There are two methods for PCB routing: automatic routing and interactive routing. Before automatic routing, interactive routing can be used to pre-route traces with stricter requirements. Input and output traces should avoid adjacent parallel routing to prevent reflection interference. Ground planes should be added for isolation when necessary. Routing on adjacent layers should be perpendicular to each other, as parallel routing can easily cause parasitic coupling.
The success rate of automatic routing depends on a well-designed layout. Routing rules can be predefined, including the number of trace bends, vias, and steps.
Typically, exploratory routing is performed first to quickly connect short traces. This is followed by maze routing, which optimizes the overall routing paths for all connections. It can break existing traces as needed It attempts to reroute traces to improve overall performance.
1. Power and Ground Line Handling
Even with excellent routing across the entire PCB, interference caused by inadequate power and ground line considerations can degrade product performance and sometimes even affect yield rates. Therefore, power and ground line routing must be treated with utmost care to minimize noise interference and ensure product quality.
Every electronics designer understands the causes of noise between power and ground lines. Here, we focus solely on noise reduction and suppression methods:
(1) It is well-known that decoupling capacitors should be added between power and ground lines.
(2) Maximize the width of power and ground traces, ideally making ground traces wider than power traces. The priority order is: ground traces > power traces > signal traces. Typical signal trace widths range from 0.2 to 0.3 mm, with minimum widths reaching 0.05 to 0.07 mm. Power traces should be 1.2 to 2.5 mm wide.
For digital circuit PCBs, a wide ground plane can be formed using wide conductors to create a ground network (analog circuit grounds cannot be used this way).
(3) Use a large copper ground plane. Connect unused areas on the printed circuit board to ground as the ground plane. Alternatively, use a multilayer board where the power supply and ground each occupy a separate layer.
2. Grounding Considerations for Mixed-Signal Circuits
Many modern PCBs no longer contain purely digital or analog circuits but instead integrate both types. Therefore, routing must account for mutual interference, particularly noise on ground planes.
Digital circuits operate at higher frequencies, while analog circuits exhibit greater sensitivity. For signal lines, high-frequency traces should be routed as far as possible from sensitive analog components. Regarding ground planes, the entire PCB presents only one external connection point. Thus, the shared grounding of digital and analog components must be managed internally on the PCB. Within the board, digital and analog grounds are physically separated and not interconnected, except at the PCB's external interface points (e.g., connectors). Digital and analog grounds share a single connection point. Some systems may opt for isolated grounds, determined by system design.
3. Routing Signal Lines on Power (Ground) Planes
When routing on multilayer PCBs, limited unused space on signal planes means adding extra layers wastes material, increases manufacturing complexity, and raises costs. To resolve this, consider routing on power (ground) planes. Prioritize power planes first, then ground planes, as maintaining ground plane integrity is optimal.
4. Handling Connection Legs in Large-Area Conductors
In large-area grounding (electrical) applications, component legs are commonly connected to them. The treatment of these connection legs requires comprehensive consideration. From an electrical performance perspective, full contact between the component leg pads and the copper surface is ideal. However, this approach introduces potential drawbacks for component soldering and assembly:
① Soldering requires high-power heaters.
② It easily leads to cold solder joints.
Therefore, to balance electrical performance and manufacturing requirements, cross-shaped pads are employed—referred to as heat shields or thermal pads. This design significantly reduces the likelihood of cold solder joints caused by excessive heat dispersion during soldering. The same approach applies to connecting pins on the ground plane of multilayer boards.
5. The Role of Network Systems in Routing
In many CAD systems, routing is determined by network systems. Excessively dense grids increase routing paths but result in overly small step sizes and excessive data volume in the drawing field. This inevitably demands greater storage capacity from equipment and significantly impacts the processing speed of host computers and electronic products. Some paths become redundant, such as those occupied by component pad footprints, mounting holes, or mounting features. Conversely, overly sparse grids drastically reduce routing paths, severely affecting routing success rates. Therefore, a reasonably dense grid system is essential to support routing.
The standard distance between pins of a component is 0.1 inches (2.54 mm). Consequently, the grid system's base is typically set to 0.1 inches (2.54 mm) or an integer multiple less than 0.1 inches, such as 0.05 inches, 0.025 inches, or 0.02 inches.
6 Design Rule Checking (DRC)
After completing the routing design, it is essential to thoroughly verify whether the routing complies with the rules established by the designer. Simultaneously, confirm that these rules meet the requirements of the printed circuit board manufacturing process. Common inspection aspects include:
① Whether the spacing between traces, between traces and component pads, between traces and vias, between component pads and vias, and between vias is reasonable and meets production requirements.
② Whether power and ground traces have adequate width, and whether power and ground traces are tightly coupled (low impedance). Identify areas on the PCB where ground traces can be widened.
③ Whether optimal measures have been taken for critical signal traces, such as minimizing length, adding shielding traces, and clearly separating input and output traces.
④ Do analog and digital circuit sections have independent ground planes?
⑤ Do graphics added post-layout (e.g., icons, annotations) risk causing signal shorts?
⑥ Have suboptimal trace geometries been modified?
⑦ Are process lines added to the PCB? Does the solder mask meet manufacturing requirements? Are the solder mask dimensions appropriate? Do character markings overlap component pads, potentially compromising assembly quality?
⑧ Are the outer edges of power/ground planes on multilayer boards recessed? Exposed copper foil on these planes risks short circuits.
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