Selecting a circuit board material for an application often comes down to a choice based on which has better specifications on the data sheet. Two different PCB materials from different suppliers might look identical in terms of key specifications, such as dielectric constant (Dk) and dissipation factor (Df), making a choice difficult. But how close are those two materials really? When can the data sheets look the same but the two materials be quite different?
Permittivity, also known as dielectric constant (Dk), and dissipation factor (Df) are among the most commonly consulted specifications when comparing different PCB materials. Both specifications are typically frequency-dependent parameters, with Dk typically decreasing with increasing frequency while Df typically increases with increasing frequency. When comparing these parameters for different PCB materials, especially from different suppliers, it is important to remember the frequency dependence, and to compare the parameters of different circuit materials for the same frequencies. Not every circuit material is tested the same way or under the same conditions, and differences in test methods can yield differences in key material parameters, such as Dk and Df, even when evaluating the same circuit material with the different methods.
Most PCB materials are anisotropic, which means that they can show different values for a parameter when measured in different directions. For Dk, for example, most circuit materials have a different Dk value in the x-y plane of the material compared to the z-axis (thickness) of the material. Since in PCBs it is the thickness of the dielectric material that provides the foundation and isolation for circuit traces and ground planes, circuit designers typically refer to a PCB material’s Dk in the z-axis or through the thickness of the material.
Circuit material suppliers may provide measurements of a material’s Dk in the x-y plane as well as in the z-axis, but in comparing different materials, it is important to compare “apples to apples” and z-axis to z-axis and to ensure that the same Dk measurement method was used for any circuit materials being compared. In remembering that Dk is a frequency-dependent parameter, measurements of Dk for different circuit materials should also be at the same frequency, since a difference in test frequency can mean a difference in measured Dk value, even for the same material in two different tests.
Quite simply, when comparing the Dk for different circuit materials from different suppliers, the Dk should be measured in the same direction (z-axis), at the same test frequency, and using the same test method. For measuring the Dk of circuit materials, Rogers Corp. uses the IPC-TM-650 126.96.36.199 clamped stripline method at 10 GHz through the z-axis (thickness) of the material. Similarly, when comparing the Df values of different materials, comparisons should also be under similar conditions: through the z-axis, at 10 GHz, and at room temperature (about +23ºC).
For the majority of commercial PCB materials, Dk is also very dependent upon temperature. Although most measurements of circuit material Dk are performed at room temperature (at or around +23ºC to +25ºC), deviations of more than a few degrees from room temperature can result in a different Dk value than the one measured for the same material at room temperature.
Look for Asterisks or footnotes
Asterisks or footnotes on data sheets can be very meaningful, since they are often alerting a reader to an exception or special case. With the number of applications expected to continue to grow strongly through millimeter-wave frequencies due to automotive safety systems and Fifth Generation (5G) wireless communications networks, more circuit designers are considering choices of circuit materials at higher frequencies, such as 60 and 77 GHz. They may also be learning about tradeoffs between smooth copper conductors and not-so-smooth electrodeposited (ED) copper conductors.
Smooth copper typically features lower loss at higher frequencies than ED copper. One key tradeoff when using smooth copper on a circuit material compared to ED copper is the lower copper peel strength because of the smooth copper surface compared to the rougher surface of ED copper, which promotes greater copper peel strength. In comparing different circuit materials for copper peel strength at these higher frequencies, however, any comparisons should consider those asterisks. Comparisons of different circuit materials for millimeter-wave frequencies should be for the same types of copper and for the peel strengths achieved for the same types of copper. Not all circuit materials are tested for copper peel strength using the same type of copper, and an asterisk for this specification on a data sheet may sometimes point out that the copper of interest is not the copper that was used when measuring the copper peel strength for that material.
Handling the Heat
Yet another circuit material parameter that can require some focus when comparing different materials is thermal conductivity. Different test methods are used to determine the thermal conductivity of a circuit material, some with the copper, some without. Obviously, copper is a good thermal conductor while the dielectric material of a PCB material is not. Inclusion of the copper in any test of thermal conductivity will impact the results, although the amount of that impact will depend upon the thickness of the dielectric material. Copper included with thinner dielectric material will increase the thermal conductivity more than copper included with a thicker dielectric material being tested for thermal conductivity.
Some may argue that having the copper is more like the way the material will be used in an actual application, with copper traces on the material contributing to the overall thermal conductivity of the PCB material. But most circuit simulation software, which calculates the effects of a circuit material’s thermal conductivity on the performance of a circuit fabricated on that material, assumes the thermal conductivity of the circuit material without the copper.
From these few circuit material parameters, it is clear that comparing different PCB materials from different suppliers may not always be so simple. When comparing circuit material data sheets, a good starting point is to make sure that the same parameters are being compared—that the same test methods at the same test frequencies and temperatures were used for basic material parameters such as Dk and Df. If they were measured using the same test methods, then it is fair to compare the different materials for those parameters, and to expect the performance from the circuit material that is finally selected!
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