Microstrip PCB Design: Controlling Impedance
How to design high-speed traces for RF and digital signals.
As signal speeds increase, a PCB trace can no longer be treated as a simple wire. It becomes a transmission line. The microstrip is the most common form of controlled-impedance trace because it is easy to manufacture and test. Understanding how the geometry of your PCB affects impedance is key to avoiding signal integrity issues.
The Anatomy of a Microstrip
A microstrip consists of a trace of width 'W' on top of a dielectric substrate of thickness 'h,' with a continuous ground plane underneath. The magnetic and electric fields travel partly through the dielectric and partly through the air above the trace. This 'mixed' medium is why we use an 'Effective Dielectric Constant' (εeff) in our calculations.
The Geometry-Impedance Relationship
Impedance is primarily controlled by the ratio of W to h. If you make the trace wider (W increases), the capacitance to ground increases, which lowers the characteristic impedance (Z0). If you use a thicker substrate (h increases), the capacitance decreases, raising the impedance. The dielectric constant (εr) of the material also plays a role; higher εr lowers Z0.
Material Selection: FR-4 vs. High-Frequency Laminates
Standard FR-4 is fine for signals up to a few GHz, but its dielectric constant can vary by 10% between manufacturers, and it has high 'loss tangent' (it absorbs signal energy). For high-performance RF designs, materials like Rogers or Teflon (PTFE) are used because they offer much tighter tolerance on εr and significantly lower losses at high frequencies.
Design Best Practices
Avoid 90-degree bends; use 45-degree miters or smooth curves to prevent impedance discontinuities. Keep the ground plane continuous under the trace—breaks in the ground plane cause massive EMI and signal reflections. Use our Microstrip calculator to synthesize the exact trace width needed for your specific stackup.
FAQ
Why is 50 ohms the standard for PCB traces?
While 50 ohms is a historical standard for RF, it also works well for PCBs because it results in trace widths that are easy to manufacture with standard processes on common substrate thicknesses like 1.6mm.
Does solder mask affect impedance?
Yes. Solder mask has a higher dielectric constant than air, so it slightly increases the effective dielectric constant and lowers the impedance, typically by 1-3 ohms. High-precision designs must account for this.
What is the difference between Microstrip and Stripline?
A microstrip is on the surface (exposed to air), while a stripline is sandwiched between two ground planes inside the PCB. Striplines offer better EMI shielding but are harder to calculate and manufacture.