In the design process, a comprehensive method is employed to enhance the routing efficiency. We provide effective strategies to improve both the layout quality and overall design efficiency. These techniques not only shorten the project development cycle for our clients but also ensure the highest possible quality of the final product.
1. Determining the Number of Layers
The board size and the number of layers should be decided early in the design phase. If your design involves high-density ball grid array (BGA) components, you must account for the minimum number of routing layers required by these devices. The number of wiring layers and the stack-up configuration directly impact the routing and impedance characteristics of the printed circuit board. The board size also plays a role in determining the stack-up and trace widths needed to achieve the desired performance.
Historically, it was believed that fewer layers meant lower costs. However, there are many other factors influencing manufacturing costs. In recent years, the cost difference between multi-layer boards has significantly decreased. It’s better to plan for more layers from the start and evenly distribute copper to avoid last-minute issues where signals don’t meet rules or spacing requirements, which could force you to add new layers. Proper planning at the beginning can save a lot of time and effort during the routing stage.
2. Design Rules and Constraints
Autonomous routing tools don’t inherently know what to do. They need to operate under well-defined rules and constraints to complete the routing task effectively. Different signal types have different routing requirements. Each signal class should be prioritized accordingly, with higher-priority signals having stricter rules. These rules affect trace width, maximum via count, parallelism, signal interaction, and layer limitations—factors that greatly influence how well the routing tool performs. Carefully defining these constraints is essential for successful routing.
3. Component Placement
To optimize the assembly process, manufacturability (DFM) rules impose restrictions on component placement. If the assembly team allows some flexibility, the PCB can be optimized for easier automated routing. These rules and constraints directly influence the layout design.
When placing components, it's important to consider the routing channels and via areas. While these are obvious to a designer, automatic routing tools typically handle one signal at a time. By setting wiring constraints and defining signal layer usage, the designer can guide the tool more effectively through the routing process.
4. Fanout Design
In the fanout phase, each pin of a surface-mount device should have at least one via to enable internal layer connectivity, online testing (ICT), and rework when more connections are needed. To maximize the efficiency of the auto-router, use the largest via size possible, with a 50-mil spacing. Choose a via type that maximizes the number of available routing paths.
When designing for ICT, keep in mind that test fixtures can be expensive and are usually ordered near full production. It's too late to make changes once the design is nearly complete. Therefore, it's best to plan for online testing early in the design process, so that the via fanout strategy aligns with both the routing path and testability requirements. Power and ground planes also play a key role in routing and fanout design. To minimize inductance in filter capacitor connections, place vias as close as possible to the component pins. Manual routing may be necessary, which could affect existing routing paths and require re-evaluation of via selection. Thus, it's crucial to understand the relationship between via and pin inductance and set clear priorities for via specifications.
5. Manual Routing and Critical Signal Handling
Although this article focuses on automated routing, manual routing remains an essential part of modern PCB design. It helps the automated tools complete complex tasks more efficiently. Regardless of the number of critical signals, it's best to route them manually first or in combination with automation. Critical signals often require careful attention to achieve optimal performance. Once routed, these signals are easier to verify by engineers. After verification, they can be fixed, and the remaining signals can be automatically routed.
6. Automatic Routing
Routing critical signals requires careful control over electrical parameters such as distributed inductance and electromagnetic compatibility (EMC). EDA vendors offer various ways to manage these parameters. Understanding how input settings affect the routing process ensures better results. Use general rules for automatic routing, define constraints, and limit the number of layers and vias used. This way, the routing tool follows the engineer’s design philosophy.
If no constraints are applied, the tool may use all available layers and create excessive vias. After setting up constraints and rules, the automatic routing should closely match expectations. Some finishing touches may still be needed, such as ensuring enough space for other signals. Fix parts of the design to prevent them from being affected by later routing steps.
The same approach applies to the remaining signals. The complexity of the circuit and the rules defined will determine the number of routes. As each signal type is completed, the constraints for the remaining signals can be relaxed. However, manual intervention may still be necessary. Modern auto-routers are powerful and typically complete 100% of the routing, but if some signals remain, they may need to be manually routed.
7. Key Points for Automatic Routing Include:
7.1 Slightly adjust settings and try multiple routing paths;
7.2 Keep basic rules unchanged, but experiment with different routing layers, line widths, spacing, and via types like blind or buried vias to see how they affect the outcome;
7.3 Allow the routing tool to handle default networks as needed;
7.4 The less important the signal, the more freedom the tool has in routing it.
8. Finalizing the Wiring
If your EDA tool can display the length of each signal, check the data. You might find that some low-constraint signals are excessively long. This issue is easy to fix—manual editing can reduce the routing length and the number of vias. During the finishing stage, evaluate which wiring is logical and which isn’t. Like manual routing, automatic routing can also be refined and edited during inspection. This step ensures the final design meets all performance and reliability standards.
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Boluo Xurong Electronics Co., Ltd. , https://www.greenleaf-pc.com