PTFE, or polytetrafluoroethylene, is a synthetic fluoropolymer that is best known as Teflon, and in particular for its use in non-stick frying pans. In the world of PCBs for RF electronics, PTFE is better known as the material of choice for many RF systems. While it is not the only available material for these circuits, it is sometimes seen as a cure-all for excessive loss in an RF system, and it is sometimes described as a mandatory material for these boards.
PTFE-based materials for use in RF PCB stackups consist of a PTFE matrix combined with various additives and fillers. These additives are what make each PTFE-based product unique, and commercially available products are precisely engineered with various fillers such that the laminate provides specific electrical, thermal, and mechanical properties. If you’ve never dived deep into getting an understanding of the role of fillers and reinforcements in these materials, we’ll provide the insight you need to understand these in this article.
What Goes Into PTFE-Based Materials
PTFE-based materials are not necessarily thick films like flexible polyimide. Instead, they are a composite material built from PTFE and a set of additives and fillers. It is the inclusion of certain additives and fillers that specializes a commercially available PTFE-based PCB material for use in a particular application. The predominant material component in PTFE-based materials is a random matrix of PTFE; all additives are bound within the PTFE matrix and, together, the two types of materials determine the electrical, mechanical, and thermal behavior of the laminate.
One important aspect of PTFE-based materials is the inclusion of additives and fillers to form a composite material. These components are selected such that they enable a range of possible properties in commercial materials. Principally, there are two types of additives in these materials:
- Reinforcements, which primarily affect mechanical behavior
- Fillers, which can affect mechanical and dielectric behavior
The primary filler used in PTFE-based material is ceramics, which are typically added to the PTFE matrix in powder form. Reinforced materials may contain a ceramic filler. Reinforcements consist of multiple possible materials that support the PTFE matrix and give it some level of structural integrity. The main set of reinforcement materials is highlighted below.
Aside from these reinforcements, additional materials used in PTFE laminate materials include resins, flame retardants, and stabilizers. These additional materials ensure the laminate material can hit flammability ratings, resist aging, and more easily bond to other layers in a stackup.
The primary filler material used in commercial PTFE-based laminates is ceramic powders. The use of ceramics provides two advantages over a woven or random glass matrix as a reinforcement. First, they provide higher thermal conductivity than the PTFE base material; this is the same reason ceramic slabs are used as some PCB substrates. Second, they can modify the dielectric properties to have much higher Dk value if desired, which is useful in lower frequency RF systems.
Although the glass reinforcement provides the best results in terms of mechanical rigidity, this reinforcement is not desirable at higher frequencies associated with mmWave systems. PTFE-based products tend to include a mechanical spread glass reinforcement, but you may still get some fiber weave effect in the design that creates undesired skew, leading to incorrect phase response.
For this reason, ceramic-reinforced laminates tend to be preferred in RF systems. They can provide the major benefits of engineered ceramic materials as well as eliminating glass fiber weave problems. Ceramics encompass a broad class of materials that can be engineered to give specific material properties, so they are very useful as the primary additive in PTFE-based materials. In particular, various ceramics can be used to produce laminates that provide:
- Higher than normal thermal conductivity
- Low CTE mismatch compared to copper
- More stable dielectric constant behavior
- Lower layer-to-layer misregistration
Overall, for thinner dielectric layers, a ceramic-filled PTFE material is probably the best choice. At very high frequencies, it should be clear that glass-reinforced should be avoided. In many instances, an unreinforced material is also acceptable, but it will be more difficult to handle during production due to its pliability. The popular vendors for these products, with Dk values ranging from 3-10, are available from Rogers, Arlon, and Taconic.
Whenever you need to design high-performance RF systems with PTFE-based PCB materials, use the complete set of PCB design tools in Allegro PCB Designer. Allegro is the industry’s best PCB design and analysis software from Cadence, offering a range of product design features with a complete set of management and version control capabilities. Allegro users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.