Printed Circuit Design & Fab - March 2009 - (Page 38) LamInate Designing with HIgH FReqUenCY MAteRIALs Understanding the primary and secondary performance requirements in a high frequency application can help designers determine the most suitable materials to use. by JohN CooNRoD Specialty high frequency circuit materials have been used in the PCB industry for decades. There are several attributes of these materials that are very unique when compared to the more traditional FR-4 materials. Understanding these attributes can help the OEM, designer and fabricator optimize the application and maximize the potential for improved electrical performance on the products that are being developed. There are additional, nonelectrical advantages to these materials as well. In order to realize the full potential that these materials have to offer, it is important to understand the specific application and design constraints that need to be addressed. Most high frequency applications will place multiple demands on the PCB material, and some of the requirements may be contradictory. For a specific application, one requirement is often paramount, while the others are secondary. Understanding the primary design considerations and specific requirements for the high frequency materials will allow the designer to choose the optimum material for the application. Several example applications will demonstrate how the various attributes of the product need to be considered in order to determine the optimum material to be used. The first application example is a small filter circuit using high frequency materials that will be soldered to a larger FR-4 circuit board, as shown in fIGure 1. The completed assembly will be housed inside a sealed enclosure and will operate in an outdoor environment that varies greatly by the seasons. Added to this is the specific heat generated by the equipment. Temperatures inside the enclosure could vary from –25 °C to +70° C. Filter circuits typically need a material with very consistent dielectric constant (Dk). This means consistent Dk within a sheet of circuit material, as well as from lot-to-lot. This is required so the filters being attached to the FR-4 circuits will have the same performance, and minimal part-by-part tuning will be necessary. This filter will be a double-sided plated through-hole (PTH) circuit, with 38 the ground plane on the bottom and the signal plane on the top, coupled to the FR-4 circuit board using a microstrip edge coupler. As previously mentioned, most high frequency applications have more than one demand on these circuit materials. In this case, the consistent Dk is paramount, but there are other demands that also need to be considered as secondary. All circuit materials have some amount of growth or shrinkage due to heating or cooling of the material, due to the coefficient of thermal expansion (CTE). In this case, the CTE may not appear to be important, but it can be critical. Once the filter is soldered onto the FR-4 circuit board, the solder joints become a rigid bridge between the FR-4 circuit and the high frequency filter. When the unit experiences a change from 0° C to +30° C (as an example), the FR-4 and the filter circuit will expand. If the CTE is significantly different between the FR-4 and the filter material, stresses can develop at the solder joints that connect the circuit board to the filter, and this may become an issue over time. In most cases, fIGure 1. Example of a PCB with a filter soldered to it. MARCH 2009 PRINTED CIRCUIT DESIGN & FAB
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