High-temperature processing is a routine part of manufacturing high-frequency circuit materials. From first forming dielectric prepreg materials to laminating the dielectric materials with conductive metals and eventually adding circuit elements, heat is an ally in producing printed-circuit-board (PCB) materials. Two types of composite materials—thermoplastic and thermoset materials—are commonly used for the dielectric layers in PCBs or as adhesives in manufacturing circuit laminates and they each have their own traits and characteristics. But how do they differ? What are the strengths and weaknesses of each type of material and why choose one over the other for an application?
Thermoplastic and thermoset materials are both processed at elevated temperatures. Thermoplastics are normally in a rigid or hardened state and soften as temperature is increased toward a material’s melting point. Thermoplastic materials can be reinforced with fillers, such as woven glass or ceramic materials. One of the best-known thermoplastic materials used in high-frequency PCBs is polytetrafluoroethylene (PTFE), which is often reinforced with some form of filler.
Thermoset materials harden as a result of a thermochemical reaction, such as the reaction that hardens the two components of an epoxy when mixed together. Because they start out in a soft or liquid state, it can be a simple matter to reinforce thermoset materials with fillers. Once hardened or “cured,” thermoset materials are typically harder than thermoplastic materials. Unlike thermoplastic materials, thermoset materials go through the thermochemical reaction to a hardened state once, and cannot be re-melted like a thermoplastic. Prior to the cure of thermoset materials, they have a limited shelf life compared to thermoplastic materials, which are stable at room temperature.
Thermoplastic materials such as early PTFE-based circuit boards were considered difficult to process, with relatively high coefficient of thermal expansion (CTE) compared to copper, contributing to the challenges of forming reliable plated through holes. Special chemical treatments of through hole walls were required to form sufficiently strong bonds between the plating metal and the thermoplastic PTFE. In contrast, thermoset materials tend to have CTEs that are much closer in value to the CTE of copper, allowing the use of standard processing methods when preparing through hole walls for plating.
Perhaps the simplest way to describe the differences between thermoplastic and thermoset materials for PCBs is that thermoplastic materials tend to provide better electrical performance but can require more elaborate manufacturing processes, while thermoset PCBs are easier to manufacture but traditionally have offered lower performance.
Thermoplastic materials typically have less electrical loss than thermoset materials, with less change in electrical performance over time and at elevated temperatures than thermoset materials. Unlike thermoplastic materials, thermoset materials can oxidize over time. The oxidation process can cause changes in a PCB material’s dielectric constant (Dk) and dissipation factor (Df) resulting in a potential for performance change at RF/microwave frequencies.
Through research and refinement, however, scientists at Rogers Corp. have improved upon the characteristics of both thermoplastic and thermoset materials for PCBs, with the choice of filler material having a great impact on electrical and mechanical performance levels. As an example, RO3000® circuit material is a thermoplastic material, a ceramic-filled PTFE composite that is available with Dk values from 3.0 to 10.2. As a thermoplastic material, it is very stable electrically and mechanically over time and temperature, with low temperature coefficient of dielectric constant (TCDk). It represents a dramatic refinement of early PTFE-based thermoplastic circuit materials, which exhibited CTE values in the z-axis of 300 ppm/°C or higher.
Even though RO3003™ circuit material is based on PTFE for low loss at microwave frequencies (dissipation factor of 0.0010 at 10 GHz), it is not plagued by the typically high CTE of PTFE-based circuit materials. It incorporates a special ceramic-based filler material to significantly reduce the CTE to 24 ppm/C, closely matched to copper at 17 ppm/C. In addition, although most thermoplastic circuit materials require special chemical treatments to prepare the walls of through holes for plating with conductive metals, RO3003 thermoplastic circuit material can fabricate reliable plated through holes using a straightforward plasma process.
In contrast, RO4835™ thermoset-based circuit material was also developed and refined through experimentation. The ceramic-filled circuit material features much higher oxidation resistance than conventional thermoset materials. It is a high-performance material for high-frequency applications but, because it is not based on PTFE, it does not require special preparation (such as a sodium etch) to enable the formation of reliable plated through holes. The material is compatible with RoHS-compliant, lead-free processing. It supports low-cost fabrication processes comparable to those used for FR-4 circuit materials, but achieves outstanding RF/microwave electrical performance.
Although thermoset materials are not noted for low electrical loss compared to thermoplastic materials, the RO4835 thermoset material achieves low dielectric loss enabling it to be used in low-cost circuit applications above 500 MHz. It has a dielectric constant of 3.48 in the z axis at 10 GHz, held to a tolerance of ±0.05. Plated through holes can be fabricated on RO4835 laminate using standard processing methods, since the material achieves a z-axis CTE of 31 ppm/°C which is close to the 17 ppm/°C of copper commonly used for plating through holes.
RO4835 circuit materials build upon the success of another thermoset material from Rogers Corp., ROHS-compliant RO4350B™ circuit materials which have become a popular starting point for designers of high-power, high-frequency amplifiers. RO4350B laminates are rigid thermoset materials that do not use PTFE but can achieve excellent RF/microwave performance over time and even at elevated temperatures. They feature excellent thermal conductivity and mechanical thermal stability for stable and reliable use in power amplifier and other higher-power RF circuits. RO4835 and RO4350B thermoset materials remain rigid and stable at room temperature and share the benefit of ease of processing, using manufacturing methods typically applied to FR-4 materials.
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