Issue link: https://iconnect007.uberflip.com/i/362552
40 The PCB Design Magazine • August 2014 TCDk properties and as a general statement, a good value would be 50 ppm/°C or less and this value would be an absolute value in the mathe- matical sense. Of course, the closer the material is to zero for TCDk the better. The comment about a field failure attracting attention to TCDk is valid, because unaware engineers may fine- tune the application in a con- trolled environment, such as a lab, only to find that it will change electrical performance as the unit goes through nor- mal temperature cycling in the field. TCDk describes how much the material will change dielectric constant with a change in temperature; the TCDk value varies among the different types of high- frequency PCB materials. In general, PTFE-based laminates have excellent electrical per- formance for loss but suffer from high (poor) TCDk. This is one reason why some PTFE laminates are filled. With the proper filler, the TCDk can be adjusted to a good, low level. For example, a nearly pure PTFE laminate may have a TCDk value of 200 ppm/°C whereas a PTFE laminate with ceramic filler can have a TCDk of 20 ppm/°C. Satellite applications are sensitive to TCDk. The change in temperature which the PCB is subjected too can be extreme and if the dielec- tric constant (Dk) changes significantly, the PCB will not operate in the manner for which it was designed. A common material used in satellite applications for its consistent and low TCDk is the Rogers TMM laminate. However, as mentioned earlier, in the formulation process of a material, there are many tradeoffs. The TMM materials are excellent for TCDk properties, but require extra attention during the PCB fabrica- tion process. A good understanding of the PCB manufacturing properties helps fabricators ad- just their processing conditions, allowing them to manufacture a robust circuit using these ma- terials. Another material property to consider which is related to change in operating environment is TCDf, the temperature coefficient of dissipation factor. This is the property of a laminate where the Df changes with a change in temperature. In many applications, the TCDk is much more im- portant than TCDf but there are some designs which are more sensitive to TCDf. Typically, TCDf is im- portant in PCB configurations where it is critical that loss performance remains con- sistent. As with TCDk, the TCDf property is often very different when comparing one type of high-frequency circuit material to another. In general, the materi- als with high Df values typi- cally have a higher TCDf. As another real-life example, a nearly pure PTFE laminate has a very low Df of about 0.0009 and the TCDf is also low at 20 ppm/°C, as com- pared to a ceramic filled hy- drocarbon laminate where the Df is about 0.004 and has a TCDf of about 50 ppm/°C. Even though 50 ppm/°C is not considered bad, the difference be- tween these material TCDf properties is more than double. If a designer is not familiar with material attributes, ensuring consistent electrical per- formance of high-frequency PCBs can be more complicated than expected. Due to this con- cern, it is always recommended that the design- er consult their high-frequency materials man- ufacturer for advice on proper materials when considering a new design. PCBDESIGN The TMM materials are excellent for TCDk properties, but require extra attention during the PCB fabrication process. A good under- standing of the PCB manufacturing properties helps fabricators adjust their processing conditions, allowing them to manufacture a robust circuit using these materials. " " lightning speed laminates John Coonrod is a market de- velopment engineer for Rogers Corporation, Advanced Circuit Materials Division. To read past columns, or to reach Coonrod, click here. ENVIRoNMENTAL EFFECTS oN HIGH-FREQUENCy MATERIAL PRoPERTIES continues