Nowadays, when we open the SoC manufacturer'sPCB Layout In Guide, you'll find that the corner angles of high-speed signal traces have become a topic of great interest.
Almost all guides emphasize avoiding right-angle turns and instead encourage using 45-degree angles; some even suggest that curved paths may be preferable.
So, what is the best angle for PCB trace turns? Is 45 degrees best, or should they be curved? And what about 90-degree right-angle turns?
This issue has plagued PCB designers for nearly two decades. In the early 1990s, Intel, a giant in the PC industry, led the development of PCI bus technology.
Since then, we seem to have entered an era of ”high-speed” system design. As electronic design and chip manufacturing technologies continue to advance in accordance with Moore’s Law, IC processes continue to improve, transistor switching speeds continue to increase, and the clock frequencies of various buses continue to rise, signal integrity issues have become a major focus of research and attention.
Why can't we use acute angles?
First, it is important to clarify that sharp corners should not be used in PCB trace routing; the answer to this question is clear. Regardless of whether high-speed signal transmission is adversely affected, sharp corners should be avoided from a PCB manufacturing perspective.
This is because at points where PCB traces intersect to form sharp angles, a problem known as ”acid corners” may occur. During the PCB manufacturing process, these ”acid corners” can cause excessive etching of the traces, leading to trace breaks.
Although we can use tools such as CAM350 to automatically detect potential ”sharp corner” issues and resolve them before PCB manufacturing, many manufacturing engineers do not necessarily understand the layout design; they simply follow a set process to fix problems without fully understanding whether the fixes might cause further signal integrity issues. Therefore, we should try to avoid sharp corners as much as possible during the design process.
The Problem with 90-Degree Corners
For high-frequency, high-speed signal traces, various PCB design guides strongly recommend avoiding 90-degree bends. This is because high-frequency, high-speed signal traces must maintain a consistent characteristic impedance.
However, using a 90-degree bend causes the trace width at the bend to change—typically to 1.414 times the normal trace width—which can result in signal reflection. Furthermore, the additional parasitic capacitance at the bend can also cause signal propagation delays. Therefore, using 90-degree bends is not a wise choice for high-frequency, high-speed signals.
The Impact of Right-Angle Corners
Right-angle turns primarily affect signals in the following three ways:
A corner can be regarded as a capacitive load on a transmission line, which slows down the rise time.
At a 90-degree corner, the trace width is approximately 1.414 times the normal trace width, causing an impedance discontinuity and resulting in signal reflection.
Electromagnetic interference (EMI) generated by right-angle tips: These tips are prone to emitting or receiving electromagnetic waves, thereby generating EMI.
For high-speed digital signals, 90-degree bends generally do not have a significant impact on narrow-width traces (such as 4–5 mil). However, for high-frequency transmission lines with wider trace widths (such as 50Ω impedance and 100-mil trace width), 90-degree bends can severely affect the signal, leading to signal reflections and delay issues. Therefore, avoiding 90-degree bends whenever possible is a general principle in high-speed design.
45-degree bevel
With the exception of RF signals and other signals with special requirements, general traces on a PCB should ideally be routed at a 45-degree angle. It is important to note that when using 45-degree angles, the length of the trace at a corner should be at least 1.5 times the trace width, and the spacing between traces should be at least four times the trace width.
Modern EDA design software (such as Cadence Allegro, Altium Designer, Mentor Graphics, etc.) offers a wide range of routing options that make it easy to set these rules. This helps prevent signal shortcuts and discontinuities.
Considerations for Using Curved Lines
For some high-speed differential signals (such as USB 3.1, USB 3.2, HDMI 2.0, PCIe Gen 4/Gen 5, etc.), the use of curved traces may be worth considering. However, for general high-speed PCB designs, curved traces may not be necessary. This is because a large number of curved traces can make the routing process very complex later on and take up more space.
Overall, PCB design is constantly evolving. As manufacturing processes and data transmission rates improve, rules that were once considered standard may no longer apply. When dealing with high-speed signal transmission and signal integrity, designers need to be flexible and select the most appropriate routing method based on the specific circumstances. In modern PCB design, there are no set-in-stone rules; instead, different routing methods must be applied flexibly based on specific requirements and circumstances.
Frequently Asked Questions
1. What are the problems with sharp corners in PCB trace routing?
Sharp corners can cause ”acid corners” in PCB manufacturing, which can compromise circuit integrity and should be avoided whenever possible.
2. Why are 90-degree bends not suitable for high-speed signal transmission lines?
90-degree bends cause discontinuities in trace width, leading to signal reflections and delay issues; therefore, they are not suitable for high-speed signal transmission.
3. Why are 45-degree bevels commonly used in PCB design?
A 45-degree cut generally preserves signal integrity while preventing signal short-cuts and discontinuities.
4. Are curved lines commonly used in PCB design?
Curved traces are used in certain special cases, such as high-speed differential signals, but they may not be necessary in general.
5. Are there any set-in-stone rules in PCB design?
In modern PCB design, there are no set-in-stone rules; designers must choose the appropriate routing method based on specific circumstances and requirements.
