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Hybrid High-frequency Multilayer PCBs

A hybrid multilayer PCB uses materials with significantly different critical properties than those associated with a traditional multilayer PCB. A hybrid could use a mix of FR-4 materials with high-frequency materials, or a mix of different high-frequency materials with different dielectric constants. Hybrid construction is becoming more popular as technology evolves, but they bring with them some benefits and challenges which need to be better understood.

The reasons for using a hybrid multilayer PCB typically fall into one of three categories:  Cost, improved reliability, or enhanced electrical performance. High-frequency circuit materials are typically more expensive than FR-4 types. Sometimes, hybrids using a combination of these two different materials are constructed to ease cost issues. Many times, a multilayer PCB will feature some circuit layers that are electrically critical and many layers that are not critical. In this case, the less expensive FR-4 material is used in the non-electrically-critical layers and the more expensive high-frequency material is used in the more critical layers.

Another reason for using hybrid multilayers is to improve reliability when one of these materials has a high CTE. Some high-frequency PTFE materials have high CTE properties and that can be a reliability concern. When an FR-4 material with a low CTE is used in conjunction with the high CTE material to make the multilayer, the composite CTE can be acceptable. 

Some hybrids use materials with very different dielectric constants that are used for enhanced electrical performance. In the case of some couplers or filters, it may be advantageous to use laminates with different dielectric constant values.

The combination of FR-4 and high-frequency materials is becoming more common since there are few compatibility issues related to using FR-4 and most high-frequency circuits materials. However, there are several circuit fabrication issues which need to be understood.

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Editor's Note: This column originally appeared in the April 2014 issue of The PCB Design Magazine.