When Selecting Low-Loss Microwave Laminates, Loss Tangent Is Only 25% Of The Story

Introduction

The optimal selection of a low-loss microwave laminate requires much more due diligence than comparing reported loss tangent (tand) or dissipation factor (Df) values of various competitive materials. The problem with reported values is that they measured under idealized conditions, which neglects moisture and processing affects on material performance and omits the importance of conductor losses (and the choice of the copper profile). Experienced microwave engineers prefer to understand the insertion loss of the material, especially in a circuit representative of their application. This approach is a much more accurate measurement that takes into consideration all the variables that impact a design (rather than just laminate dielectric loss) and results in designs optimized for high-performance applications.

Although loss tangent is good starting point, determining the highest-performance, lowest-loss microwave laminate requires further analysis. For example, a comparison of all IPC-4103/06 style laminates reveals a range of loss tangent values from 0.0012 to 0.0025. One would expect the 0.0012 laminate to have the lowest insertion loss, but various independent tests have shown that the material with the 0.0025 loss tangent results in the lowest insertion loss in its class, despite having higher dielectric loss than alternative products. Based on our evaluations at Arlon, the performance differences can be attributed to lower moisture absorption, improved fabrication to resist processing chemicals, and lower conductive loss. With this example, utilizing special low-profile copper in combination with a unique resin formulation provides lower conductive loss, while still providing excellent copper bond. Although the same copper could theoretically be used for all laminates (thus equalizing conductive loss), copper bond strength could be sacrificed to the point where fabrication is no longer feasible. Therefore, in practice, this results in a higher conductive loss, which ends up offsetting the lower loss tangent advantage.