A Low Loss LTCC System for Wireless Applications

Heraeus Inc.
W. Conshohocken, PA

A new low temperature co-fired ceramic (LTCC) material system has been developed to address emerging new communications market demands such as wireless telephones, automotive components and Bluetooth modules. Unlike other materials for microwave applications, the system relies on a combination of excellent dielectric and conductor properties. By optimizing both conductor and dielectric materials it is possible to produce a system for circuit fabrication that is low cost with highly conductive silver lines (better than 2 mW /sq), fine line definition (75 µm printed and 50 µm by photo processing), and excellent dielectric properties (µr = 9.1 ±0.1, low Tf of < 10 ppm/°C and low loss tangent tan d = 10­3 ). The material can be supplied in a wide range of fired thicknesses from 25 µm up, with ±2 percent thickness control and a wide processing window. The material system is comprised of CT2000 LTCC tape, TC2302HQ co-firing silver conductor paste, TC2301PI photo-sensitive silver conductor paste for co-firing and TC7301 co-firing silver via fill paste, and is intended for wireless circuit applications up to 5 GHz.

The system takes advantage of the fact that conductor losses far exceed dielectric losses, particularly at low microwave frequencies. As shown in Figure 1 , the CT2000 tape provides significant performance advantage over low loss LTCC with a standard conductor and at 2 GHz is comparable to that of alumina.

The Heratape™ CT2000 LTCC tape features a dielectric constant of 9.1 ±0.1 at 450 MHz and a dissipation factor of 1 * 10?3 and 2 × 10?3 at 450 MHz and 2 GHz, respectively. Thermal expansion is 8.5 ppm/°C over the 25° to 300°C range. The material is silver conductor compatible, and lead and cadmium free, with excellent handling properties. It is dimensionally stable on 850°C refire and features a high Q and near zero Tf . CT2000 is available in sheet or roll form in thicknesses of 2 to 3.9 mils. Shrinkage after firing is typically 11.5 percent in the x and y dimensions and 14 percent in the z dimension.

The TC2302 HQ co-firing Ag conductor paste is a pure silver conductor with low resistivity and superior fine line print resolution. It features excellent compatibility with Heratape CT2000 during the co-firing process. TC2302HQ is optimized for screen printing of conductor tracks and is ideal for printing fine line patterns. The conductor material features a resistivity of = 2 W /sq at 15 µm. Figure 2 shows a typical application using 3 mil line and 3 mil space inductor traces.

TC2301PI photo-imageable co-firing Ag conductor paste features similar properties to the printed conductor paste. However, this paste is designed for photo imaging of the conductor tracks. The material has a wide processing latitude and can be imaged with or without a nitrogen purge. The paste is exposed with a light source wavelength of 300 to 400 nm. Exposure time at ultraviolet intensity consists of a 60 second nitrogen purge followed by a 15 second exposure at 30 to 40 mW/cm2 . Exposure without the nitrogen purge is 45 seconds. Glass masks should be used for best results. Developing consists of a 20 to 40 second spray at room temperature using 0.8 to 1.2 percent sodium carbonate in a deionized water solution. Line resolutions of 50 µm imaged (38 µm fired) are achieved. Figure 3 shows typical results for a 2 mil line/2 mil space application.

The TC7301 co-firing Ag via fill paste is also a pure silver conductor with excellent compatibility with CT2000 tape. The material is optimized for stencil printing of vias and features low resistivity.

Heraeus Inc.,
W. Conshohocken, PA
(610) 825-6050.