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Lightning Speed Laminates: PCB Materials that Empower Signal Integrity

Signal integrity is a broad study of issues related to electrical performance. It can be associated with RF applications; however, it is more commonly related to high-speed digital concerns. Several attributes associated with circuit materials can contribute to good signal integrity.

Impedance control for PCBs is typically a concern when considering signal integrity. The factors impacting impedance control are circuit thickness, conductor width and spacing, copper thickness and the dielectric constant (Dk) of the material. There are interactions between these variables, but one point to consider is that the thickness of the circuit has much to do with how sensitive the other factors are to affecting impedance changes. A simple example is to consider a double-sided microstrip 50 ohm transmission line at two different thicknesses. Using a circuit with 10 mil thick material and a Dk of about 3.9, a conductor width change of 1 mil will cause about a 2.8% change in impedance. For a thinner circuit using the same material 4 mils thick, a conductor width change of 1 mil results in an impedance change of 6.8%.  

Also considering the same example, the Dk control of the material is less sensitive to impedance change regardless of the circuit thickness. A difference of 0.1 for the Dk in the previous example will yield a difference in impedance of 1.1% for the 10 mil circuit and also 1.1% for the 4 mil circuit.Read the full column here.Editor's Note: This column originally appeared in the August 2013 issue of The PCB Design Magazine.