Printed Circuit Design & Fab - March 2009 - (Page 39) LamInate fIGure 2. Standard FR-4 material. one excursion from 0° C to +30° C is not a problem, but with repeated cycling, the solder joints become stressed and the electrical connection can be compromised. Another critical issue is that of the material’s temperature coefficient of dielectric constant (TC Dk). Each type of circuit material has a property where the Dk value can change with a temperature change, and that is the temperature coefficient of dielectric constant. With certain circuit materials, the Dk may appear to be good and consistent when tested at room temperature; however, as the temperature changes, the Dk can change and cause the filter to perform significantly different than as designed. An additional consideration is the assembly operation, where the filter is soldered to the FR-4 circuit board. Since the filter has plated through-hole vias, the z-axis (thickness) CTE can become a concern during the soldering operation. Depending on the design of the filter and the FR-4 circuit board, the dwell time during soldering may need to be adjusted, particularly if thick materials and heavy copper are used. If the circuit material has a higher z-axis CTE than the filter material, an extended dwell at elevated temperatures can cause damage to the plated through-hole vias and degrade the electrical connection from the signal plane to the ground plane. Lastly, if the filter is intended to operate over a wide range of frequencies, it is important to understand that most circuit materials will have different Dk values at different frequencies. It should also be considered that some materials are more stable than others. In summary for this application example, this filter circuit should use a material that has a tight tolerance for Dk, a CTE that is closely matched to FR4 in the x-y plane, a low TC Dk, a low z-axis CTE and a stable Dk vs. frequency curve. There are many high frequency circuit materials to choose from but finding the right circuit material, which fills all of these requirements, will reduce the available choices. A comparison between the properties of a standard FR-4 circuit material and the material under consideration for the best choice is highly recommended. For this example, the typical values regarding the properties of interest for a standard FR-4 material are shown in fIGure 2. The user will need to determine the fitness for use of the selected materials by conducting appropriate short-term and long-term reliability testing as dictated by the needs of the application. MARCH 2009 fIGure 3. Second example of a simple microstrip, bent around a mandrel. Additionally, if it were critical to reduce the physical size of the filter, a different circuit material with a higher dielectric constant may have been the right choice. A material with a higher dielectric constant will allow the circuit to be reduced in size while still performing the same function. The high frequency circuit material used for this filter application may not actually be used at a relatively high frequency. The material may have been selected due to its tight Dk values and the other properties mentioned. Another item where most high frequency materials have an advantage over more traditional FR-4 circuit materials is minimal moisture absorption. Having low moisture absorption is important for sev- PRINTED CIRCUIT DESIGN & FAB 39 http://www.chemcut.net http://www.chemcut.net
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.