RF & Microwave PCB
FR-4, based on epoxy resin and glass reinforcement, is the most popular laminate material for printed circuit board industry for a long time. However, PCB industry also use other material for different application. In RF/microwave products, low loss and special controlled dielectric constant material like PTFE (Teflon) was widely used. These material was developed long time ago. Partly because of low volume in production, it is quite expensive in the past. When wireless became popular in consumer products years ago, the demand for low loss material was went high. But the material cost is still high. Even some new developed material were trying to get involved, it seems none of them can reduce the material cost dramatically. How to reduce the PCB cost became an essential problem for the designer. One of the solution is the mixed dielectric design.
Because the low loss material is not needed for all wireless system, it mostly designed-in, from my understanding, for the circuits from antenna to power amplifier. To reduce the PCB cost, designer used multiple PCB and only the front end of the receiver subsystem need high cost low loss material. However, the cost is still high because of multiple PCB’s and the connector between them. Beside, the Teflon PCB is soft and is comparably difficult in assembly due to warpage.
Mixed dielectric PCB cosist of different dielectric material in on multilayer construction. For example, it can be a 6 layer board with layer 1 to 2 made by Teflon and the rest of layers made by FR-4. There is cost anvantage by using one PCB instead of two or more. No connector is needed and the product dimension can be shrunk. The electrical performance can be improved also for no connector used and signal path is closer together. For assembly work, the mixed dielectric multilayer board is much stiff and easier to manufacture.
The manufacture of mixed dielectric MLB has been quite standard for many PCB fabricator. The difficulties in manufacturing is to get an optimum production parameter for two or more different materials. Since most of the design are not balanced in construction, the warpage problem need to be carefully managed. It relates to material choice and design as well as the lamination process. Sometimes, use the exotic material on the bottom layer to balance the design can be a solution. But usually it’s not necessary and only bring a lot of cost.
To meet electrical perormance, the mixed dielectric multilayer is deisgned with blind/buried via quite frequently. In some case, it can bond with metal and used in the power amplifier application.
The application for the mixed dielectric MLB is not only for high frequency products. For high speed digital design, it may also help. For example, if there are some critical transmission lines need to pass long distance in PCB and the Df (dissipation factor) of FR-4 material is too high and cause signal integration problem, use some low loss material in part of the inner layer may have great help. It can save some cost instead of using low loss material in all layers.