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60 The PCB Design Magazine • April 2014 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 materi- als with high-frequency materials, or a mix of different high-frequency materials with differ- ent 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 electri- cal performance. High-frequency circuit ma- terials 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 electri- cally critical and many layers that are not criti- cal. 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 multilay- ers 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 multi- layer, the composite CTE can be acceptable. Some hybrids use materials with very dif- ferent dielectric constants that are used for en- hanced electrical performance. In the case of some couplers or filters, it may be advantageous to use laminates with different dielectric con- stant values. The combination of FR-4 and high-frequen- cy materials is becoming more common since there are few compatibility issues related to us- ing FR-4 and most high-frequency circuits ma- terials. However, there are several circuit fabri- cation issues which need to be understood. The type of high-frequency material used in a hybrid construction can make a big difference in the degree of special processing needed for circuit fabrication. PTFE-based high-frequency materials can be more problematic for circuit fabrication due to special drilling and plated through-hole (PTH) preparation requirements. Hydrocarbon-based laminates can easily be fab- ricated using standard FR-4 circuit board pro- cessing techniques. by John Coonrod rOgers COrPOrATiOn LIGHTNING SPEED LAMINATES column Hybrid High-Frequency Multilayer PCBs

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