Detailed Explanation of PCB Wiring And Signal Integrity Issues

PCB wiring for high-speed signals

Nowadays, whenever you open the PCB Layout Guide of the original SoC factory, it will mention the issue of the corner angle of high-speed signal routing. It will be said that high-speed signals should not be routed at right angles, but at a 45 degree angle, and it will be said that using a circular arc is better than a 45 degree corner.

Is this the case? How should the PCB wiring angle be set, and is it better to follow a 45 degree or a circular arc? Is 90 degree right angle wiring feasible?

Everyone began to struggle with the corner angle of PCB wiring, which is something that has happened in the past decade or so. In the early 1990s, Intel, the dominant player in the PC industry, led the customization of PCI bus technology. It seems that starting from the PCI interface, we have entered an era of "high-speed" system design.

Electronic design and chip manufacturing technology are advancing according to Moore's Law. As the process of IC manufacturing continues to improve, the transistor switching speed of ICs is also getting faster, and the clock frequency of various buses is also getting faster. The issue of signal integrity is also constantly attracting research and attention from everyone.

In the early days, PCB cable laying bacteria were relatively simple, simply pulling and smoothing the circuit, clean and beautiful, without worrying about various signal integrity issues. For example, the HP classic HP3456A multimeter circuit board shown in the figure below has a large number of 90 ° angle wiring, almost intentionally straight angles, and the vast majority of areas are not covered with copper.

The upper right corner of the PCB board not only runs at right angles, but also reduces the line width after turning, which can cause signal reflection problems and affect signal integrity.

This article discusses the issue of routing corner angles for high-frequency/high-speed signals. We take a look at the advantages and disadvantages of various routing corner angles, starting from acute angles to right angles, obtuse angles, arcs, and all the way to any angle.

Why cannot PCBs be wired with sharp corners?    

The answer to whether PCB can be wired with sharp angles is negative. Regardless of whether using sharp angle wiring will have a negative impact on high-speed signal transmission lines, from the perspective of PCB DFM alone, it is necessary to avoid the occurrence of sharp angle wiring. This is because at the intersection of PCB wires to form sharp angles, it can cause a problem called acid traps. In the PCB making process, during the etching process of the PCB circuit, excessive corrosion of the PCB circuit can occur at the "acid traps", leading to the problem of virtual breakage of the PCB circuit.

Although we can use CAM 350 for DFF Audit to automatically detect potential issues with "acid traps", avoiding processing bottlenecks during PCB manufacturing. If the PCB factory process personnel detect the presence of acid traps, they will simply stick a piece of copper into this gap.

Many engineering personnel in PCB factories do not actually understand layout. They only repair acid traps from the perspective of PCB engineering processing. However, it is unclear whether this repair can bring further signal integrity issues. Therefore, when laying out, we should try to avoid acid traps from the source as much as possible.

How to avoid sharp corners during wire drawing, which can cause acid trap DFM problems? Modern EDA design software (such as Cadence Allegro, Altium Designer, etc.) comes with comprehensive layout routing options. When we use these auxiliary options flexibly in layout routing, we can greatly avoid the occurrence of "acid trap" phenomenon during layout. The outlet angle of the solder pad is set to avoid the angle between the wire and the solder pad forming sharp angles, as shown in the example below.

By utilizing the Enhanced Pad Entry function of Cadence Allegro, we can minimize the formation of angles between wires and solder pads during wiring, thus avoiding the DFM problem of "acid traps".

Avoid crossing two wires to form sharp angles.

Switching the "toggle" option when flexibly applying Cadence Allegro wiring can avoid sharp angles and angles when pulling out T-shaped branches of wires, and avoid causing "acid traps" DFM problems.

Can PCB layout be wired at 90 °？  

High frequency and high-speed signal transmission lines should avoid running at 90 ° corners, which is a strong requirement in various PCB Design Guides. High frequency and high-speed signal transmission lines need to maintain consistent characteristic impedance, and using 90 ° corner running will change the line width at the transmission line corner. The line width at the 90 ° corner is about 1.414 times the normal line width. Due to the change in line width, it will cause signal reflection. At the same time, the additional parasitic capacitance at the corner will also have a delay effect on signal transmission.

Of course, when the signal propagates along a uniform interconnect line, there will be no reflection or distortion of the transmitted signal. If there is a 90 ° corner on the uniform interconnect line, it will cause a change in the PCB transmission line width at the corner. According to relevant electromagnetic theory calculations, this will definitely have an impact on signal reflection.

The impact of right angle routing on signals is mainly reflected in three aspects:

The corner can be equivalent to a capacitive load on the transmission line, slowing down the rise time

The line width at the 90 ° corner is approximately 1.414 times the normal line width, causing impedance discontinuity and resulting in signal reflection

EMI generated by a right angled tip, where the tip is prone to emitting or receiving electromagnetic waves, resulting in EMI

The parasitic capacitance caused by the right angle of the transmission line can be calculated using the following empirical formula:

C=61W (Er) 1/2/ZO

In the above equation, C refers to the equivalent capacitance at the corner (in pF), W refers to the width of the line (in inches), Er refers to the dielectric constant of the medium, and ZO is the characteristic impedance of the transmission line.

For high-speed digital signals, a 90 ° corner can have a certain impact on the high-speed signal transmission line. For our current high-density high-speed PCBs, the general wiring width is 4-5mil, and the electrical capacity of a 90 ° corner is about 10fF. After calculation, the cumulative delay caused by this capacitance is about 0.25ps. Therefore, a 90 ° corner on a 5-mil wire width will not have a significant impact on the current high-speed digital signal (100 psc rise time).

For high-frequency signal transmission lines, in order to avoid signal damage caused by skin effect, wider signal transmission lines are usually used, such as a 50 Ω impedance and a 100 mil line width. The line width at the 90 ° corner is about 141 mils, and the signal delay caused by parasitic capacitance is about 25 ps. At this time, the 90 ° corner will cause a very serious impact.

At the same time, microwave transmission lines always hope to minimize signal loss. Impedance discontinuity at the 90 ° corner and parasitic capacitance outside can cause phase and amplitude errors in high-frequency signals, input and output mismatches, and possible parasitic coupling, leading to deterioration of circuit performance and affecting the transmission characteristics of PCB circuit signals.

Regarding the 90 ° signal routing, Lao Wu's own opinion is to try to avoid routing at 90 ° as much as possible.

45 degree oblique tangent

   In addition to RF signals and other signals with special requirements, the wiring on our PCB should preferably be run at 45 °. It should be noted that when running at a 45 ° angle with equal length, the length of the wiring at the corner should be at least 1.5 times the line width, and the distance between the winding of equal length lines should be at least 4 times the line width. As high-speed signal lines always transmit along the impedance path, if the spacing between the winding of equal length lines is too close, due to parasitic capacitance between the lines, high-speed signals may take shortcuts, resulting in inaccurate length. The winding rules of modern EDA software can easily set relevant winding rules.

Arc routing with arc  

If it is not explicitly required by technical specifications to use curved lines or RF microwave transmission lines, I personally believe that it is not necessary to use curved lines because of the layout of high-speed and high-density PCBs, a large number of curved lines are very troublesome to repair in the later stage, and a large number of curved lines are also time-consuming. For high-speed differential signals like USB3.1 or HDMI2.0, I believe that circular arcs can still be used.

  Of course, for RF microwave signal transmission lines, it is still preferred to use circular arcs, or even use "45 ° external oblique cutting" lines for routing.

Conclusion

With the development of 4G/5G wireless communication technology and the continuous upgrading of electronic products, the current PCB data interface transmission rate has reached 10Gbps or 25Gbps or above, and the signal transmission rate is constantly moving towards high-speed direction. With the development of high-speed and high-frequency signal transmission, higher requirements are put forward for PCB impedance control and signal integrity.

For digital signals transmitted on PCB boards, many dielectric materials used in the electronics industry, including FR4, have been considered uniform during low-speed and low-frequency transmission.

But when the electronic signal rate on the system bus reaches the Gbps level, this assumption of uniformity no longer holds. At this time, the local variation in the relative dielectric constant of the dielectric layer caused by the gaps between the glass fiber bundles interwoven in the epoxy resin substrate cannot be ignored. The local disturbance of the dielectric constant will cause the delay and characteristic impedance of the line to be spatially correlated, thereby affecting the transmission of high-speed signals.

The test data based on FR4 test substrate shows that due to the relative position difference between microstrip lines and fiberglass bundles, the measured effective dielectric constant of the transmission line fluctuates greatly, with a difference of up to Δ ε r=0.4. Although these spatial disturbances appear small, they will seriously affect the differential transmission line with a data speed of 5-10Gbps.

In some high-speed design projects, in order to address the impact of glass fiber effect on high-speed signals, we can use zig zag routing technology to mitigate the impact of glass fiber effect.

Cadence Allegro PCB Editor 16.6-2015 and subsequent versions have brought support for zig zag wiring mode.

In the Cadence Allegro PCB Editor 16.6-2015 menu, select "Route ->Unsupported Prototype ->Fiber Weave Effect" to open the zig zag routing function.

Twenty years ago, our PCB layout did not need to worry about whether to follow curved lines or the impact of glass fibers on high-speed signals. There is no fixed PCB layout rule, and with the improvement of PCB manufacturing technology and data transmission speed, it is possible that the correct rules now may no longer be applicable in the future.