Electrical Properties

Characteristic Impedance (Surface Microstrip)

Impedance is an electrical property controlling the flow of an electrical signal. Impedance is measured in ohms. Typical designs have characteristic impedance of 20 - 75 ohms end to end and/or 80 - 125 differential. Impedance is a function of:

• Dielectric constant of the insulator
• Width and thickness of the conductor
• Distance (thickness) between signal and shield (ground)
• Amount of adhesive
• Shield configuration
• Signal speed
• Loss tangent

Relationships within a Controlled Impedance Microstrip Design

EMI / RFI Shielding (Typical Shielding Concepts)

Equivalent American Wire Gauge (A.W.G.) Size

Flexible Design Considerations: Impedance and Capacitance Control

Double-sided and multi-layer flexible circuits are ideally suited for providing interconnections that are
specifically designed to provide desired levels of signal integrity.
Construction techniques commonly referred to as “stripline” or “microstrip” are particularly well suited for these applications.   

Microstrip

• Typically a 2 layer flex – 1 signal, 1 ground
• Generally used in lower frequency applications less than 500MHz. Signal speeds, where crosstalk is a concern, should have a reference plane. Microstip construction are flexible enough for dynamic applications.
• Benefits: More flexible, better for power lines, and typically less expensive.

Stripline

• Typically a 3 layer flex – 1 signal and 2 ground
• Generally used in higher frequency applications >500MHz because of crosstalk concerns. Mostly used for less dynamic or static applications.
  
• Benefits: Better signal integrity characteristics (cleaner signals). Cons: More expensive and less flexible.