-ceramic construction. Inductance values for the LL1005-FH series, rated at ±5% tolerance, range from 1 to 27 nH, and Q is typically greater than 80 at 1.9 GHz. Self-resonant frequencies are greater than 6 GHz. The inductor costs $0.18 (10,000). Toko's larger, 1005 FSLP2520 is a wirewound inductor that offers values from 1 to 47 µH. Toko asserts that its proprietary wirewound structure yields dc resistance approximately half that of typical wirewound inductors. The FSLP2520 costs $0.25 (10,000).

Though not exactly minuscule, some new transformers are small, and they're surface-mountable. The SEP series from Associated Components Technology, for example, offers 1000- to 10,000-mH inductance (in coil or transformer configurations) in a surface-mount form factor that measures 0.029×0.367×0.322 in. These devices cost $2.60 (5000). A surface-mount current transformer (for current sensing) from GFS Manufacturing handles 8A continuous current in a package that measures 0.45 in. sq by 0.25 in. high. Finally, a family of surface-mount audio transformers from Pico Electronics handles audio power from 100 mW to 3W in a package that's 0.2 in. high.

Combating EMI

An assortment of surface-mountable RC and inductive components in tiny formats helps you control EMI in your board and interconnect designs. AVX, for example, offers a family of integrated passive components for EMI filtering and decoupling on I/O lines. Magnetic filtering components, too, are available in chip form. Three recent introductions give you a choice of microminiature inductive design-ins.

A multilayer EMI suppressor from Associated Components Technology offers 300 ohms impedance at 100 MHz. The 0603-size JCB-0603 handles currents to 200 mA and specs 0.5 ohms series dc resistance. It costs $0.11 (100,000). The 0805-size CMF EMI filter from MMC Electronics offers capacitance values from 22 to 2200 pF, allowing you to design filters for several megahertz to beyond gigahertz, with attenuation reaching 45 to 60 dB at the resonant frequency.

If you prefer to roll (rather, wind) your own magnetic components, a surface-mountable ferrite core from MMG North America measures just 0.162 in. high. It offers an alternative to traditional through-hole devices for configuring EMI suppressors or inductors. The nickel-zinc-ferrite cores offer metallized terminals with a tin or tin/lead outer surface for reflow soldering. You can use the cores to configure inductors or EMI suppressors that require maximal impedance at frequencies greater than 200 MHz.

For the ultimate in passive-component density, you can specify silicon-based RC or RLC networks. California Micro Devices (CMD) and the giant Philips are making major efforts in network technology to configure both standard and custom passive-component arrays.

The P/Active family of integrated passive components from CMD uses a silicon substrate to provide an interconnect function, to provide a controlled-impedance ground plane, to afford ESD protection, and even to integrate Schottky diodes where needed. A couple of off-the-shelf products illustrate the capabilities of the process.

PAC T, a lowpass filter, uses 25-mil-pitch QSOP packaging to cut board space by more than 70% compared with discrete configurations. The device integrates resistors, capacitors, and diodes to provide frequency response greater than 1.8 GHz and ESD protection greater than 2 kV. The PAC T is available in a variety of line-impedance and attenuation combinations. Several PAC products from CMD provide bus termination. For example, the PAC AC terminates 18 data lines with RC networks. PAC products cost $0.62 (1 million).

Philips, too, has major ambitions for silicon-based passive arrays and networks. The Philips devices integrate silicon-nitride (Si₃N₄) capacitors and polysilicon resistors. Resistor values range from 5 to 500 kilohms; capacitors cover 1 to 500 pF. According to Philips, future products will also incorporate inductors in the range of 1 to 100 nH. To gain a feel for the attainable pc-board real-estate savings, consider a typical octal T-filter design using 24 0603-size discrete components. The discrete design consumes 84-mm² of board space; an integrated, SSOP20 package uses 50 mm².

The latest generation of minuscule passive components, arrays, and networks allows you to design ultracompact systems at a reasonable cost. Your choice of miniature components depends on your company's manufacturing prowess, the components' characteristics (for example, maximum power rating), and the available board space.

References

1. Elliott, Heidi, "Honey, they've shrunk the passives--again," Electronic Business Today, October 1996, pg 91.

2. Schweber, Bill, "Good things come in small (IC) packages," EDN, Feb 3, 1997, pg 73.

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