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New Materials for By Jim Wellbrock While low-loss materials such as the various PTFE's have been in the marketplace for some time, they have historically been aimed at the military and high-end commercial users. Today's demands are increasing movement toward a lower cost, high-performance material. While flat-panel and patch antennas are key applications for low-cost, high performance materials, there is also a need for these materials as printed circuit substrates in other wireless system components. For instance, an RF receiver that performs satisfactorily using conventional printed circuit materials such as FR-4 at frequencies in the 800-900 MHz region will have borderline performance at 2 GHz and will be unacceptable at 10 GHz or higher. This 2 to 10 GHz range is a key market niche for low-loss, low-cost materials. Similarly, the RF transmit section of a wireless system may be using significant power and low-loss materials to become critical in limiting signal loss and improving overall system performance.
Permeability: The ability to support a magnetic field. This is related to inductance. The units for this number are Henrys/meter, and it is denoted by m. Characteristic Impedance: The impedance (potential to kinetic energy ratio) seen by an electromagnetic wave propagating in a medium. For a lossless transmission line (or short sections of a lossy line), it is defined as:
Insertion Loss: The amplitude difference between input and output signals through a transmission medium. It is usually measured in decibels and denoted a. There are two separate factors that contribute to insertion loss: reflection and dissipation. A transmission line is a combination of resistance, conductance, capacitance and inductance. Dissipation, also known as attenuation, is power loss in the system due to heat. Power is the product of electric potential (voltage or EMF), current and a power factor. Power factor is the cosine of the phase angle between the EMF and current waveforms. Since there is a 90-degree phase angle difference between EMF and current in inductors and capacitors, the power factor for these components is zero. Therefore, there is no dissipated power due to inductance or capacitance. It is a function of a medium's resistance and conductance alone, where EMF and current are in phase. The second type of loss is reflection. This loss occurs when source and load impedances are not perfectly matched to the transmission line connecting them. If the impedances are not matched, a portion of the signal applied will be reflected back to the source and lost, even if attenuation is low or non-existent. Insertion loss then is the sum of attenuation (dissipation) and reflection.
Sheldahl, Inc. - ComClad HF This plastic gives the system a completely homogeneous dielectric with
very favorable electrical properties. The dielectric constant is 2.6, and the dissipation factor is 0.0025 over
a frequency range of 1 to 10 GHz. This results in optimum signal performance at a low cost. The copper peel strength
of the laminate is 6.0 lbs/sq-in when measured using the IPC-2.4.9B method. The glass transition temperature of
ComClad HF is 140°C using RSA II. Dimensional stability is dependent on the thickness of the dielectric, amount
of copper etched off each side and the type of processing used. Large amounts of copper on each side will limit
the amount of distortion. Details of these properties can be accessed at www.sheldahl.com. ComClad HF offers many advantages for manufacturers of wireless systems. The favorable dielectric constant of 2.6 and low dissipation factor of 0.0025 make it ideal for use in a variety of high-frequency systems. The use of Noryl plastic allows the designer to hot and cold form, mold, bend and potentially even insert-mold the material in a plastic injection process. Noryl also offers very good value to the customer and helps keep total system costs down in the very competitive wireless market.
Soldering must be approached carefully and Sheldahl recommends the use of shielding and fixturing to limit the amount of the board that is exposed to the SMT or wave solder profile. Hand soldering is possible, but care must be taken to form the joint quickly and properly. Circuit design (the use of larger pads and connection points) can limit the potential soldering problems. Leaded components are recommended instead of SMT-components if a choice is available. Leaded components will add to the strength of the joint by providing a mechanical connection to reduce stress on the component/board interface. Low melt temperature solders are available and might be considered for use with this product. Alternative joining metals are being investigated and show good early results. Materials such as tin-bismuth may work well with ComClad HF. Sheldahl has recently begun development of a new product called ComClad
XF. This product is a combination of very smooth 0.5 ounce copper and a polypropylene base substrate, which has
very good electrical characteristics. The dielectric constant of this material appears to be in the range of 2.2
to 2.3 and dissipation factor approximately 0.0005. More testing is underway to confirm these numbers. This product
has excellent characteristics for very high frequencies (>50 GHz). ComClad XF is under development and anyone
with specific questions or applications for this type of product is encouraged to contact Sheldahl for more information.
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Today's rapidly
expanding wireless market is in a constant search to find high-performance and low-cost technologies to satisfy
customer demands for smaller and lighter packaging while maintaining a high level of system performance. Because
of this, low-loss, low-cost substrate materials are gaining in popularity with manufacturers of wireless systems
such as direct broadcast satellites, cellular base stations, PCS base stations, wireless local loops, GPS units,
antennas for these systems and other wireless devices.
The materials used in printed circuit boards, microstrip and stripline elements as well
as flat panel and patch antennas are typically constructed of copper laminated or bonded to a base substrate material.
There are several key product characteristics that are important when making decisions on high-frequency materials:
With several standard thicknesses, panel sizes and custom configurations available, there
are many solutions available to the designer. Sheldahl also provides engineering and design support to assist customers
with application-specific details regarding the product. There are also several assembly options available with
ComClad HF. While the product does require special handling in joining processes such as soldering, Sheldahl has
application engineering resources ready to work with customers to meet their needs and develop solutions for specific
applications.