In today's age of high speed Internet, high frequency wireless and wireline communications, conventional materials are reaching their performance limits. Copper-based electrical connections are being limited by the dielectric properties of the insulating layer.
Solutions using optical connections are available but are too expensive. Liquid crystalline polymers (LCP) are a unique class of high temperature thermoplastic materials able to technically fill the gap between conventional materials that have reached their limits and expensive optical connections. LCP materials have low and stable dielectric constants and low loss, and are capable of handling electrical transmissions at high speeds.
In the past, LCP materials were not suitable for use as circuit board materials because of poor adhesive and dimensional characteristics. However, these shortcomings have now been overcome and LCPs are destined to become the next major material platform for the high frequency electronics industry.
One of the biggest challenges today is to develop materials to build multi-layer PCBs that exhibit exceptional dimensional stability while offering excellent high frequency electrical properties. The ZYVEX™ family of LCP materials has been specifically developed to enable the manufacture of high performance monolithic multi-layer PCBs for high frequency applications.
What is LCP?
LCPs are quite different from conventional crystalline thermoplastic materials in that conventional materials exhibit two phases of matter, solid at room temperature and liquid when heated to a molten state. LCPs can exhibit a third phase that has both solid and liquid properties.
The ZYVEX material is made using a thermotropic LCP that exhibits a crystalline melting point of 280°C. Its polymer backbone consists mainly of tightly packed rigid rod molecules that produce materials with unique properties. This structure, combined with its liquid crystalline packing, results in excellent electrical, dimensional and barrier properties. In addition, the ZYVEX polymer system has extremely low water absorption and excellent resistance to almost all chemicals. Unlike conventional semicrystalline polymers, the continuous-use temperature of LCPs is well above the glass transition temperature.
Material Properties
The ZYVEX product family of LCP circuit materials is comprised of single clad and diclad laminates, bondply and cover film products. ZYVEX material properties and technical data are listed in Table 1 . As shown, the laminate products offer excellent dielectric properties that appear unmatched by today's high performance polyimides. The material allows the use of thinner layers with no signal distortion. Its good dimensional stability reduces stress due to movement giving it the ability to form higher integrated designs. Its low moisture absorption reduces processing bake times and helps maintain stable electrical and mechanical properties. The material also has an adjustable in-plane coefficient of thermal expansion (CTE), thus minimizing stress during processing and allowing it to lay flat for handling. It may also be used in more applications because of its ability to match CTE to copper or silicon.
In addition, the material has a low modulus for those applications requiring mechanical flexibility. Once the circuit is formed into the required shape, it is capable of maintaining the shape and intended performance. On the other hand, the material can be used as a circuit material in hybrid constructions together with other rigid circuit materials such as FR-4. It is an adhesiveless laminate that finds use in simple applications where other conventional materials are marginal (where ease of processing, low moisture absorption and good dimensional stability are required, for example).
The excellent high frequency properties allow the use of thinner layers with no signal distortion. It also results in fewer design changes and increased reliability. ZYVEX also has excellent barrier properties, thus causing no storage problems in a manufacturing environment and providing no pathway for corrosive chemicals to penetrate, permitting near hermetic packaging. Finally, it is flame resistant for reduced environmental concerns. The diclad laminates can be manufactured in a range of copper and LCP thicknesses, and is available in roll or sheet formats.
Applications
The ZYVEX LCP material's excellent dielectric constant (Dk ) and low loss (Df ), as well as its low moisture absorption, lends itself to use in high frequency and high speed circuits where maintaining stable electrical properties is critical. Uses in controlled impedance interconnects, high frequency antennas and high frequency layers in multi-layer PCBs are typical.
The material's good dimensional stability and low moisture absorption are critical for fine line definition and reliable circuit performance, giving rise to its application in substrates for high density circuits. In addition, its good chemical resistance permits it to be exposed to corrosive chemicals that find use in automotive applications and other harsh environments.
Conclusion
As with all new technology platforms, the initial cost of entry is high and this new LCP material is no exception. However, over time the cost will be reduced while the performance benefits will far outweigh the initial costs. The excellent electrical properties of ZYVEX diclad material combined with its dimensional stability and low moisture absorption make its use in high frequency, high circuit density PCB applications incomparable. Additional information may be obtained from the company's Web site.
ZYVEX™ LCP is a licensed trademark of Rogers Corporation.
Rogers Corp.,
Advanced Circuit Materials Div.,
Chandler, AZ (480) 961-1382,
www.rogerscorporation.com.
Circle No. 301