4 Replies Latest reply on Jun 3, 2016 9:28 AM by dave.barber

    Stack up issue

    sandeep.kumar

      Hi Everyone,

       

      Can some one tell me regarding the differences between folllowing while editing stack up:

       

      Thickness(um), Test Width(um),

      Z0 Ohm & Target Z0 ohm.

       

      Kindly explain the difference. how are they different from each other?

       

      Thanks

        • 1. Re: Stack up issue
          milostnik

          Hi Sandeep,

           

          This is related to impedance calculations.

           

          First you have to understand that the best place to calculate impedances is with a dedicated tab with a trace segment, since the stackup editor is only the first aproximation of impedance calculation.

           

          IF you are in the Basic tab, then thickness is the amount of copper or prepreg or core between the layers. The test width is an imaginary trace of this dimension used to calculate the Z0 ohm impedance of this imaginary trace.

          In the Z0 planing tab, If you are planing for single traces, you can type the Target Z0 impedance or this imaginary trace and let the tool calculate for you the needed width.

           

          The best and more precise way to calculate the correct traces is to make a simple schematic, and put three trace segments on it.

          The first will be used for calculations on single ended traces. Then couple the second and third for a diff pair calculation.

          Now using the right mouse select "edit type and value". You can now calculate the correct values and try different traces and insulation to optimize the calculation.

           

          See attached pictures.

           

          Hope this helps.

            Matija

          • 2. Re: Stack up issue
            sandeep.kumar

            Thanks a lot

             

            One more thing for AIR  Dielectric is 1 & for prepeg we will choose according to the material used

            but for copper(conductor) what will i choose?

            • 3. Re: Stack up issue
              milostnik

              Hello Sandeep,

               

              that is easy: You don't.

              In the stackup editor in the Metal tab, you will see that the field for Er for copper is set to <Auto>, if you click on the Calculate Er for metal layer from surrounding dielectrics. The tool will take care of the right value.

               

              Hope this helps

                Matija

              • 4. Re: Stack up issue
                dave.barber

                I find the "Calculate Er for metal layers" a bit peculiar. If I select this then deselect it will just pick up the Er for the nearest layer. It doesn't seem to calculate anything special.

                I've found using the "calculate Er" gives different results to our manufacturers calculations. If I use 1 for the Er value this yields much closer results.

                stackup.jpg

                Even the CES quick guide says: "Since only dielectric constant values greater than 1 affect the layer, to reduce calculation times, set the dielectric constant to 1.0 to omit that material layer from the stackup in the solver. "

                 

                I'm still in a bit of a quandary as to what settings in stack-up editor yeild the most representative results when it comes to manufacture