Defining the stackup early in the design helps constrain the layout and prevent downstream manufacturing issues. Primarily, a trace width based on the finest pitch component will act as an input to the impedance field solver software; this step ensures manufacturability and coordination with the characteristic impedance necessary for different transmission protocols. While most traces will only use a 50/100Ω scheme for single/double-ended lines, many standard formats like USB and HDMI require impedance value deviations from the typical. In any case, promoting signal integrity for the characteristic impedance of transmission lines is a design necessity for optimal board performance.
Defining Characteristic Impedance of Transmission Lines
Every stackup structure will carry a characteristic impedance that defines the voltage drop and any phase shifts between the electric and magnetic waves; it’s valuable to remember that impedance is the vector sum of the real-valued resistance and imaginary-valued reactance. The loop impedance defines the transmission line itself: recall that current always has to flow in a closed loop, meaning the return current propagates antiparallel to the direction of the driven signal on the nearest plane. That’s why the characteristic impedance of a transmission line varies according to the physical dimensions of the trace itself (width, height, gap for differential lines, etc.) and the normal distance to its reference plane.
For this reason, characteristic impedance calculations can vary from simplistic to stunningly complex expressions. Perfectly uniform transmission lines can use the form
where v is the propagating speed of the signal, and CL is the capacitance per unit length. While helpful as a starting point, this format lacks some real-world rigor inherent to transmission line calculations. For PCBs, transmission lines often focus on microstrips and striplines. The former are surface-facing transmission lines (i.e., found on the top or bottom layer), while the latter exists between two reference planes within the PCB stackup. General manufacturing techniques produce trapezoidal traces due to the difference in the time the etchant solution contacts the top and bottom of the copper foil; however, emerging additive manufacturing will allow for rectangular prism traces as the technology matures and scalability increases.
IPC-2141 Characteristic Impedance Approximations for Common Transmission Lines |
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|---|---|---|
| Microstrip |  𝜀r: relative permittivity, h: dielectric height, w: trace height |
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| Stripline |
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| Asymmetrical* stripline |
h1: shorter distance from trace to plane, h2: longer distance from trace to plane |
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*: The stripline’s vertical position between the reference planes is at an offset.
It’s worth noting that even as an industry standard, some designers may find these approximations lacking the necessary accuracy for their design. More robust characteristic equations in various journals and texts can be helpful when transmission line design departs considerably (think varying trace widths, multiple dielectric heights, shapes with non-uniform cross-sectional areas, etc.).
Design Techniques for Consistent Impedance
The reliability requirements of modern PCBs leave little room for design or manufacturing error – for this reason, designs descend from a controlled impedance framework. Modern field solvers are potent simulation tools that can quickly determine the impedance of the geometric shapes used to build transmission lines. The characteristic impedance is analogous to the instantaneous impedance encountered by the signal during transmission, the importance being that a signal needs to “see” the same impedance throughout its passage to prevent reflections that can reduce signal integrity, sap power, and potentially damage components. A few tricks are at a designer’s disposal to ensure impedance remains consistent during transmission:
- Minimize stub lengths – Stubs occur anywhere a transmission line does not terminate, with the classic example being unused vertical layers in a via transition. Consider a four-layer board with a signal originating on the top layer and exiting on the third layer: the copper existing between layers 3 and 4 is extraneous. When the signal travels from the top layer to the third layer, some of the signal diverts into this unterminated portion of the line and, with nowhere else to go, reflects the source. Fabricators can use backdrilling (removing the unused portion of the via barrel post-plating) to prevent this situation; typically, this is only required on high-speed boards or critical signals more susceptible to reflection.
- Termination – To minimize discontinuities between the driver and receiver in a signal transmission path, designers can use a resistor to match the impedance at the driver to the characteristic impedance of the trace. Series and parallel resistor configurations are available, and while both have their pros and cons, a shunt resistor is generally more applicable as it can absorb reflections at the far end of the transmission line. Despite less loss in the series arrangement (only Ohmic loss, as there is no separate current branch from the driver to the transmission line), its reduced ability to manage reflections may preclude it from some designs.
- Routing topology – While finding the most direct path between two points during layout is straightforward, nets spanning three or more pads rise increasingly. Daisy chaining simplifies the transmission path electrically, offering reflectionless (assuming a consistent characteristic impedance) performance as the signal passes through every node on the net sequentially. Nonlinear topologies possess greater redundancy and shorter transmission paths, but splitting the signal among shared transmission lines increases the likelihood of reflection and distortion.
Your Contract Manufacturer’s Characteristics? Reliability and Quality
The characteristic impedance of transmission lines is a superficially simple engineering topic that gradually gives way to significant complexity as transmission line design departs from idealized and typical forms. While most heavy mathematics lifting will go on “behind the scenes” with field solver software, designers should still prioritize their transmission lines to reduce reflections for performance and system stability; an experienced contract manufacturer can aid this process by reviewing stackups, schematics, and board files. At VSE, our engineering team is committed to building electronics for our customers. With over forty years of manufacturing knowledge in bringing life-changing and life-saving products to market, we’re well-versed in DFM.
