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56 The PCB Design Magazine • February 2014 Q: Can high-frequency circuit materials be mixed with FR-4 materials for hybrid multilay- er PCBs? A: Yes, this is often done, and there are fewer compatibility issues than one may assume. A multi- layer hybrid PCB using many layers of FR-4 and a few layers of high-frequency circuit ma- terials is commonly utilized for several different reasons. In some cases, the reason is cost-related; the more ex- pensive high-frequency lami- nates are used only in the layers where better electrical performance is needed. In other cases, a mix of materials may be used for improved reliability. Some PTFE-based laminates have high coefficient of thermal expansion (CTE), which can be a concern when a circuit is built with many layers of this ma- terial. Hybrid multilayers have been built with very thermally stable FR-4 (with a low CTE) used on non-electrically critical layers to help offset the higher CTE of some PTFE laminates. As a side note, not all PTFE laminates have a high CTE and many with ceramic fillers have extremely good and low CTE values. Another case is when it is necessary for a multilayer PCB to use materials of significantly different dielectric constant. In those cases, dif- ferent high-frequency materials may be mixed together to achieve the goal. With many hybrid builds, the materials are often compatible, how- ever special circuit fabrication processing is nec- essary. The material suppliers and circuit fabri- cators should always be involved when hybrid builds are required. Q: Are high-frequency circuit materials more difficult to fabricate than standard FR-4 materials? A: Like so many questions and answers in the engineering world, the answer to this ques- tion is "it depends." There are several different types of materials used for high-frequency ap- plications. The two most common circuit ma- terials used are PTFE-based (such as DuPont's Teflon) and filled hydrocarbon. Both of these categories have subcat- egories and the following gives some distinction as it relates to circuit fabrication. PTFE materials that are un- filled or nearly pure PTFE are typically more problematic for the PCB fabricator, because it's a soft material with a high CTE, and there are dimension- al stability issues and special plated through-hole (PTH) processing challenges. PTFE laminates that are reinforced with woven glass negate many of the problems with softness, handling, and dimensional stability. When a laminate is a filled PTFE substrate, the CTE can be significantly improved, and the PTH preparation process is more forgiving. For ease of fabrication, the rec - ommended material is a woven glass-reinforced ceramic-filled PTFE laminate. This laminate of- fers extremely good electrical performance and can be relatively friendly for PCB fabrication. The different versions of filled hydrocarbon laminates can vary quite a bit, but in general they can be processed in the same processes as a FR-4 laminate. However, different parameters will be necessary for these processes, but usually the same equipment can be used. A few differ- ences worth mentioning include the need for different speeds/feeds at drill, potentially less drill life, different parameters for PTH prepara- tion using plasma or permanganate, and lami- nation parameters which are often very differ- ent from FR-4 prepreg. Q: Why does the high-frequency industry need laminates with so many different dielec- tric constant options? A: The need for different dielectric constant relates more to RF applications than digital ap- lightning speed laminates With many hybrid builds, the materials are often compatible, however special circuit fabrication processing is necessary. The material suppliers and circuit fabricators should always be involved when hybrid builds are required. " " FAQ: MICRoWAVE PCB MATERIALS continues