A Thin, High Frequency Laminate for Multilayer PCB Applications
Smaller, lighter and more integrated seem to be the direction toward which today's high frequency PCB technology is moving. The microwave portion of the circuit must now be an integral part of a multilayer PCB assembly containing RF, IF, baseband and digital circuitry in one neat package. This push for total integration has placed new demands on high frequency laminate material for PCB fabrication. In addition to low loss and stable dielectric properties at microwave frequencies, the material must be thin, mechanically and thermally stable, and easily machined and processed.
The GML1100 thin-core copper laminate is just such a material. With a dielectric thickness of 9.3 or 3.5 mils, this new high frequency material is designed specifically for use as the RF portion of multilayer PCB assemblies and in thin circuit applications. The new material features a dissipation factor of 0.003 and 0.004 at 2.5 and 10 GHz, respectively, and a dielectric constant of 3.29 ±0.05. The hybrid polyester resin construction is a result of quantitative structural property relationships and an advanced molecular modeling process used in the biotechnology industry. The material maintains FR-4 process ability and thermoset plastic properties, and is environmentally friendly with no heavy metals or added fire retardants. The laminate features standard glass construction and good drilling and PCB processing characteristics. Table 1 lists performance data for the 9.3-mil-thick material.
The thinner RF laminates require a higher degree of precision in thickness tolerance. The standard tolerance for most high quality RF laminates is ±0.001". Using this tolerance on a laminate of 0.0035" thickness represents a 30 percent variation in material thickness, which results in unacceptable dielectric variation. The GML1100 material thickness tolerance is 0.0005" to 0.0007", even thinner than a human hair, which measures approximately 0.002" thick. This thickness represents a considerable measurement problem at the laminator or board shop. However, during manufacture, a continuous process, shown in Figure 1 , allows for improved thickness control due to computer feedback loops and on-line adjustments. In addition, specialized equipment is used to improve the measurement accuracy of the laminate, as shown in Figure 2 .
Hybrid Multilayer Construction
As stated previously, RF devices are becoming smaller and more complicated and need to move into multilayer construction. For example, feed networks are being combined with patch antennas on the multilayer PCB surface to receive and transmit RF signals. High speed computers are utilizing clock and processor speeds approaching the microwave range. Hybrid multilayer PCB constructions are being used to keep the digital and analog portions of the circuitry on standard laminate layers while integrating the RF portions on a lower loss laminate layer. The GML1100 material fits these applications to a tee. It can be used with low dielectric constant and low loss bonding sheets while exhibiting good plated-through-hole (PTH) stability with FR-4 coefficient-of-thermal-expansion performance. Figure 3 shows a cross section of a PTH after multiple thermal cycles. The material also features a 94 V-0 fire-retardant rating.
Multilayer applications are now being utilized in antennas, cellular/PCS/GSM equipment, pagers, Global Positioning System and other satellite communications, auto collision avoidance and navigation devices, medical telemetry, RF identification tags, high speed computer and wireless local area network equipment as well as home automation, sensing alarms, controls, remote monitors and Internet linkages. Some form of RF/microwave circuitry is involved in each of these applications, which utilize increased levels of hybrid PCB integration. Thus, all of the applications can benefit from the advantages offered by the GML1100 laminate.
Copper-clad GML1100 material is currently available in 18" x 24" panels. Samples are available upon request.