Controlled Impedance

Characteristic Impedance

The characteristic impedance of a transmission line is dependent on the relationship of the conductor width, conductor thickness, dielectric thickness between conductor and ground-power reference planes, and the dielectric constant of the dielectric medium. It is recommended that the designer contacts GPV Printca A/S to discuss impedance needs during the initial design phase. This will enable mutual understanding of requirements and impact of material characteristics, such as specific Dk´s and manufacturing processes, on needed impedance targets and tolerances.

The actual impedance may have to be tested via a small prototype build. This is often necessary when tight impedance tolerances are required, or in the case of small line widths and dielectric thicknesses, which are more sensitive to variations. A tolerance swing due to etching variations will be more significant for a 0.127 mm line width than for a 0.254 mm line, for example.

The recommended impedance tolerance is +/- 10 %. A lesser tolerance is often achievable, especially with fully embedded Microstrip and Stripline structures.

Surface Microstrip

The microstrip line is a popular transmission line structure for high speed digital circuits. The Surface Microstrip location on the external layer is subject to potentially greater impedance variables. This is due to the additional copper electro-plating it receives, resulting in increased line thickness and line width tolerances.

For critical applications, the Microstrip line can be embedded in dielectric material. The impedance can be calculated from the Surface Microstrip formula. Then for each 0.001 inch below the surface, subtract 1% of the impedance calculated. This derating factor provides good results for embedding up to approximately 0.381 mm. A thicker embedding has little additional effect.

Triplate stripline

The stripline is embedded in dielectric material and is sandwiched between two reference planes. This configuration significantly reduces cross talk effect. This structure is most suitable for improving impedance tolerances.

Impedance Test Pattern
Actual Impedance will be measured via the TDR (Time Domain Reflectometry) method. Test coupon will be generated by GPV.