Opening the door to wireless innovations
03/01/2000
Advances in LTCC technology lead to new applications in wireless packaging.
D.I. AMEY
M.T. DIRKS
R.R. DRAUDT
S.J. HOROWITZ
C.R.S. NEEDES
The second generation (2G) wireless phone will soon be history, replaced by the 2.5G handset offering a range of new features, including improved Internet and e-mail access, and many of the features of a personal digital assistant (PDA). The growth of wireless applications, combined with these new features, continues to challenge radio frequency (RF) designers to incorporate more functions into a smaller space while at the same time increasing battery life, improving display quality, lowering cost and achieving new levels of manufacturing robustness. While the level of integration of ICs continues to be important, integration at the module or substrate level is increasingly viewed as an important enabler.
Low temperature co-fired ceramic (LTCC) is rapidly becoming the technology of choice for RF components and modules for portable wireless applications. LTCC has demonstrated that it can enable the high density integrated packages needed to meet the size, performance and cost requirements for portable wireless. It is also positioned to meet the high density interconnect requirements of GHz clock rate signal processors.
The LTCC Process
LTCC technology uses thick film dielectric materials cast as a tape in place of screen-printable dielectric compositions. Each layer of tape is blanked to size, and registration holes punched. Vias are formed in the dielectric tape by punching or drilling, while conductor traces and via fills are screen printed. Recent advances have combined screen printing with photopatterning to extend the feature formation capability of the thick film process.1 Using a silver composition, it is also possible to pattern 50 micron lines and spaces on the inner layer or top layer of a co-fired interconnect structure. This technology can be used for high density interconnects, or for patterning spiral embedded inductors or interdigitated capacitors.
When all circuit layers have been punched, printed and inspected, they are registered, laminated and co-fired. The co-fire process (i.e., dielectric and conductor fired at the same time) involves few firing steps, and allows inspection of punched and printed layers prior to lamination, with greater layer capability (layers > 50 demonstrated) than conventional thick film technology (Figure 1).
Inner layer, top layer and via fill conductors can be gold or silver based, depending on the application and assembly technology specified. Military and aerospace applications have historically used gold or gold alloy conductors, and portable wireless applications generally use silver and silver alloy conductors. For applications requiring wire bonding, silver inner layer metallizations can be reliably combined with gold top layer conductors. For top layer assembly using solder, silver alloy or gold alloy conductors are available.
High-k dielectric layers or embedded thick film resistors can be screen-printed on any layer during a fabrication process to incorporate passive components. Combining embedded resistors, capacitors and inductors gives designers the capability of embedded functions (e.g., resonators and filters), not just individual passives.2
Features and Benefits of LTCC
Table 1 summarizes the features of ceramic LTCC solutions, and the benefits to the OEM that designs and manufactures handsets for the portable wireless market.3 High Q, or low loss, is an important feature of LTCC tape technology that contributes to enhanced performance. Lower losses mean reduced power consumption and extended battery life, critical for portable applications.
Figure 2 shows the 50 ohm microstrip attenuation (dB/inch) of two LTCC tape systems compared to a 99 percent alumina substrate metallized with thin film gold and a copper clad polytetraflouroethylene (PTFE) printed wiring board.4 The first tape has attenuation below 0.5 dB/in. up to about 12 GHz, while the second tape performs like a copper metallized PTFE laminate. The figure shows improved attenuation over the thin film on 99 percent alumina for frequencies up to 20 GHz, the limits of this measurement technique. This data was generated using a T- pattern microstrip resonator.5 Cavity and open resonator studies have shown that the dielectric constant and loss tangent of the second tape are insensitive to frequencies up to 40 GHz.4 It is a second generation LTCC system developed specifically to meet emerging needs for wireless applications above 12 GHz.6
Ceramic solutions can provide a good platform for mixing analog, digital, and RF technologies, and they are hermetic, providing the designer with options to integrate highly reliable hermetic packages as part of the interconnect structure. Incorporating photopatterning enhances the capability to define precise features, thus improving impedance control, providing higher Q components and increasing packaging density.1
The low thermal expansion of the tape can minimize performance variations from changes in temperature. The good coefficient of thermal expansion (CTE) match to Si, GaAs and SiGe facilitates bare chip attach without underfill.
LTCC solutions are also [well-established] in the military, aerospace and automotive markets, and are often used for harsh environment applications.
LTCC also offers good feature size, and overall physical and electrical property stability. In addition, active trimming of passive components, long a feature of conventional thick film processing, imparts the ability to dynamically adjust circuit performance. The ability to design in three dimensions (i.e., to incorporate interconnect and passive components and functions in a single integrated block) differentiates LTCC from organic laminate technologies that dominate many of today`s high density interconnect applications.
Recent Wireless and BGA Applications
Figure 3 is a Bluetooth radio module. (Bluetooth is a global specification for short-range wireless connectivity and data transmission originally proposed by Ericsson.) This low-cost microwave radio technology is intended to replace cables between various portable devices to simplify person-to-person and [machine-to-machine] interaction. This radio module is geared to create [personal-area] networks in which any electronic device within a radius of 3 meters can seamlessly share data. It will be used in a broad range of electronic devices for voice and data communication, including mobile phones, modems, laptop and desk computers, fax machines, printers, wireless headsets and digital cameras.
The prerequisites for the initial hardware module design are small size, low cost, reliability, low power and the ability to function in many applications. Ericsson used flip chip assembly for the IC and LTCC for the substrate because of its flip chip compatibility, small size, and robust high frequency and mechanical properties. With this combination, the company integrated microwave structures for the antenna filter and transmit
eceive baluns into the transceiver substrate.
The high density interconnect design of IC packages for wireless applications incorporates embedded decoupling capacitors and other RF passive structures, including a voltage controlled oscillator resonator. This design eliminates many external components, has a micro ball grid array footprint and is cost-competitive with standard packages.
Summary
Applications requiring multilayer controlled impedance interconnect, high-density component and function integration, direct chip attach, and stable dimension and electrical properties over a broad range of environmental conditions continue to emerge. LTCC technology opens the door to a wide range of wireless packaging innovations, including next-generation portable wireless and GHz clock rate applications.
References:
1. M. Skurski et al., "Thick-Film Technology Offers High Density Packaging," Microwaves & RF, Feb. 1999.
2. D. Wilcox et al., "The Multilayer Ceramic Integrated Circuit(MCIC) Technology: An Enabler for the Integration of Wireless Radio Functions," Advancing Microelectronics, July/Aug. 1999, pp. 13-18.
3. D. Amey et al., "Ceramic Technology for Integrated Packaging for Wireless," 1999 RFIC Symp., June 1999.
4. D. Amey and S. Horowitz, "Characterization of Low Loss LTCC Materials at 40 GHz," 1999 Symp. on Microelectronics, Oct. 1999.
5. D. Amey and S. Horowitz, "Tests Characterize High-Frequency Material Properties," Microwaves & RF, Aug. 1997.
6. P. Donohue et al., "A New Low Loss Lead Free LTCC System for Wireless and RF Applications," 1998 Intl. Conf. on Multichip Modules and High Density Packaging, pp. 196-199, 1998.
D.I. AMEY, research fellow, M.T. DIRKS, market development manager, R.R. DRAUDT, product manager, S.J. HOROWITZ, marketing manager, and C.R.S. NEEDES, research manager, can be contacted at DuPont Microcircuit Materials, 14 T W Alexander Drive, Research Triangle Park, NC 27009; 919-248-5752; Fax: 919-248-5715; E-mail: [email protected].
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Figure 3. Bluetooth radio module.
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Figure 1. LTCC process flow.
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Figure 2. 50 ohm microstrip attenuation.