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Impedance

When designing pcb:s for wave impedance, the following must be known:

1. Materials with different thicknesses have different dielectric constants. The constants for materials we use can be seen here.
2. The operating frequency of the interface. The relevant dielectric constant can be found from the previously linked data sheet.
3. The nominal copper thickness of inner layers decrease in our process by 3 µ. E.g., the copper thickness in case of an 18 µ folium inner layer will be 15 µ after production. Meaning that when calculating wave impedance, 15 µ must be entered into the program.
This value depends on the processes used by the producer and may differ from our 3 µ.
4. The copper thicknesses of outer layers increase, due to galvanic copper plating, by at least 35 µ. Meaning that both base folium and galvanic copper plating must be taken into consideration (12 µ + 35 µ =48 µ).
5. The topography or uniformity of the copper area of the outer layers. Uneven topography means that copper thickness on tracks in a sparser copper area will be up to two times bigger than in a filled area.
6. The declension of tracks during the etching process. The width of a track decreases, at minimum, by the thickness of the base folium.
7. The availability of necessary inner layers and prepegs at the supplier of the printing circuit boards. It is reasonable to determine, during the design stage, which materials are available at the warehouse of the selected supplier and take this into consideration when preparing the design. Our selection of materials can be viewed here. In case your required build up and the build up offered by the supplier do not match, recalculate the wave impedance using the supplier’s build up!
8. Always add to the files a specification that would show the type of wave impedance (single, diff pair); impedance value in ohms and tolerance; values for signal and support layer (signal, ref); and track width and gap (D-code would also be helpful). One possible specification has been given below:

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