Thermoset and Thermoplastic PCB Materials

High-temperature processing comes into play during the manufacturing of the high-frequency PCB boards. It starts with the lamination of prepreg on the dielectric substrate followed by adding the circuit layer. Heat-processing is the primary source to combine all these layers together to form a well-defined board. Thermoplastic and Thermoset materials are generally used for the dielectric layers in PCBs or as adhesives in manufacturing circuit laminates. These materials have their own strengths and weaknesses.

A Thermoplastic material is also called thermo-softening plastic. It is a polymer, which converts to liquid when heated and attains a glassy hard state when cooled.

Characteristics of Thermoplastic Material

• Processed at elevated temperatures
• Originally, available in rigid/ hard form, but softens when the temperature increases towards its melting point.
• These materials can be reinforced with fillers, such as woven glass or ceramic materials.
• Polytetrafluoroethylene (PTFE) is one of the best-known thermoplastic material and it is often reinforced with some filler.
• They provide good electrical performance, but require complex processing procedures.
• Less electrical loss with less change in electrical performance over time.

Issues associated with Thermoplastic Material: Their processing is difficult due to their high CTE value.

Thermoset is also called thermosetting plastic. It is also a polymer material that is cured by applying heat (above 200 centigrade) through a chemical reaction. Before curing, this material is in liquid or malleable form.

Characteristics of Thermoset Material

• Processed at elevated temperatures
• They achieve their hard state due to a thermochemical reaction. Such a reaction hardens two components when they are mixed together.
• Once hardened, they are harder than thermoplastic materials.
• Their CTE value is close to the CTE value of Copper, which is suitable for standard processing procedures.
• Prone to more electrical loss
• These materials oxidized over time, result in changes of a PCB’s dielectric constant (Dk) and dissipation factor (Df), which is eventually responsible for PCB’s change in performance at RF/ microwave frequencies.

Issues associated with Thermoset Material

• Once hardened through a thermochemical reaction, thermoset material cannot be re-melted back to their original state.
• Their electrical performance is not good as compared to thermoplastics, but can be processed easily.

The characteristics of both thermoplastic and thermoset materials can be improved with the choice of accurate filler material. This choice of filler material greatly impacts their electrical performance levels.