The Challenges of Right-Angle PCB Traces
The 90° trace angle of PCB traces causes the outside corner of a trace to be more narrowly etched than the standard trace width.
The three aspects of right-angle PCB traces—parasitic capacitance effect, signal reflections, and EMI—affect high-frequency signal transmission in circuits.
If right-angle PCB traces carry electromagnetic waves in the forward and backward direction, it causes signal reflections and standing wave resonance.
Right-angle PCB traces pose challenges in circuits handling high-frequency signals
As a designer, you might have been taught to avoid routing right-angle PCB traces. In some circuit layouts, right-angle PCB traces pose challenges for engineers and manufacturers alike. In this article, we will take a look at right-angle PCB traces, what makes them so challenging to work with, and their effects on circuit board performance.
Right-Angle PCB Traces and Circuit Performance
The skill of a PCB designer can often be judged just by seeing how they choose to place components and route their board. From experience, a skilled PCB designer understands how to route the board to ensure improved performance.
The performance of a PCB is directly influenced by the quality of the trace. Right-angle PCB traces impact signal transmission. In right-angle PCBs, the trace takes sharp right-angle turns. The 90° trace angle causes the outside corner of the trace to be more narrowly etched than the standard trace width. In the worst-case scenarios, right-angle PCB traces fail to be fully etched and may form shorts in the circuit board.
Another consequence of right-angle PCB traces is impedance discontinuity. Right-angle PCB traces are associated with trace width changes. A change in the trace width introduces impedance discontinuity. The chances of strong electromigration cannot be ruled out in right-angle PCB traces. High current density and electric field density in these traces produce electromigration, which requires large creepage and clearance distances in high voltage designs. Right-angle PCB traces affect the signal integrity of the circuit above certain frequency ranges.
Let’s look at how right-angle PCB traces influence signal transmission.
Right-Angle PCB Traces and Signal Integrity
Signal integrity can be affected by right-angle PCB traces. The three aspects of right-angle PCB traces–parasitic capacitance effect, signal reflections, and EMI–are not bad enough to affect the signal transmission of the circuit board in normal frequency ranges up to GHz. However, right-angle PCB traces become troublesome when the frequency of the signals is increased further.
Parasitic Capacitance Effect
The right-angled corner of a PCB trace produces an effect equivalent to that of a transmission line connected with a capacitive load. The parasitic capacitance offered by a right-angle PCB slows down the rise time of the signal in the transmission line. The parasitic capacitance effect is not that significant with normal trace widths. However, the parasitic capacitance effect can be intensified by factors such as trace width, the dielectric constant of the medium, or the characteristic impedance of the transmission line.
The corners of a right-angled PCB experience high electric field density compared to other trace angles and geometry. There is impedance discontinuity in right-angle PCB traces, which causes signal reflections as well. With an increase in the line width of right-angle PCB traces, the impedance decreases and signal reflections increase. Typically, right-angle PCB traces are responsible for only 8% to 19% of impedance variations in circuit boards.
Electromagnetic Interference (EMI)
As mentioned above, there is an impedance mismatch between dielectric and right-angle PCB traces. If right-angle PCB traces carry electromagnetic waves in the forward and backward direction, this causes signal reflections and standing wave resonance. In circuits where the standing wave resonance of a right-angle PCB trace gets excited, the electromagnetic interference becomes severe.
Right-Angle PCB Traces and Acid Traps
Acid traps are often discussed when the circuit layout includes acute-angle and right-angle corners. During the PCB etching process, these corner angles accumulate acid and hold onto it for a duration greater than what the design requires. This causes connection issues, trace width variations, trace breaks, and excessive surface roughness variations on the traces.
Right-angle PCB traces can cause challenges for PCB manufacturers who cut corners instead of using newer etchant solutions. For engineers, right-angle PCB traces are of concern only when the circuit board has to handle the high voltage of high-frequency signals at hundreds of GHz.
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