The PCB Design Magazine

PCBD-Aug2017

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58 The PCB Design Magazine • August 2017 The question of when to transition from FR-4 material to a higher performing material is typically not as straightforward as one may as- sume. 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 fabrica- tion process. The move from FR-4 to a high-frequency material is often necessitated by the loss per- formance 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 fac- tor (Df) and it is rather subjective for what is considered low-loss, mid-loss and high-loss ma- terial. From my experience, I would categorize materials by Df as the following: high-loss ma- terial 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 materi- als 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 switch- ing to high-frequency materials. These changes are mostly due to FR-4 materials having a di- electric 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 nec- essary. Additionally, the high-frequency lami- nate 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. by John Coonrod ROGERS CORPORATION Transitioning From FR-4 to High-Frequency Materials LIGHTNING SPEED LAMINATES

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