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What is Automatic Routing? Definition,Types and Process

31 Jan 2026 10:46:45 GMT Tyson From www.hycxpcba.com

dense multilayer PCB generally refers to a complex circuit board that has three or more copper layers, stacked dielectric insulation layers, and features high component mounting density and high interconnection density.

A smaller PCB footprint usually leads to higher component density, thereby enabling more functions per unit area and reducing the need for external wiring.

The compact density of this multi-layer PCB stems from the spatial optimization of the multi-layer segments. Designers achieve a more compact design by routing signal traces to accommodate more circuits within a very small footprint. The density results from the integration of the following core features:

High-Density Interconnect (HDI) Device Applications: Widely use packages such as BGA and QFN with fine-pitch pins (e.g., pin pitch < 0.5mm), resulting in a sharp increase in the number of nodes that need to be interconnected per unit area.

Micro-refined traces and pitches: Thanks to advanced manufacturing processes, the line width/line spacing can reach 3-4 mils (approximately 75-100 microns) or even smaller, providing a physical foundation for wiring but also imposing stringent requirements on rule setting.

Complex interlayer interconnection structure: extensive use of blind vias, buried vias, and microvias to achieve three-dimensional spatial connections between different signal layers, which is the key to freeing up routing space and achieving high density.

Multilayered Power Distribution Network (PDN): Through dedicated power and ground planes, it provides stable power supply to numerous components and controls signal return paths, which is crucial for Signal Integrity (SI) and Electromagnetic Compatibility (EMC).

Multilayer circuits typically appear in high-end communication, medical devices, automotive electronics, and other fields, where these devices benefit from a smaller PCB footprint. Some advanced communication boards, especially those related to telecommunications, networking, or radar modules, may reach 20+ PCB layers. However, this also poses a core challenge to routing: meeting all electrical performance, signal integrity, and manufacturability requirements within extremely limited space.

What is PCB Automatic Routing (Autorouting)?

PCB Automatic routing is more than a wiring tool; it is a process for post-processing the layout of a PCB, which is governed by algorithms driven by software. The auto router independently determines and establishes the geometric layout of traces and vias according to predetermined physical and electrical constraints, which are generated from the imported netlist obtained from the schematic.

In other words, the process of auto routing is software-based, determining the physical layout of signal paths and vias for multi-layer printed circuit boards using the netlist description of the circuit. The success of this process relies on the completeness and accuracy of the rules that define the constraints. Constraints generally follow predefined formats that form the foundation for the process of autorouting, including:

Physical Rules: Trace width, Trace spacing as per IPC standards, and the size and limitations for different types of vias (through hole, blind, and buried vias).
Electrical Appliance Rules:
- Impedance control: Define the desired impedance for high-speed signals (such as DDR and PCIe) and enforce it through trace width, dielectric thickness, and reference planes.
- Timing constraint: Perform equal-length routing or length matching for signal groups that need synchronization, like timing domains and data buses, in order to control the signal delays.
Topology Rules: Specify the wiring order of essential networks to address congestion issues in planning routes and layouts.

The primary aim is to achieve the generation of the routing solution that can fulfill the electrical constraints, optimize signal integrity, and fulfill the Design for Manufacturability(DFM) constraints.

How PCB Automated Routing Works?

PCB automated routing is a multi-stage process of optimization, and its common workflow is as follows:

Preprocessing and Constraint Setup:

Include the optimized component layout, as inefficient component layout is a major cause of routing failure.
Load the set of rules for electrical constraints (trace/space), parameters for differential pairs, impedance targets, and rules for the thickness and diameter of vias.
These rules form the basis of all the following operations performed by the algorithm.

Network Priority Sorting:

The tool distinguishes between net types based on criticality (such as clock nets, differential pairs, high-speed signals) and based on the complexity of the routing (such as BGA escape routing).
High-priority nets get preference in the allocation of routing resources.

Path Exploration and Routing (Core Algorithm Phase):

The routing engine uses a variety of algorithms to search for paths in the routing resource grid of the PCB. The main aim is to find the best path (minimizing a certain cost function, possibly taking into account length, via count, congestion, etc.) that exists between the two points without violating design rules (DRC).
Combination of Hybrid Algorithms: Modern automatic routers use a hybrid approach rather than a pure algorithm. Thus, for example, a graph-based shortest path algorithm (such as A*) may be employed in initial connectivity routing; then a congestion-conscious algorithm may be employed in global routing; finally, push-and-shove or shape-based routing may be employed in trace geometry detail.

Via Strategy and Optimization:

The algorithm automatically adds vias when layer transitions are required. An experienced router takes into account signal return paths, thermal analysis, and planes in preventing overuse of vias, especially in high-speed traces.

Iterative Optimization and Conflict Resolution:

For those nets which do not have a routing path or do not follow auto routing rules, it starts an “unrouting and rerouting” process and then retries until convergence or reaches a limit of iterations.
Post-processing and Validation: After the autorouting process has been completed, the Design Rule Check (DRC) and Layout Versus Schematic (LVS) checks are done.

The crucial simulation closed-loop:

The output of PCB automatic routing cannot be directly used for production. Reflection, crosstalk, and timing must be checked through signal integrity (SI) simulation; the impedance and noise of the power supply network must be verified through power integrity (PI) simulation. Simulation results often need to be fed back to constraint rules or layout for multiple rounds of iterative optimization.

Types of Auto Routing Algorithms Used In PCB

Algorithm Types	Features	Use Cases
Maze Routing	Search the space of routing on each cells to determine if there exists a legal path between two pins in given constraints.	Initial path discovery in PCB & VLSI routing
Graph-Based Shortest Path	Finds minimum-cost paths based on length, vias, and congestion penalties	Subroutine inside autorouters
Rip-Up and Re-Route (R&R)	Removes existing routes and re-routes them with updated costs	Global routing optimization
Negotiated Congestion Routing	Gradually penalizes overused resources until congestion resolves	Global routing in dense designs
Push-and-Shove Routing	Moves existing traces to create space while preserving DRC	Interactive and automatic PCB routing
Shape-Based Routing	Routes using actual trace shapes instead of grid cells	Modern PCB autorouters (fine pitch, HDI)
Topological Routing	Determines relative routing order and topology before geometry	Early-stage routing planning
Hierarchical Routing	Breaks large designs into regions and routes hierarchically	Large multilayer boards
Genetic Algorithms	Evolving routing solutions over generations	Research prototypes
Monte Carlo Tree Search (MCTS)	Explores routing decisions using stochastic sampling	Emerging EDA research

Limitations of PCB Automatic Routing

A poor component layout will render any autorouter ineffective
Optimal component placement may lead to unsolvable routing congestion
Analog, RF networks, power circuits, and power converters that are sensitive to noise, require specific matching or shielding, need to rely on engineers for manual routing or guided routing to ensure signal integrity
Algorithms typically seek “acceptable solutions” rather than mathematically global optimal solutions, which may result in an excessive number of vias or non-ideal routing, increasing effects such as ground reflection and crosstalk, and degrading electrical performance
Overly complex or contradictory design rules may cause the algorithm to stagnate or produce unforeseen routing patterns.

Best Practices When Using Auto Routing

Constraints First, Layout Collaboration: Develop detailed and consistent constraint rules before autorouting, and collaborate with PCB layout for optimization.
Manual pre-routing of critical networks: Manually pre-route clocks, critical differential pairs, sensitive analog signals, etc., lock their paths and layers, and set “main roads” for automatic routing.
Hierarchical and zoned strategy: Plan the main routing directions for each signal layer (e.g., horizontal routing for layer X and vertical routing for layer Y), and use power/ground planes for effective isolation.
Phased and selective routing: First run high-priority networks, check the results, and then route general networks. For extremely complex modules, consider routing by region.
Simulation-driven iterative optimization: Establish a closed-loop process of “routing – simulation – rule/layout adjustment – re-routing”, which is the only reliable path to achieving high-performance design.

Conclusion

Autorouting of dense multi-layer PCBs is a complex technology that integrates strict design rules, efficient search algorithms, and in-depth engineering experience. Understanding its working principles, mastering constraint settings, and clearly recognizing its limitations are the keys to achieving efficient human-machine collaboration. Only by combining the powerful computational capabilities of PCB automatic routing with engineers’ professional judgment in system architecture, signal integrity, and manufacturability can we efficiently and reliably meet the challenges of PCB design for today’s high-performance, high-density electronic products.

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