Low Cost, High Performance Prepregs for PCB Cavity Designs
Two new Speedboard® prepregs have been introduced that are specifically designed to offer superior performance in applications involving cavity designs in multilayer printed circuit boards. These materials exhibit very low loss and low dielectric constant (Dk) to be compatible with high performance RF and microwave laminates.
Many of today’s complex and compact high frequency multilayer circuit board assemblies require cavity structures to permit high power devices to be mounted on to proper heat sinks or sensitive devices to be mounted directly on ground planes. These designs are extremely difficult to fabricate to the exacting mechanical tolerances required for proper operation. However, the new prepregs feature repeatable, low X-Y resin flow with a high Tg for superior reliability, rendering them ideal for these applications.
The new prepregs are being produced in two material variations, Speedboard LF (Low Flow) and Speedboard EF (Environmentally Friendly, a halogen-free material). They share many high performance characteristics such as low loss, low Dk, and stable Dk and loss from 1 MHz to 40 GHz. Figure 1 shows a comparison of insertion loss vs. frequency performance when combined with various laminates to form 50 Ω stripline traces. The low dielectric constant of Speedboard LF and EF results in wider traces and superior thickness uniformity for controlled impedance layers and reduced crosstalk with increased routability. In addition, the high Tg allows for multiple laminations, coining or lead-free solder reflows and the material is micro-reinforced for superior crack resistance. Both types show excellent adhesion to all commercial laminates, are compatible with Omega-ply and exhibit low outgassing for space applications.
Figure 2 shows the edge of a hybrid PCB that is fabricated using Speedboard LF and halogen-free FR4 laminate. As can be seen in Figure 1, this combination (blue) offers a halogen-free solution with significantly improved performance compared to an all-FR4 (green) design. The material is excellent for filling buried vias during the lamination process and is capable of laser drilling up to five times faster than glass prepregs. The low Dk also makes for thinner boards and improved plated-through hole aspect ratios, while minimizing trace “print-through.”
Speedboard LF prepreg consists of a standard BT resin in a continuous toughening matrix and features a Dk of 2.6 and a loss tangent of 0.009. The repeatable X-Y axis resin flow makes it ideal for cavity modules and PCBs. Speedboard EF consists of a high Tg halogen-free resin in a similar matrix, and is ideal for RF/microwave boards and high speed digital circuitry. Both combine with standard or halogen-free FR4 cores for superior cost vs. performance. Table 1 lists the typical material properties of both types. Typical applications include base station power amplifiers and other cavity-type PCBs.
Speedboard LF is currently in high volume production. Speedboard EF has been fully qualified and is available for delivery. Additional information may be obtained from the company’s Web site.
W.L. Gore & Associates,
Elkton, MD (800) 445-4673, www.gore.com/electronics.