Effective battlefield communications, advanced unmanned aerial vehicles (UAVs), and high-resolution radar all have one thing in common—they require more (technology) with less (size, cost, weight, and power consumption). Circuit and systems designers are constantly breaking new ground to meet the needs of modern defense systems but, to do so, they have relied on an often-overlooked component: circuit materials. After all, impressive advances in circuit materials technology have armed aerospace and defense designers with the tools they need to meet demanding, modern requirements.
A key requirement for military components and systems in recent years has been the reduction in form factor, while also increasing functionality. Unfortunately, this combination usually results in an increase in operating temperature, making thermal management an important concern for military circuit and system designers. One way that most electrical engineers have traditionally dealt with the problem of temperature rises at the circuit-board level has been by specifying printed-circuit materials with lower dissipation factors. But the dissipation factor is just one characteristic of a printed-circuit-board (PCB) material from a list of parameters that can also provide insights into how a material can impact that need for more functionality from less size, weight, and cost.
In comparing materials with different dissipation factors, for example, the loss-tangent difference can be an order of magnitude between two PCB materials, such as epoxy glass and polytetrafluoroethylene (PTFE) circuit laminates. For low-loss circuit materials, the thermal conductivity (TC) of the material is often the main differentiator when the thermal management of a design is a concern. Selecting materials with the higher thermal conductivity will have the largest impact in reducing temperature from a PCB perspective.
How can the TC of a PCB material impact the performance of a military system? To better understand how PCB materials with high TC values could benefit some designs, a study was performed by circuit-materials supplier Rogers Corp. on a number of different PCB materials (Table 1). Samples of each material 0.020 in. (0.5 mm) thick with 50-Ω transmission lines were evaluated with a 1.9-GHz signal source set to five different power levels, with a top level of 26 W. Figure 1 details the results of these measurements. These materials represent a fairly wide range of commercial materials, with much different material characteristics. For example, by comparing FR-4 to a polyphenylene-oxide (PPO) circuit material, the impact of reducing the loss tangent by a factor of 5 can be observed. When comparing modified PPO circuit materials with RO4350B materials from Rogers Corp.—with both considered low-loss RF circuit materials—the effects of using RO4350B and its higher TC results in a close to 50% reduction in temperature rise. For optimal thermal management, selection of a material with both low loss tangent and high TC is desired.
In a quest for the best balance of low loss and high TC, a material like RT/duroid 6035HTC from Rogers Corp. features a low loss tangent of 0.0014 at 10 GHz and a TC of 1.4 W/m/K. The laminate is based on a PTFE resin with high-TC ceramic filler. As the measured response of Fig. 1 shows, the level of RF power to the PCB based on the 6035HTC material has minimal impact on temperature rise. The combination of low-loss tangent and high TC is the reason for this: While other PTFE materials are available with lower loss tangents (as low as 0.009), their TC values are also much lower, in the range of 0.2 to 0.3 W/m/K. This will result in far inferior results in terms of thermal management compared to RT/duroid 6035HTC material.
Cost is also a consideration when selecting a PCB material. In the case of the low-loss materials, the resin used in these materials is either a thermoset type (such as epoxy, PPO, or butadiene) or PTFE based. The materials using PTFE tend to be more expensive. Processing these materials [such as the use of plated-through holes (PTHs) for connections through the PCB] and special handling requirements can also be pricier. Among the thermoset materials that would present cost advantages, RO4350B laminate has the best thermal performance. While greater by a factor of 3 over RT/duroid 6035HTC material in terms of TC, it still is significantly lower than other potential thermoset RF material choices.
The importance of reliable PCB performance on the battlefield cannot be overemphasized; mission success can depend on an electronic function such as communications. Modern communications systems, such as software-defined radios (SDRs), are designed not only for reliable, high-speed communications, but to do so securely even in hostile operating conditions. Such radios are used not only by personnel, but also on board satellite systems, airships, and on UAVs. All of these advanced military systems have depended on new developments in high frequency materials, which have been critical in reducing weight and size while also increasing functionality.
In the past, foam-based PCB materials have been developed to provide advantages in weight, but such materials were difficult to process using traditional PCB handling methods (for example, no PTH capabilities). Ultimately, these materials were removed from the market.
For space and airborne applications, many programs have relied on materials based on PTFE/random glass (such as RT/duroid® 5880 material from Rogers Corp.) or PTFE/ceramic filler (RT/duroid 6002 material from Rogers Corp.) because of either the low dielectric constant and loss tangent (lower electrical loss) or low Z-axis coefficient of thermal expansion (CTE, for high PTH reliability) and stable temperature performance. But increasing system demands for lighter-weight materials motivated further PCB materials development for additional savings in weight. This has resulted in the development of a material such as RT/duroid 5880LZ from Rogers Corp., which combines the benefits of low dielectric constant with the low Z-axis CTE of the RT/duroid 5880 and RT/duroid 6002 materials, but with a 30% reduction in density (see Table 2). The RT/duroid 5880LZ material is suitable for light-weight antennas requiring a low dielectric-constant material. Bond layers for the material can be either thermoplastic or thermoset films (Table 3 shows three options for use with RT/duroid 5880LZ).
The unique properties of RT/duroid 5880LZ are achieved through the use of a select filler system, which also makes possible the excellent thermal cycling reliability of PTHs formed in the material. To evaluate the thermal cycling reliability of PTHs formed in RT/duroid 5880LZ, testing was performed on 0.060-in.-thick material with 0.0198-in.-diameter viaholes. Samples were exposed to 500 air-to-air thermal shock cycles at −55 and +150°C. No failures were found in any of the 125 PTHs tested.
The growing use of mobile data has impacted electronic design in commercial as well as in military circles. According to a report by a leading data firm,2 the amount of mobile data is projected to practically double every year through 2016. These demands fuel the need to develop faster electronic systems that can handle not only the mobile data portion of a network but also data from fixed sources, as networks move towards serial data rates of 40 Gb/s.
These trends hold true in the case of both commercial and military systems, with an increasing amount of defense-related data produced by remote sensors, surveillance systems, and a growing number of military electronic systems in general. The increasing speeds of digital signal processors (DSPs) and field-programmable gate arrays (FPGAs) in military electronic systems has made these systems more intelligent, but also more reliant on the capability of processing more data faster.
As serial data rates have increased from 2.5 Gb/s in the early 2000s to 9.8 Gb/s, and now to 40 Gb/s, the push has increased for PCB materials with the loss and dispersion characteristics that can support these data rates. High-speed digital signals, which can be viewed as a combination of fundamental and harmonic frequencies, are extremely broadband in nature and require PCB materials that can handle broadband signals. For high-speed digital projects, designers are turning to PCB materials that have traditionally been used in RF/microwave circuits, such as RO4350B LoPro copper foil material in 0.004-in. thickness. This PCB material provides very stable performance over a wide bandwidth.
Figure 3 shows test results from 8 to 50 GHz for RO4350B LoPro material, where the dielectric constant dispersion is only 1.2%. This low value ensures that the shape of a high-speed digital pulse is preserved through a PCB, since the pulse’s various signal components (fundamental, harmonics) travel with minimal time differences through the PCB’s signal path.
Another material characteristic that is important for maintaining signal integrity is loss. Materials are characterized in terms of both dielectric and conductor losses. For many applications, it is important to select a material with minimal conductor losses, since any gains made by choosing materials with low dielectric losses could be lost by increased conductor losses. Figure 4 details the improvement in insertion loss for RO4350B LoPro material. At 20 GHz (the fundamental frequency of a 40-Gb/s signal), reduction in insertion loss is about 30% over traditional RO4000® material, helping protect the amplitude of the overall signal. For high-speed digital applications, the use of RO4350B with LoPro foil enables circuit designers to not only preserve signal integrity but, with the 0.004-in. thickness of the material, to accommodate complex multilayer designs while keeping overall thickness low.
These material advances represent just a handful of the improvements made in PCB materials in recent years. These have been motivated by the needs of electronic designers not only for military applications, but in commercial, industrial, automotive, and medical electronics industries to do more with less: to achieve increased electronic functionality from smaller, lighter, and less-expensive PCB materials.
- Horn, Caisse, Willhite, “Measurement and Modeling of the Effect of Laminate Thermal Conductivity and Dielectric Loss on the Temperature Rise of HF Transmission Lines and Active Devices,” Design-Con 2012.
- “Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update, 2011-2016,” February 2012.