Many PCB fabricators and designers will often 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 information between data sheets may be misleading since there are many variables that go into collecting the data.
One major issue with understanding the information on data sheets and making appropriate 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 correct—answers. Most PCB materials are anisotropic, which means the Dk is not the same on the x-, y- and z-axis, where the z-axis is the thickness 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 evaluates the z-axis of the material for Dk, and get a different answer when testing that same material 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 properties 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 comparison. All materials will have a lower (better) Df at lower frequencies, and if a comparison is done with material A at 2.5 GHz and material B was tested at 10 GHz, then it is not an equal comparison because the lower-frequency test will naturally have a lower Df when compared to testing that same material at a higher frequency.
To read this entire column, which appeared in the June 2017 issue of The PCB Design Magazine, click here.