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Generally speaking, the general PCB design software includes the ability of autorouting, but in reality, no autoroute can completely replace the skills, experience, and flexibility of the PCB layout engineer.
However, you can use the autoroute in the following situations:
a. Once all the components have been placed, you can check your completion rate with the autoroute job, and if it's below 85%, you'll need to adjust your component placement.
b. When wiring, bottlenecks and other critical connection points can come off the cracks and can be identified using the auto-routing feature.
c. When you don't know how to start wiring or get stuck, you can use autoroute as a source of inspiration.
Before you begin laying copper traces, take some time to consult with your manufacturer to see if they have requirements regarding minimum trace width, trace spacing, and the number of PCB layers they can assemble.
By understanding this information in advance, you can set the trace width and spacing values in your design rules, avoiding the need to re-route the entire PCB layout.
The geometry (thickness and width) of the traces ensures that the circuit works properly under all environmental and load conditions. Since PCB traces are used to transmit electrical signals, they must be of a width compatible with the current passing through them.
PCB layout engineers must determine the minimum width of each trace to avoid the risk of the circuit board overheating; this parameter directly affects the routing process as it reduces the available space on the PCB.
If space is not an issue, it is recommended to use traces wider than the minimum value, thereby improving heat management and reliability of the circuit board, with outer layer traces allowing for better heat exchange and possibly having smaller widths.
It's critical to maintain adequate space between PCB traces and pads to avoid short circuits during PCB manufacturing or assembly.
Generally, it's recommended to leave appropriate gaps between each adjacent trace and pad; they must always have enough space around them, with no traces or pads, to avoid the risk of electric shock.
The placement of components determines the success of the PCB design. To correctly place components, you must fully understand their characteristics.
For example, heat-sensitive electrolytic capacitors must be kept away from heat-generating diodes, resistors, and inductors.
Below are some simple guidelines:
a. Pay attention to components with multiple pins, as these components occupy large spaces.
b. Keep component orientations consistent.
c. Consider the function of each component and its relationship with other components before placement.
d. If components have already been procured, it is recommended to print the layout on paper according to the dimensions and check if the components fit.
Traces carrying digital signals, especially high-frequency signals on a PCB, must be kept separate from those carrying analog signals.
Separating the analog and digital signal traces reduces the possibility of mutual interference, thereby enhancing circuit stability and reliability. When analog and digital signals share the same trace, the following issues may arise:
The high-frequency components of the digital signal may interfere with the analog signal, reducing the precision of the analog signal.
Digital signals inherently contain noise, which can affect the analog signal when both share the same line.
Digital signals exhibit a certain delay during transmission, which may cause distortion in the analog signal.
Every PCB needs at least one ground layer, as it provides a common reference point for measuring voltage across all traces.
Conversely, if you choose to route each trace individually to the ground rather than using a ground plane, you get numerous different ground connections, each with its own resistance and voltage drop.
The simplest and most linear solution is to create a solid ground plane, which could be an entire copper area. In multilayer boards, this could even be an entire layer.
Placing a ground layer under signal traces helps to reduce impedance and improve noise immunity. It is recommended to place the power and ground layers on the innermost layers of the board, keeping them symmetrical and centered to prevent PCB warping.
During component placement, after placing all sockets, have you left enough space between other components and all the traces that connect them?
If not, there might be an electric shock risk, and relying solely on the solder mask as an insulator cannot guarantee safety.
When using sockets, remember to leave a ring of space outside the physical dimensions of the mounting holes to protect it from nearby components and traces.
If most traces on one layer follow a particular direction (e.g., horizontal), prioritize the vertical direction for traces on an adjacent layer to reduce crosstalk between tracks.
Alternating trace directions also enhances signal stability. Traces in the same direction may experience signal reflections, attenuation, and distortion due to the interaction of capacitance and inductance between signal lines.
However, alternating trace directions may also increase routing complexity and cost, so it should be weighed and considered in practical design.
To reduce capacitive coupling generated by traces placed above and below large ground planes, ensure that traces allocated for power and analog signals are arranged on dedicated layers.
In addition, capacitive coupling can be avoided or the effects of capacitive coupling can be reduced in the following ways:
Smaller capacitance values reduce the impact of capacitive coupling. Therefore, designing circuits with the smallest possible capacitance can minimize this effect.
Increasing the impedance of relevant signals in the circuit can reduce capacitive coupling. For instance, adding appropriate resistors at the signal input or output reduces the effect of capacitive coupling between the signal source and load.
Differential signal lines can reduce capacitive coupling to some extent. Since differential signal lines consist of two lines, and signals are transmitted through the difference between them, single-line capacitive coupling issues are avoided.
Placing thermal vias can improve the heat dissipation efficiency of the PCB. Heat dissipation holes can introduce airflow into the interior of the PCB and increase the surface area of the PCB, making it easier for heat to dissipate. Additionally, heat dissipation holes can reduce air bubbles on the surface of the PCB and the accumulation of gas during soldering.
Placing solder pads can improve the reliability of the PCB. In the design of solder pads, it is necessary to consider the soldering process and quality, as well as the mechanical strength and stability between components and the PCB. By optimizing the design and layout of solder pads, soldering quality can be improved, soldering defects reduced, thereby enhancing the reliability and performance of the PCB.
The traces related to power and ground signals need to be thicker than those carrying digital or analog signals, allowing them to carry larger currents and be easily identified through simple visual inspection, thereby reducing the possibility of connection errors between signals and power lines.
A common rule is to use a width of 0.040 inches for power and ground traces, and 0.025 inches for all other traces.
If you do not make the power and ground traces wider than the average width, a large amount of heat trying to flow through those narrow spaces may eventually burn the wires and damage the PCB.
Compared to the traces connecting to integrated circuits, you can see that the width of the +5V power trace is larger.
The silkscreen layer that comes with the PCB can be used to mark the information you want to mark. But at the same time, pay attention to the following:
a. Avoid using too much text, which takes up space.
b. Not all available information needs to be written down, for example, resistance values are absolutely not necessary to label.
c. If allowed, the text can be larger, making it clearer when printed.
d. Do not place labels on exposed copper pads to be soldered, as the ink may hinder the flow of solder, resulting in poor joints.
Engineers are generally aware that acute angles and right-angle curves can cause problems at high frequencies, leading to discontinuities and, consequently, damaging signal integrity by increasing crosstalk, radiation, and reflection.
When traces run through the entire PCB and around components, the optimal angle is 45°.
