Coplanar Waveguide
Understanding Coplanar Waveguide
CPW has become the dominant transmission line technology for MMICs and advanced packaging because it keeps all conductors on the same surface, simplifying fabrication and enabling reliable flip-chip integration.
CPW Characteristics
- Impedance: Determined by the ratio of center conductor width to gap width. 50 ohms with various dimension combinations.
- No vias: Ground is on the same surface as the signal. Shunt connections (stubs, bypass caps) connect directly to the ground plane without via holes.
- Mode control: Air-bridges or bond wires must connect the two ground planes at regular intervals to suppress the slotline (odd) mode.
CPW vs Microstrip
| Parameter | CPW | Microstrip |
|---|---|---|
| Via holes needed | No | Yes |
| Backside ground | Optional | Required |
| Shunt components | Easy | Need via |
| MMIC suitability | Excellent | Good |
Frequently Asked Questions
What is CPW?
CPW is a planar transmission line with center conductor and ground planes on the same surface. No backside ground needed. Ideal for MMICs and flip-chip. Easy shunt connections without vias. Impedance determined by conductor/gap ratio.
Why is CPW preferred for MMICs?
MMICs processed on thin substrates (100-150 um) make microstrip via holes difficult and lossy. CPW keeps everything on one surface: no via holes, compatible with flip-chip mounting, and easy probing for on-wafer measurement.
What are air bridges in CPW?
Air bridges connect the two CPW ground planes over the center conductor to suppress the parasitic slotline mode. Without air bridges, the slotline mode can propagate and cause unexpected resonances and coupling.