Murata and The Shrinking World of Ceramic Capacitors

A look at how ceramic technologies are complementing semiconductor advances to enable more sophisticated and smaller wireless electronic systems.

By Mark Porter, Engineer, Murata Manufacturing Co., Ltd.

Moore’s Law is given undue credit for the shrinking size of electronic systems and products. True, the claimed doubling of transistor density within integrated circuits every 18 months or so has had a key role. But other technologies, including those involving ceramic components, devices, and modules have made a vital contribution too. Improvements in ceramic materials technology have led to dramatic size reductions in components and sub-assemblies in recent years. This trend is so marked that the limiting factor for some designs is not the size at which particular components are available but the ability of automatic assembly equipment to handle the smallest types. Nowhere is the demand for miniaturization more prevalent than in the design of wireless devices, both for cellular and noncellular

Cellular and Non-Cellular Wireless:
While many people think of wireless in terms of cellular networks, the world is rapidly becoming much more complex. Figure 1 shows Murata’s view of the likely wireless connectivity that will appear in cellular phones over the next few years. GPS satellite, mobile television (DVB-H), FM transmitters that provide a link to car radio receivers, 802.11x WiFi transceivers, Bluetooth transceivers, wireless security keys, and e-money transceivers (Near Field Communications), ultra-wideband (UWB) wireless links, Wibree, FM radio receivers, and WiMax transceivers are some of the options. That’s a lot of wireless technologies, plus, of course, the cellular network radios themselves, which may be tri- or quad-band GSM plus UMTS.

Figure 1: Murata’s Estimates for the Take-Up of Various Technologies (A Wireless Handset May Need to Cover 13 MHz to 6 GHz in the Future)

In the not-too-distant future, we could be looking at handsets with wireless capabilities stretching from 13 MHz to 6 GHz. The challenge for handset makers is to get the right combination of technologies for intended markets without making the size, power consumption, and cost unacceptable. Smaller components, higher levels of functional integration, and the use of hybrid modules or other building blocks all have a part to play.

Developments in Components:
When surface mount multilayer ceramic capacitors first became available 20 years ago, the standard size was 1206 (3.2 x 1.6 mm). Today, the most common values of decoupling capacitors in RF circuits (1000 pF, 0.01 μF, and 0.1μF) are all available in an 0201 package measuring just 0.6 x 0.3 mm—a size reduction of 96%. At the lower end of the capacitance range, a 47 pF capacitor, which is a popular standard value used in oscillators and filters, can be just 0.4 x 0.2 mm (the 01005 size). That’s 98% smaller than its 1206 counterpart.

Package Length Width Maximum Dielectric Rated
EIA Ref. (mm) (mm) Capacitance voltage
01005 0.4 0.2 1000 pF X5R 6.3 V
0201 0.6 0.3 0.22 μF X5R 4 V
0402 1.0 0.5 2.2 μF X5R 4 V
0603 1.6 0.8 10 μF X5R 6.3 V
0805 2.0 1.25 47 μF X5R 4 V
1206 3.2 1.6 100 μF X5R 6.3 V
Figure 2: Maximum Capacitance by Package Type for Murata Ceramic Capacitors (January 2008)

At the other end of the scale, there are 100 μF multilayer ceramic capacitors manufactured at the 1206 size and components up to 47 μF in the smaller 0805 size (2.00 x 1.25 mm). Of course, lower operating voltages of semiconductors have contributed to the trend of smaller passive components because dielectric layers can be thinner while still providing sufficient dielectric strength. On the other hand, the more stable dielectrics often needed in wireless applications tend to have lower dielectric constants than general purpose types, limiting the capacitance that can be packed into a given size. Figure 2, on page 16, shows the maximum values available from Murata within the standard sizes, regardless of dielectric material. In practice, it is the challenges of component handling, inspection, and re-work that limits the adoption of some of the smaller packages. The cellular phone industry is in the process of moving from 0402 to 0201 for components where suitable capacitance values are available. Similar size reductions have been achieved in other passive components. Chip inductors for power supplies filtering through to coils for use at microwave frequencies have been shrinking year on year. For wireless devices, the 01005 package is the smallest in general use. Coils up to 120 nH are available in this size, rated at 150 mA, or with a self-resonant frequency of at least 600 MHz. Coils up to 5.1 nH have a typical self-resonant frequency of over 6 GHz. As a result of these improvements, EMI filters comprising combinations of ferrites, wound inductors, and capacitors have all shrunk too.

Developments in Devices:
With respect to wireless applications, ceramic devices, which are distinct from the simplest components, include dielectric antennas, connectors, piezo speakers, surface acoustic wave (SAW) filters, isolators, ceramic resonators, ceramic duplexers, and gyro sensors. Great advances have been made recently in devices for combining, filtering, and separating signals, and rapid progress continues (Figure 3). Only recently, single frequency SAW filters were in an industry-standard 2.5 x 2.5 mm package. Now you get the same device in a 1.4 x 1.1 mm package.

Figure 3: Within Two Years, a 7-Band Antenna Switch Will Measure Just 2.5 x 2.5 mm, Including Filters

SAW duplexers and diplexers have followed similar size reduction trends. Murata’s Switchplexers™ are RF diode antenna switches on ceramic substrates, with integral lowpass filtering on the transmit side, and a diplexer to split the GSM channels. Today, a 6-band device that will handle 3 x GSM and 3 x UMTS bands comes in a 4.5 x 3.2 mm package, but this is expected to become a 7-band part with its size reduced to 2.5 x 2.5 mm within two years. This is no small feat when you consider the challenges of isolating individual filters to prevent signal leakage and crosstalk between channels. These advances in ceramic-based devices are being achieved through improvements in materials technology, design, and processing techniques.

Migration to Modules:
The growing complexity of both baseband and RF technologies within wireless systems, together with the imperative of reducing costs, is driving handset OEMs and others towards a platform approach to system design.

Semiconductor manufacturers produce ASICs that handle various circuit functions within industry-defined standards. ASIC development within the wireless OEM companies themselves has become too expensive for most to consider, mainly due to rising costs resulting from shrinking geometries in semiconductor fabrication. Much of the differentiation of final products is created in software or through the design of system packaging. In fact, for mobile phone makers, some 80% of their design effort is now software development. This trend is further evidenced by the growth in the use of pre-assembled modules. In other words, system designers are specifying more pre-fabricated circuit blocks in order to focus their efforts on achieving rapid time-tomarket and product

In order to produce competitive modules, companies such as Murata now work as closely with
semiconductor manufacturers as they do with OEMs. The design of modules and the implementation of successful reference designs based around them means ensuring passive components work well with their associated semiconductor devices. This has been a relatively easy task in the past, but as wireless operating frequencies rise into the GHz region, even apparently simple capacitors take on greater complexity. For example, every conductor, including the component terminations, contributes inductance to a circuit. One thing to be considered with capacitors is the impedance and equivalent series resistance (ESR) varies with frequency. A 2.5 GHz 10 pF capacitor from one company can be a different component to a 10 pF capacitor from another, even where similar dielectric characteristics are claimed. For example, specifying a COG (or NPO) dielectric based on its stated temperature characteristics may not fully describe how the component will function in a given application.

Figure 4: The World’s Smallest Complete Bluetooth Module is Just 6.15 x 6.15 mm

Wireless modules are assembled on ceramic substrates, so it is no surprise companies with their origins in ceramic electronic components now dominate the ceramic modules business. A well-designed module based on standard low temperature co-fired ceramics (LTCC) will require only half the XY area of a conventional printed circuit board (PCB). Today, the Bluetooth transceiver module, shown in Figure 4, is available in a 6.15 x 6.15 mm package—complete with crystal and EPROM. 802.11B transceivers come in modules measuring 8.4 x 8.2 mm. A new non-shrink LTCC technology has been developed by Murata to produce a further 20% space-savings. The non-shrink substrate enables more aggressive design rules to be used, with circuit interconnect and components laid out closer together. As a result, 2008 will see a combined Bluetooth and 802.11x module appear in a tiny 10 x 10 mm package. The growth in transistor density within ICs and the capabilities of the resulting chips are well documented. Equally important, but perhaps less well appreciated, are the advances that have been made in materials and processes related to ceramic technologies. These are the technologies enabling electronic systems designers to take even greater advantage of progress in the semiconductor industry.

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