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70 DESIGN007 MAGAZINE I FEBRUARY 2021 thickness of the circuit can be influenced by the PCB fabrication process. e substrate thick- ness issue depends on the circuit construction, and in the cases of a foil lam construction, the thickness control is influenced somewhat by the PCB fabricator. e Dk of the material is relatively fixed by the material supplier, however for some mate- rials there can be a slight alteration of the Dk value due to material interaction with certain PCB processes. With these considerations in mind, the PCB fabricator should do studies with these variables, using a circuit material of choice, to define data which can be used in the material library for impedance modeling. e circuit material library will have data that is tai- lored to the circuit material being processed at that specific PCB fabricator. e circuit material supplier can certainly assist by giving the fabricator more detailed information for the material properties, as the material is tested at the supplier using the material supplier's test method. Many of the high frequency circuit material suppliers use the IPC-TM-650 2.5.5.5c test method to report Dk at 10 GHz. is test method is a raw mate- rial test and is not affected by circuit fabrica- tion. As an example, the IPC test method uses a clamped fixture and there is some amount of entrapped air to be expected. e entrapped air has a very small effect, but air does have a Dk of about 1. e reported Dk with the clamped test method will oen be lower than if the material is tested in circuit form without entrapped air. Another reason for the PCB fabricator to build their database based on their process is the fabricator will oen have a choice of what copper foil to use in a foil lam construction. e bond surface of the copper foil, and specif- ically the roughness of that surface, can impact the "circuit perceived Dk" or as Rogers Corpo- ration calls it, design Dk. Again, with the PCB fabricator having the ability to alter the circuit performance due to the choice of copper foil, they should be defining the Dk values used per their process and with the copper they use. e properties which the circuit material library should use from the material supplier are typically things like CTE, T g , peel strength, moisture absorption, thermal conductivity, etc. ese properties are innate to the mate- rial and they are typically not affected by the PCB fabrication process. Peel strength could be the exception on this list, where certain PCB processes can cause this property value to change. Peel strength (or bond strength) may be a good property value for the fabricator to gather information from the material supplier and then perform their own study and com- pare results with the material supplier. When using data from a material supplier, the PCB fabricator should be very familiar with the test methods used by the supplier. A note- worthy example would be the material prop- erty of thermal conductivity. ermal conduc- tivity can be determined by several different test methods and the results may or may not be appropriate for how the circuit fabricator will apply the data. One test method for thermal conductivity used for laminate evaluations will include the effects of copper. A different test method will not include the effects of the cop- per. Since copper has a very high thermal con- ductivity, the influence of copper can make the results significantly different when comparing these two test methods for an evaluation of the same laminate. Depending on how the PCB fabricator will use the thermal conductivity information, they may or may not want to have the copper effects included. It is always good advice for the PCB fabrica- tor to work closely with the material supplier and that is especially true when the fabricator is developing a material database. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coonrod, click here.