In the process of designing a printed circuit board (PCB) stackup and computing transmission line characteristic impedance, obtaining accurate dielectric material properties from reliable sources is critical. A key factor in this regard is the relative permittivity, also known as the dielectric constant (Dk). Dissipation factor (Df), or loss-tangent (tanδ), is another crucial parameter needed for determining dielectric loss.
The values of Dk can vary based on the specific test method used. Some methods provide results from in-plane measurements, where the electric fields are parallel to the test sample. Conversely, other methods derive Dk from out-of-plane measurements, where the electric fields are perpendicular to the test sample. This variability can significantly impact the accuracy of impedance calculations.
Failure to design the stackup properly can lead to issues. If the Dk value used in field solver impedance simulation is incorrect, it can result in time domain reflectometer impedance test failures and potentially lower PCB fabrication yield.
Copper clad laminate (CCL) panels used for PCB construction are a mixture of fiberglass and resin, cladded on one or both sides with copper. CCL suppliers use various test methods to determine Dk and Df, which are eventually published in their construction tables. PCB fabricators and signal integrity (SI) engineers rely on these values to design PCB stackups and perform SI analysis.
There are over a dozen test methods specified in Institute of Printed Circuits specifications. These methods were designed for quality control and do not guarantee accuracy for design applications. CCL suppliers often include disclaimers in their construction tables to this effect.
All glass weave reinforced laminates are anisotropic, meaning dielectric properties will be different along different axis. Several papers in the past have studied laminate anisotropy. Unfortunately, the publication of Dk by laminate suppliers does not include anisotropic properties required for precise impedance prediction and signal integrity modeling.
Attempts have been made to determine anisotropic properties through elaborate HFSS models. However, these techniques are computationally impractical for many designers due to long modeling time and expensive computing processor and memory overhead. Plus, accurate values of Dk of glass and resin are required.
Heuristics are mental shortcuts, rules of thumb or problem solving techniques that help people make decisions and solve problems quickly and efficiently. Heuristics do not guarantee absolute accuracy or completeness. Instead, they are based on past experiences and allow one to use readily obtainable information to come up with solutions when more exact information is not easily available.
Understanding the anisotropic properties and accurate measurement of Dk is vital for reliable PCB design and fabrication. By considering these factors, designers can ensure better performance and higher yield in PCB manufacturing.
This DesignCon 2024 paper introduces a new process utilizing heuristics to determine laminate anisotropy based on Dk/Df construction tables provided by laminate suppliers.
The paper referenced here received the Best Paper Award at DesignCon 2024. To read the entire DesignCon 2024 paper, download the PDF.
