Selecting PCB materials for high-frequency applications

John Coonrod, Rogers Corporation -July 11, 2012

The greatest concern during PCB assembly is due to the effects of thermal stress from soldering. Other sources of thermal stress during PCB assembly are from solder rework or exposure to multiple thermal cycles. In terms of circuit materials, effects from thermal stress can typically be projected by comparing the CTE values for different materials, as shown in Table 3.

Table 3: Typical CTE values for materials commonly used in high frequency PCBs.

In general, a circuit material with a lower CTE will be more robust and handle the thermal stress of PCB assembly better than a material with a higher overall CTE. This is one reason why multilayer PCBs typically use more than one type of circuit material. Materials which might provide good electrical performance may have characteristics (such as high CTE) that make them less than robust to handle the thermal stress of PCB assembly.

By using a combination of materials, some with good electrical properties and others with lower overall CTE, a robust multilayer PCB construction can be designed and assembled. Such a construction is known as a hybrid multilayer PCB, which can provide cost as well as performance benefits. More information about hybrid multilayer PCBs can be found on a paper presented at PCB West 2010. [1]

In general, a circuit material with CTE value of 70 ppm/°C or less is considered robust for PCB fabrication and assembly. As Table 3 shows, however, one of the materials with the best electrical performance also has the worst CTE. This is one reason why ceramic-filled PTFE laminates were formulated. They combine excellent electrical performance with very good CTE. Unfortunately, they exhibit poor dimensional stability, since the material is soft and circuit dimensions can be easily distorted. To provide good electrical performance and CTE with improved dimensional stability, ceramic-filled PTFE laminates with woven glass reinforcement were developed.

When making a choice in high-frequency circuit materials based on fabrication issues, the clear-cut favorite would be ceramic-filled hydrocarbon material with woven glass. These materials feature a low dissipation factor typically on the order of 0.003 and are robust in terms of most circuit fabrication processes. If better electrical performance is required, the choice would be ceramic-filled PTFE with woven glass. These materials typically have a dissipation factor in the range of 0.002 and are generally fabrication-process friendly.

The major concerns in fabricating PCBs with these materials relate to drilling and PTH preparation. For the best electrical performance, the choice is micro fiber glass PTFE, although this material can be difficult in terms of fabricating more complex circuit constructions. The material, which is nearly pure PTFE, is often used for simple high-frequency circuitry such as microstrip filters and couplers. Additionally this material is often used in a hybrid multilayer circuit, in which it supports critical functions, while other materials more friendly to circuit fabrication processes are used for the remainder of the multilayer PCB.

Choosing materials based on end-use applications
There are several different concerns for choosing materials for high frequency applications. A good example in chart form is given from the Rogers Corporation Product Selector guide on the website and a portion of this is shown in Table 4.

Table 4: A tabular format for comparing high-frequency materials.

Table 4 provides a quick comparison of different circuit materials based on key electrical performance parameters, including dielectric constant (Dk), dissipation factor (Df), thermal conductivity, and CTE. Two values of Dk are listed for each material: process and design.

The process Dk refers to the value determined by industry-standard IPC test method, IPC-TM-650 at 10 GHz. This value is used as a process control for making the substrate. The test method is reliable and well proven, but the Dk value is specific to that test methodology and that test frequency. The test method uses a clamped stripline resonator and is a fixture mechanism allowing large volumes of materials to be tested, which is necessary for a laminate manufacturer. However, the fixture is not representative of an actual stripline circuit or a microstrip circuit, and the use of process Dk values in computer-aided-design software simulation tools has been known to yield erroneous results.

In some cases, process Dk values may not be ideal for design purposes. For that reason, a second set of Dk values, the design Dk numbers shown for each material in Table 4, were determined using actual microstrip transmission line circuits, across a wide frequency range. These values are more appropriate for circuit design and modeling.

Table 4 also lists tolerance values for Dk for each material. Some high-frequency applications have very tight specifications for impedance control and the Dk tolerance is a good indicator of how well this material may be suited for those applications. In addition, Table 4 shows values for Df for each material, which is related to dielectric losses. For an application that requires low-loss performance, a material with lower Df value would be a logical choice, although this choice should also be weighed against the ease or difficulty of PCB fabrication with that material.

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