Here are some of the Hybrids and Mixed Dielectrics PCB.
In contrast, some high-frequency circuit materials which are supported polytetrafluoroethylene (PTFE) have high CTE values and may suffer from excessive material expansion when subjected to the elevated temperatures of some circuit fabrication processes. Combinations of low-CTE FR-4 and high-CTE PTFE circuit materials are sometimes wont to fabricate composite circuits, with the various circuit handling different circuit functions.
The composite circuit with the 2 materials yields a suitable CTE for handling the temperatures of the many fabrication processes also as some end-use applications. Figure 1 shows an example of such a multifunction, multilayer circuit, where high-frequency microstrip-based circuits are separated by FR-4 control circuitry.
PTFE laminates are commonly used for RF/microwave circuits thanks to their excellent electrical performance at higher frequencies. PTFE circuit materials typically exhibit a dissipation factor of around 0.002, signifying very low dielectric loss, whereas FR-4 materials typically have a dissipation factor of around 0.020 for much higher dielectric losses.
The CTE values of the 2 materials also are much different, but the relatively low CTE of FR-4 are often a plus during a multilayer circuit construction. The CTE for PTFE material is about 200 ppm/°C, compared to about 50 ppm/°C for a few high-Tg FR-4 materials. But when the 2 circuit materials are combined, as within the multilayer circuit of Fig. 1, the CTE of the FR-4 layers helps to lower the general CTE of the multilayer circuit.
The dimensional stability of FR-4 circuit materials also can benefit hybrid multilayer constructions also using PTFE. Those PTFE circuit laminates which aren’t reinforced with woven glass the way of FR-4 are less stable dimensionally than PTFE laminates with such reinforcement. But during a hybrid multilayer circuit with PTFE, the added FR-4 layers are typically reinforced by woven glass, improving the general dimensional stability of the multilayer construction.
DuPont Teflon fluoropolymer resins and other PTFE materials have a reputation for being “non-stick” in nature, where organic materials won’t adhere to the PTFE. With such a reputation, it’s easy to know why many would believe that it might be difficult to bond FR-4 materials to PTFE, but this is often not necessarily the case.