New Ceramics for Wireless 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.
The New LTCC TapeThe 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.
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
The New Silver Conductor SystemWhile 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).
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.
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
Numerous AdvantagesThe 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.
AcknowledgementsThe 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 InformationFor 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, firstname.lastname@example.org, email@example.com or firstname.lastname@example.org.
*The A6 family of LTCC products is a trademark of Ferro Electronic Materials.