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Lightning Speed Laminates: High-Frequency Laminates for Hybrid Multilayer PCBs

A hybrid multilayer is a PCB construction that uses dissimilar materials. Reasons for using dissimilar materials include: Improving reliability, reducing cost, optimizing electrical performance, and improving manufacturability. For the past several years, hybrid multilayer PCB construction has flourished in the arena of high-frequency RF applications.

Multilayer reliability concerns are often related to the coefficient of thermal expansion (CTE) of the circuit material. Some laminates that display excellent electrical performance at high frequencies also have high CTE values. These laminates are often non-filled PTFE-based substrates that have a very low dissipation factor (tangent delta), low dielectric constant, and very good electrical performance at microwave and millimeter-wave frequencies. However, the high CTE of these laminates can cause problems when the PCB undergoes thermal cycling, such as soldering.

A high-CTE laminate grows at a different rate than the copper during elevated thermal exposure, and this difference can cause delamination of the copper-to-substrate interfaces. Also, when a PTFE-based circuit reaches elevated temperatures, it will expand and put stress on the plated through-hole (PTH) vias, which may in turn cause them to fracture. It is typically desired to have a substrate with a CTE similar to that of copper, which is approximately 17 ppm/°C. But some PTFE substrates have a CTE in the range of 200 ppm/°C or more.

Nowadays, most multilayer constructions perform multiple electrical functions. With some multilayer circuits, only a few copper layers are considered electrically critical for high-frequency performance, while the rest are not critical. These circuits could use PTFE-based laminates for high-performance layers, and a low-CTE material in the remainder of the board. Combining the high-CTE and low-CTE materials will yield a hybrid multilayer with a composite CTE that can be acceptable for thermal reliability and good electrical performance.Read the full column here.Editor's Note: This column originally appeared in the September 2013 issue of The PCB Design Magazine.