Integrated Passives... Are We There Yet?

The drive for passive integration began almost a decade ago, in 1996, when research institutes at IZM Berlin, IMEC and the University of Arkansas began showing that resistors, capacitors, inductors and diodes could be fabricated into a single integrated passive device (IPD) by using thin-film technology. During the past nine years, this research has grown into a massive body of work, and has now begun to become a mainstream technology for fabrication of commercial devices for portable electronics.

It was clear a decade ago that these IPDs met at least three of the four key criteria for commercialization (faster, smaller, lighter and cheaper). The circuits were certainly smaller, lighter and of better performance than their discrete counterparts, but would they ever be cheaper? Unless these solutions became price-competitive, they would not become mainstream technology.

While IPD component price may still be higher today, IPD prices have continued to drop as order volumes have increased. In addition, it is clear that the assembled cost of an IPD can compare very favorably with the discrete solution it is intended to replace. The total assembled cost includes the price of the discrete components and placement cost, as well as the cost of procurement and storage. The assembly cost for discrete chip components is nearly always significantly higher than the cost of the components themselves.

In addition to the clear assembly savings, the proliferation of chip-scale and wafer-level packaging (WLP) technologies (where the package size is only slightly larger than the die size) has been quickly assimilated by the IPD manufacturing community, which is keenly aware of the significant savings in board space that these low-profile chip scale packages can provide circuit designers of portable devices. Figure 1 shows an example derived by Bourns, where the discrete components needed to construct a six-channel ESD/RF filter are compared to its chip-scale IPD counterpart.

1. Discrete vs. an integrated passive device solution for six-channel ESD/RF filters are compared. (Source: Bourns)

The application driver is mobile communication devices. While the mid-to-late '90s were a major growth period for mobile phones and laptops, it has only been in the past five years or so that the premium has been placed on added functionality, as well as the development of other mobile consumer products, such as PDAs, digital cameras and digital music devices (i.e., iPODs and portable GPS devices), driving IPDs to volume manufacturing.

The ~400 components present in today's dual-band cell phones are expected within three years to be reduced to <150 components. Knowing that 95% of the total components are passives and that they represent ~70% of the total board area, one must conclude that cell phone manufacturers' demand for IPDs will continually increase through this decade. Space on a cell phone board, or other portable device, is at a premium. The use of IPDs is clearly one of the few ways to slim down the size of the device or add more features or benefits.

Many of Bourns' ESD and EMI/RFI filtering IPDs in chip-scale packages integrate up to 20-30 components, including resistors, capacitors and diodes. In some instances, they also include oscillator functions and transistors. Volume usage for these devices has increased significantly over the past few years to the point where they are now considered standard product lines. Many of these devices are available in chip-scale and wafer-level packages.

CMD's ASIP (application-specific passive device) technology allows for the integration of spiral inductors, resistors, capacitors and/or ESD protection diodes onto IPDs to provide EMI filtering, ESD protection and power management solutions for mobile products, many in their CSP/WLP IPD format.

2. CMD Praetorian technology includes spiral inductors. (Source: CMD)

A big seller for CMD has been its IEEE 1284 filter for parallel ports. This device includes 25 resistors, 17 capacitors and 17 ESD protection circuits in a 28-pin QSOP.

To provide ESD protection and EMI filtering for a cell phone's LCD interface typically requires as many as 70-80 discrete resistors, capacitors and diodes. CMD reports that its CM1423 EMI filter and ESD protection array for Secure Digital interfaces offers a space savings of 90% and a cost savings of 50% vs. discrete implementation.

STMicroelectronics has been commercializing its IPAD (integrated passive and active device) technology for several years, and offers a wide array of integrated passives for mobile phone, PDA, digital camera and notebook computer wireless functions, such as ESD protection diodes, EMI low-pass filters, line terminations, pull-up or pull-down resistors, signal switches and RF components.

A number of applications are now available as cost-competitive thin-film IPD devices:

• Low-pass filter — Devices such as cell phones often have data and/or audio ports for connection to external devices. By their nature, cell phones generate RF noise, which can be coupled into the data/audio port. The use of integrated passive low-pass filters attenuates the RF noise, which could otherwise interfere with the circuitry of the cell phone.
• ESD protection — Many handheld devices have external ports, which are potential paths for ESD to enter the handheld device and damage the internal circuitry. Integrated passive and active devices are a very suitable solution for this type of problem where board area is an issue. Bourns uses a proprietary back-to-back Zener diode arrangement to provide ESD protection.
• Low-pass filter with ESD protection — This is a combination of low-pass filter and ESD protection fabricated into a single component.
• Line termination — System bus speeds are increasing all the time, making line termination a more important consideration. Transmission line effects, such as reflections, must be controlled in order to prevent data errors. Terminating bus lines with high-speed Schottky diodes is an effective method to eliminate this issue.
• Voltage-controlled oscillator — Bourns has designed an 800 MHz VCO for use in the receiver section of a cell phone. This VCO solution is fabricated in a 1.5 × 1.0 mm format.
• Diplexer — Dual-band cell phones have a requirement to switch between two transmit frequencies. Use of a diplexer assists the frequency selection, while at the same time providing impedance matching and band-pass filtering.
• EMI filter for LCDs — In mobile phones equipped with color LCDs, the connections between the graphic controller and the LCD are exposed to the electromagnetic interferences coming from the antenna. It is necessary to filter these frequencies to avoid disturbing the video signals. In addition, mobile phones and PDAs must be protected from potential ESD damage to the ASIC LCD controller.
• EMI filter and ESD protection for speakers and microphones — EMI/RFI IPDs have become essential for mobile phone audio circuits where antenna radiation can cause audible interferences. In addition, most audio amplifiers integrated in the base band or mounted as standalone devices are sensitive to ESD when the circuit is exposed to the external headsets, speaker ports or hands-free devices. IPDs in use in digital cameras are shown in Figure 3 .

3. IPDs are available for digital camera functions. (Source: STMicroelectronics)

Packaging foundries coming on board

A sure sign that a technology has "made it" is when it is offered by key foundries. STATS ChipPAC licensed the SyChip integrated passives technology a few years ago and currently offers it as its chip-scale module packaging (CSMP) technology. Its foundry service includes fully characterized resistor, capacitor, inductor, filter and BALUN libraries complete with electrical models. It features resistors to 100,000 Ω, capacitors to 1000 pF, inductors to 80 nH and compact BALUNs for RF applications. Packaging is available in LFBGA, FLGA and QFN formats.

Intarsia, a joint venture between Dow Chemical and Flextronics, started addressing the integrated passives issue in 1997, developing its thin-film-on-glass and thin-film-on-silicon technologies for a variety of wireless and RF applications. Unfortunately, funds ran out, and Intarsia closed its doors in the spring of 2001 amid the general downturn in the wireless industry. The Intarsia film technology package and its Passport design library are in the process of being acquired by Research Triangle Institute (formerly the Microelectronic Center of North Carolina [MCNC]), which intends to revive the technology and offer integrated passive development and prototyping activities to go along with its current bumping and 3-D through-via technologies.

There is every indication that progress in electronic miniaturization will continue to fuel growth in the development of effective medical implants. Because so much of the body's operation is electrical in nature, nervous response, sensory input, and muscle control are all likely areas for treatment, enhancement or replacement using miniaturized electronics.

Medical devices incorporating miniaturized electronics are causing a rethinking of chronic illness and disability treatment. Hearing aids are one of the first devices to take advantage of electronic miniaturization. Current areas of implementation include neuromuscular stimulation, artificial vision and disc replacement.

For instance, Theken Disc and Valtronic have developed an artificial disc to replace spinal fusion. The embedded circuitry for this device has several ICs for performing data acquisition, processing, storage, data transmission, and power management functions and >80 surface-mount components, mainly passives. One can easily predict that passive integration will be a major driver for further reducing the size of such implantable medical devices.

This article was provided by the IEEE Components, Packaging and Manufacturing Technology (CPMT) Society. www.cpmt.org