Stripline and multilayer circuits are often fabricated on polytetrafluoroethylene (PTFE) circuit materials for their outstanding electrical characteristics, even though the process of forming those circuits requires some way to keep multiple circuit layers in one piece. The benefits of stripline, with its excellent low-loss, high-frequency performance, and multilayer circuit assemblies, which provide a high density of circuit functionality in a small package, are clear. But fabricating PTFE-based stripline and multilayer circuits can require special handling of the circuit materials for optimum results.

Three approaches are commonly used for bonding multiple layers of PTFE-based circuit laminates, such as Rogers RT/duroid® 6000 series and RO3000®series materials. These three approaches rely on thermoplastic films, thermoset prepregs, and direct bonding methods, such as fusion bonding processes. The first two techniques require additional films or prepreg materials which function like glue to keep the multiple layers in one piece. The third approach employs heat and pressure to bond the multiple PTFE-based material layers into one piece.

Thermoplastic bonding films, such as Rogers’ 3001 bonding film, make it possible to clamp and bond circuit layers together using bonding films between the circuit layers. Ideally, these low-dielectric-constant (low-Dk) bonding films are also used with low-Dk laminate materials, such as RO3000series high-frequency circuit materials and RT/duroid6000 series circuit materials from Rogers Corp. Such bonding films have a melting point that is lower than that of the PTFE laminates, so that a melt temperature can be reached where the thermoplastic films, with uniform pressure applied to the circuits by a press, will flow and fill spaces between copper features on the circuits, solidifying upon cooling to form a single, stripline or multilayer circuit structure. The 3001 bonding film is chemically inert with good high temperature resistance.

The second approach to fabricating stripline and multilayer circuits is with the use of thermoset bonding prepreg materials. As with the bonding films, the multiple circuit boards are heated in a clamped assembly with thermosetting prepreg materials between the circuit laminate layers. Thermoset prepreg materials typically have a bond temperature (< +450°F or +232°C) that is lower than the melt temperature of the PTFE laminates, enabling the formation of a multilayer structure. As with the bonding films, the prepreg resin will flow and fill spaces between the different circuit copper features, aided by pressure applied to the circuit layers by the clamped assembly. Prepregs are often used in multilayer circuit assemblies based on a mix of circuit materials, such as FR-4 and PTFE circuit materials.

The third method for forming stripline or multilayer circuit assemblies from multiple layers of PTFE-based circuit materials is by direct bonding or fusion bonding, where the layers are joined together by means of high temperature and precisely controlled pressure, without the addition of bonding materials. This method has its challenges, such as the care needed to control the clamping stress when the layers are joined at an elevated temperature. But the results can be quite rewarding, since the resulting circuit assembly features a fully homogeneous dielectric-constant structure, with no interfaces formed of materials, such as bonding films or prepregs, with different Dk values than the circuit laminate layers being bonded together. For direct or fusion bonding with PTFE-based materials, a multilayer assembly achieves a single, uniform Dk value throughout the assembly, which can be an extremely useful material characteristic for high-frequency applications that must meet critical performance requirements.

How do the three bonding approaches compare? In forming a stripline circuit, for example, with thermoset bonding films and RT/duroid 6002 laminates, Rogers’ 2929 is a compatible thermoset bonding material with similar electrical properties as the RT/duroid 6002 laminates. Compared to a process like fusion bonding, forming stripline circuits with laminates and bondply films is straightforward and simpler, although the presence of the bonding films can increase signal losses and possibly compromise broadband performance. 

In contrast, a stripline circuit formed by fusion bonding multiple RT/duroid 6002 laminate layers can provide uniform Dk and electrical characteristics throughout the stripline or multilayer circuit assembly with minimal dispersion and excellent wideband performance. In a fusion bonding process, one RT/duroid 6002 laminate layer is used for the stripline inner layer signal plane and bottom ground plane, and the other RT/duroid 6002 laminate layer is used for the top ground plane. The copper layer on the other side of the laminate is completely etched away prior to fusion bonding the two laminate layers together, with the stripline conductors sitting on top of the bottom laminate layer and between the two dielectric layers. Although this process requires tremendous care in handling the laminates and controlling the pressure and temperature on the laminate layers when forming the multilayer structure, the end result is a circuit structure without additional bonding materials (and their associated Dk mismatches and potential circuit losses).

Forming stripline with two circuit laminates having the same Dk value will typically provide circuitry capable of low dispersion with good broadband performance. But if stripline is fabricated with top and bottom layers having different Dk values, dispersion can result, with poor wideband response. Similarly, if attempting to form a stripline circuit with high-Dk materials, such as RT/duroid 6010, with a Dk of 10.2, or RO3010™ laminate, with a Dk of 10.2, and a bonding film or prepreg is used to join the circuit layers, ideally that bonding material would closely match the Dk value of the laminates. But even proven high-frequency bonding films and prepregs exhibit Dk values of about 2 to 4, or much lower than the values of these high-Dk laminate materials, resulting in Dk mismatches within a stripline or multilayer circuit assembly. With a direct bonding or fusion bonding process, two laminate layers, such as RO3010 laminates, are joined together without the added bonding films or prepregs, to avoid the problems of Dk mismatch, added loss, dispersion, and uneven broadband response.

In some cases, a form of fusion bonding may be used that is not a direct bonding process, where a bonding material is also used in the process. For example, if the melt temperature required for fusion bonding may result in distortion of the copper circuit traces on the laminate because of movement of the laminate’s dielectric materials at that high temperature, a bonding material which has a lower melt temperature than the laminate may be used to keep the circuit layers together without distorting critical circuit patterns.

Of the approaches listed for fabricating multilayer circuits, fusion bonding may be the most expensive, but it also offers the potential for the highest performance. Fusion bonding is particularly well suited for multilayer circuit assemblies where certain performance levels are critical, such as minimizing dispersion in high-frequency filters. With the right circuit materials and a circuit fabricator experienced in the fusion bonding process, fusion bonding can help produce repeatable performance results in densely packed RF/microwave multilayer circuit assemblies.

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