The basics

Electrical Impedance: A measure of opposition to time-varying electric current in an electric circuit

Simplified: To slow down an electrical circuit

Why: As flex circuit design and components become more complicated, smaller and faster, it becomes necessary to slow certain circuits down, allowing specific functions of components to perform before others. The increase in processor clock speed and component switching speed on modern flex circuit means that the interconnecting paths (traces) can no longer be regarded as simple conductors.

Electrical impedance control

Design factors affecting flexible circuit impedance control

  • Impedance (Zo) is measured in Ohms (Ω) and should not be confused with resistance – you cannot measure impedance with an ohmmeter
  • Target impedance is usually between 25Ω, and 125Ω
  • Typical result of a 75Ω controlled impedance on a 0.007” trace is a signal slowdown of approximately 166ps/in
  • Conductor width increase, impedance decreases
  • Copper thickness (weight) increase, impedance decreases
  • Laminate thickness increase, impedance increases
  • Dielectric constant increase, impedance decreases
  • Inductance increase, impedance increases
  • Capacitance increase, impedance decreases

Physical factors affect flexible circuit impedance control

Physical characteristics of impedance trace:

impedance control - physical characteristics of trace

  • Height of trace (T1)
  • Width of trace at top (W2)
  • Width of trace at bottom (W1)
  • Distance to other copper features (H1)
  • Dielectric constant of PCB material (Er)

Simple impedance models

Simple impedance models

Complicated impedance models

Some PCBs may have multiple impedance requirements! More impedance requirements mean more impedance coupons, which can decrease the amount of usable panel space for PCBs.

Complicated impedance models

Flexible design considerations: impedance and capacitance control

Double-layer 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 groundMicrostrip and impedance and capacitance control
  • Generally used in lower frequency applications less than 500MHz signal speeds; where crosstalk is a concern, should have a reference plane; microstrip 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 groundStripline and impedance and capacitance control
  • 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

Relationships within a controlled impedance microstrip design

Relationships within a controlled impedance microstrip design

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