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Lightning Speed Laminates: Transitioning From FR-4 to High-Frequency Materials

The question of when to transition from FR-4 material to a higher performing material is typically not as straightforward as one may assume. Also, when the transition is obvious, a lot of unexpected issues may need to be addressed. Some of these issues can be related to the circuit design and others are related to the PCB fabrication process.

The move from FR-4 to a high-frequency material is often necessitated by the loss performance of a circuit. The acceptable amount of insertion loss for a particular circuit can vary greatly from one application to another. The material’s loss is categorized by dissipation factor (Df) and it is rather subjective for what is considered low-loss, mid-loss and high-loss material. From my experience, I would categorize materials by Df as the following:  high-loss material has a Df of 0.015 or greater and that is a pretty common value for many FR-4 materials. There are some high-performance FR-4 materials which have better loss, and some of these have Df value of 0.010. I would consider these materials to be categorized mid-loss materials. Low-loss materials typically have Df values of 0.004 or less and extremely low-loss materials have a Df of 0.002 or less.  There is a grey area between low-loss and mid-loss material where the range of Df is from about 0.004 to 0.010.

Besides the differences in loss, significant design changes may be necessary when switching to high-frequency materials. These changes are mostly due to FR-4 materials having a dielectric constant (Dk) value of about 4.2 to 4.5, while many high-frequency laminates have considerably lower Dk values. The change in Dk can cause impedance differences, so design changes to the conductor routing are often necessary. Additionally, the high-frequency laminate may not have the same substrate thickness offerings as the FR-4 materials and the thickness difference can require design changes in order to maintain the desired impedance and other critical electrical properties.

Another issue is related to PCB fabrication processing. Some high-frequency materials are PTFE-based, and those types of materials can require very different processing parameters as compared to FR-4 materials.

To read this entire column, which appeared in the August 2017 issue of The PCB Design Magazine, click here.