Consumer applications of microwave devices—particularly wireless communication devices—have grown rapidly in the last few years. Low temperature co-fired ceramic (LTCC) offers an ideal solution for these applications.
Consumer applications of microwave devices—particularly wireless communication devices—have grown rapidly in the last few years. These applications require packaging materials with high performance (especially in terms of microwave losses), high volume manufacturing capabilities and low cost. Traditionally, organic or polymer materials have been the packaging material of choice for these devices because they meet the manufacturing and cost requirements. But such materials are limited in performance and durability, especially at the higher frequency ranges required with today’s technologies. For this reason, manufacturers are constantly looking for new materials that will enable them to push the envelope on consumer wireless communication performance.
Low temperature co-fired ceramic (LTCC) offers an ideal solution for these applications. LTCC was developed as an advancement of thick film hybrid technology in the 1980s1,2 and has been significantly improved over the years.3 It provides high reliability, as well as the design flexibility to realize true three-dimensional structure (unattainable with polymers and conventional ceramic materials) and to incorporate capacitive and resistive components within the hermetic structure. Certain types of LTCC, such as M-grade A6* (A6M), also feature the low microwave loss characteristics required for wireless communication devices. However, while A6M LTCC has been used in high frequency military and aerospace applications, it could not be manufactured in high volumes and has traditionally been unable to meet the cost requirements for consumer applications due to the use of expensive conductor materials.
Researchers recently developed a new LTCC tape that can be manufactured in high volumes. In their quest to overcome the cost barrier of LTCC, researchers also discovered a way to further minimize the microwave loss of the LTCC system by using silver (Ag)—the least expensive and most conductive air-firable metal—as the conductor material. Tests have shown that the new LTCC tapes incorporating the silver conductor system offer both the required cost reduction and a significant electrical performance advantage, making it suitable for use in consumer wireless applications.
Table 1. Typical physical properties.
The New LTCC Tape
The new A6 LTCC tape, designated A6S, is composed of glass in the Ca-B-Si-O system.4
New glass manufacturing processes using much lower temperatures as well as new tape casting processes have been developed, making the production process suitable for high volume applications. The new tape also incorporates a new proprietary organic binder system that shows excellent cuttability, also increasing the ease with which the new tape can be produced. Tables 1 and 2 compare the properties of A6S and A6M.
Table 2. Typical electrical properties.
Due to the low electronic polarization of the component ions, A6S has a low dielectric constant (5.9 at 1 MHz), which provides the high signal speeds sought within the wireless communications industry. The dielectric properties at low frequencies (such as 1 MHz) were measured using the parallel plate capacitor configuration and an impedance analyzer. All test coupons were fabricated using the standard LTCC process described in previous publications.5,6
A typical firing process consisted of organic binder burnout at 450°C for two hours and sintering at 850°C for 10 to 12 minutes.
The dielectric properties (dielectric constant and loss tangent) of the fired A6S ceramic at frequencies >1 GHz were measured using cavity perturbation, resonance post and various open resonator methods. Details of the measurement techniques are reported in a separate publication.7
Figure 1. Microwave properties of A6S.
The dielectric constant and dielectric loss of A6S at a frequency >1 GHz are shown in Figure 1. A6S has a dielectric constant of 5.72 (±0.16) and a loss tangent of 0.0011 (±0.0003) over a 1 to 60 GHz frequency range, and a loss tangent of less than 0.002 up to 95 GHz, enabling it to meet the higher frequency demands of today’s broadband technology. In the past, the maximum achievable frequency range was 1 to 10 or 1 to 15 GHz.
Figure 2(a). Leached areas of Ag (left) and Ag/Pd (right) conductor pads (80 x 80 mil) in 2Ag62Sn36Pb solder.
The New Silver Conductor System
While combinations of silver and platinum (Ag/Pt) or silver and palladium (Ag/Pd) have been traditionally used as surface conductors in LTCC applications to improve solder leach resistance, adding Pt and Pd increases the cost and reduces the electrical performance of the LTCC system. For these reasons, efforts have been made to reduce the usage of Pt and Pd in the conductor formulation. Studies have shown that an Ag conductor has a leach resistance equal to the Ag/Pt and Ag/Pd conductors in a 2Ag62Sn36Pb solder composition (see Figure 2a).
Figure 2(b). Leached areas of Ag (left) and Ag/Pd (right) conductor pads (80 x 80 mil) in 60Sn40Pb solder.
However, when eutectic Sn-Pb solders are used, Ag/Pd and Ag/Pt show better solder leach resistance than an Ag conductor with no Pt or Pd addition (see Figure 2b). Similar results have been reported for hybrid applications using fired alumina substrates.8
The results reported here are consistent with that earlier report.
The optimized Ag surface conductor shows excellent electrical resistivity (<1.0 mW/square normalized to 1 mil fired thickness [the theoretical value is 0.64]), and good initial and aged adhesion.
Figure 3. LTCC structure using A6S and Ag surface conductor, via and internal conductor (via is 10 mil in diameter).
A new generation Ag internal conductor and via fill were also developed to take advantage of the three-dimensional capabilities of the new tape. Figure 3 shows the cross-sectional view of an LTCC structure using A6S and the new Ag surface, via and internal conductors. Excellent compatibility between the via and surface conductors, and between the via and internal conductors, is visible. Unlike traditional polymer or ceramic materials, which would have quickly failed in this type of application, the 10-mil via is crack-free and has excellent thermal and electrical conductivity.
The microwave performance of the new generation LTCC system was also characterized using the micro-strip technique.5,7 In this technique, power transmission over a micro-strip line (from point 1 to point 2) is expressed in decibels,9 as shown in the following equation:
-10 log (P2/P1) dB
For example, a 0.5 dB insertion loss would have a power ratio (P2/P1) of ~89%, which indicates that 11% of the power is lost. In this case, the battery life would be 89% of that of a system with no loss materials (0 dB).10
Figure 4. Insertion loss of the new LTCC system (solid circle: surface Ag; solid triangle; internal Ag; cross: Ag/Pd).
In measuring the microwave performance of the new system, ring resonators were built and the resonance characteristics were determined using a network analyzer from 1 to 25 GHz. Figure 4 shows the attenuation of the new silver surface conductor and the new internal conductor. The attenuation of an Ag/Pd surface conductor is also shown for comparison. New surface and internal conductors show insertion losses of <0.20 dB/in. up to 12 GHz and <0.35 dB/in. up to 22 GHz, leading to a longer battery life and greater durability for consumer wireless devices.
The new generation LTCC tape and silver conductor system offers tremendous advantages in wireless packaging applications. Tests have shown that the Ag surface conductor shows improved electrical conductivity, excellent solder leach resistance, and initial and aged adhesion with reduced cost. Additionally, the overall LTCC system offers superior microwave performance. Such developments will help ensure that ceramics remain an essential part of both current and emerging consumer wireless technology.
The authors would like to thank Richard Nguyen, Jeff Holthus and Cristina Lopez for their contribution in the experimental work, and Simon Turvey and Bob Gardner for their input. We are grateful to Mike Janezic and Richard Geyer and their coworkers at NIST, Tatiana Starr and Doug Paulson of Tristan Technology, and Ben Kelsall and Nick Damaskos for their contribution to the dielectric measurements. Correspondence with Paul Shepherd of CTS Corp. also added to this article.
For More Information
For more information about the new LTCC tape and silver conductor material, contact the authors at 1395 Aspen Way, Vista, CA 92083; (760) 305-1000; fax (760) 305-1100; or e-mail email@example.com
*The A6 family of LTCC products is a trademark of Ferro Electronic Materials.