Microwave Journal

Low Cost Substrates for High Speed, High Frequency, High Volume PCBs

Substrate materials offering good electrical stability and performance for high volume PCB applications in the 1 to 3 GHz frequency range

May 1, 1997


Arlon Materials for Electronics Division

Rancho Cucamonga, CA

New substrate materials for high speed, high frequency commercial electronic circuits have been introduced. The 25N substrate materials for high volume commercial circuits feature many of the performance advantages of polytetraflouroethylene (PTFE) materials with the cost/processing advantages of traditional thermoset materials. These materials offer good electrical stability and performance advantages for high volume PCB applications in the 1 to 3 GHz frequency range. Low loss, dielectric constant er stability over temperature and frequency makes the 25N materials suitable for circuits in many personal communications network applications such as cellular telephones, downconverters, low noise amplifiers, antennas and others where the higher costs of PTFE materials are prohibitive, yet instability, electrical loss and other shortcomings of lower cost FR-4 materials are unacceptable. Figure 1 shows the 25N material's er vs. temperature.

The electrical and mechanical properties of 25N materials - combined with conventional, economical processing - open a broad spectrum of applications possibilities. For example, 25N is the first multilayerable, low loss material with its own prepreg (B-stage bonding ply), permitting production of dual offset stripline circuitry in a totally homogeneous dielectric medium to maintain signal integrity for performance and reliability. Also, the materials are suitable for production of double-sided PCBs. Other unique advantages offered by the 25N materials include low er and loss properties with low thermal coefficient of dielectric constant (TCer) and good physical stability characteristics. These advantages permit the higher speed circuits required by many wireless devices to be produced.

The materials are available as copper-clad laminates or prepregs in a woven-fiberglass-reinforced, low polarity thermosetting resin system modified with a proprietary microdisperse ceramic. Electrical performance is good, with an er of 3.25 and loss tangent tand of 0.0025 to 0.003 in the 1 MHz to 10 GHz frequency range. Essentially, TCer = 0 between -40¡ and +140¡C, and water absorption is 0.08 percent, further enhancing frequency stability under a wide variety of ambient conditions. Coefficient of thermal expansion is 17 ppm/¡C in the x/y-axis and 70 ppm/¡C in the z-axis.

In most respects, processing for the 25N material is straightforward and compatible with conventional multilayer board technology. Through-hole drilling and plating characteristics are also similar to processes used for traditional epoxy and polyimide laminates.

The 25N materials are described as a crossover of traditional multilayer PCB technology for high speed, microwave and near-microwave applications. The material is designed to fill the niche for high speed, performance and reliability commercial-quantity circuit materials precisely within the company's full spectrum of thermoset and thermoplastic electronic materials. These specialty laminates include woven-fiberglass fabrics impregnated with polyimides, cyanate esters, high glass transition Tg multifunctional epoxies, and woven and nonwoven PTFE systems.

For large volume commercial applications, the 25N materials eliminate many concerns associated with PTFE materials with regard to cost, while providing performance and reliability superior to conventional FR-4 materials at competitive pricing. Table 1 lists 25N material properties compared to those of other PCB laminates. As a result, the new 25N materials offer a suitable solution for large-scale commercialization of multilayer and double-sided PCBs, particularly in the frequencies employed for commercial wireless and high speed digital applications. Additional technical information or applications assistance may be obtained from the manufacturer's Web site at arlonmed.com or e-mail: substrates@arlonmed.com.

Arlon Materials for Electronics Division
Rancho Cucamonga, CA
(800) 635-9333 or (909) 987-9533