“Right-angle traces should be avoided in PCB design.” I believe most PCB beginners and engineers know this requirement. But is it true that the traces on the PCB cannot go at 90° corners? Is the impact really that terrible? Today, we might as well talk about this topic together.
Can PCB traces be routed at 90° corners?
There is a separate chapter in the PCB design guide manuals of many IC manufacturers that specifically states: “For key high-speed signal lines, the routing cannot be at 90° corners, but must be implemented at two 135° angles. In the PCB Layout software, it is Select 45° routing mode.”
Why is there such a provision? Will 90° corners have a negative impact on signal quality? The answer is yes. However, how serious this impact is depends on the conditions.
The impact of 90° corner wiring on signals
The impact of 90° corner wiring on signals is mainly reflected in three aspects:
First, the corner can be equivalent to a capacitive load on the transmission line, slowing down the rise time;
Second, impedance discontinuity will cause signal reflection;
The third is EMI generated by the right-angle tip.
The reason why 90° corner traces cause signal integrity problems. The reason is that since the width of the copper foil at the right-angle corner of the transmission line is wider than that of the straight line, it will cause a sudden change in the characteristic impedance at the right-angle bend. This does not cause problems for ordinary signal wiring, but for high-speed wiring For transmission lines, it will cause signal reflection. If the reflection is serious, it will cause signal integrity problems.
We know that when designing high-speed circuit boards, we must pay attention to changes in characteristic impedance. Changes in characteristic impedance will cause signal reflection, making the signal quality worse. By routing the traces in a 45° corner pattern instead of making corners in a 90° pattern, there will be less sudden changes in copper foil width at the corners, which can reduce signal reflections caused by changes in wiring width and signals caused by this additional discrete capacitance. Change in rise delay.
In addition, many people think that right-angle tips can easily emit or receive electromagnetic waves and produce EMI. This is also one of the reasons why many people think that right-angle wiring cannot be done. However, many actual test results show that right-angle traces do not produce significantly more EMI than straight lines. Perhaps the current instrument performance and testing level have restricted the accuracy of the test, but at least it illustrates a problem. The radiation from the right-angle wiring is already smaller than the measurement error of the instrument itself.
In general, 90° corner wiring is not as scary as imagined. However, for the sake of safety, it is recommended that you prohibit right angles when doing PCB Layout. Whether it is an ordinary signal line, a high-speed digital signal transmission line or an RF transmission line, including copper-clad copper sheets, they must be formed into obtuse angles.