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PCBD-June2017

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56 The PCB Design Magazine • June 2017 by John Coonrod ROGERS CORPORATION When Comparing Data Sheets, the Devil is in the Details LIGHTNING SPEED LAMINATES Many PCB fabricators and designers will of- ten compare the properties listed on material data sheets to assist with the material selection process. Of course, this is a wise thing to do when choosing materials, however data sheets need to be reviewed carefully. Comparing infor- mation between data sheets may be misleading since there are many variables that go into col- lecting the data. One major issue with understanding the in- formation on data sheets and making appropri- ate comparisons is the test method category. It is possible to test the same piece of material for dielectric constant (Dk) using two different test methods, and arrive at two different—but cor- rect—answers. Most PCB materials are anisotro- pic, which means the Dk is not the same on the x-, y- and z-axis, where the z-axis is the thick- ness of the material. Some test methods will test only the z-axis and other test methods will test the x-y plane. So it is possible to test the same piece of material, using a test method that eval- uates the z-axis of the material for Dk, and get a different answer when testing that same mate- rial using a procedure that tests the x-y plane of the material. Both answers could be correct, but this can cause confusion when comparing data sheets. It is very important to ensure the test methods are the same when comparing similar materials. Electrical properties can be misleading since the Dk and dissipation factor (Df) of all circuit materials is frequency-dependent. These prop- erties will naturally change with a change in frequency . Again, when comparing data sheets from similar materials and ensuring the test methods are the same, the subtle fact that the test frequency is different can alter the com -

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