1. Determine the Number of Layers for the PCB

The board size and the number of routing layers must be determined early in the design process. If the design involves high-density Ball Grid Array (BGA) components, the minimum number of routing layers required for connecting these components should be taken into account. The number of routing layers and the stack-up method directly affect routing and impedance characteristics. The board size helps determine the stacking method and line width needed to achieve the desired design.

Historically, it was believed that fewer layers would reduce costs. However, there are many other factors influencing the manufacturing cost of the PCB. In recent years, the cost difference between multilayer PCBs has narrowed significantly. It is advisable to use more layers early in the design process and distribute copper evenly. This helps prevent situations where the signals fail to meet defined rules and space requirements, which would otherwise necessitate adding new layers later. Careful planning at the outset can prevent many routing challenges down the line.

2. Design Rules and Constraints

The automated routing tool relies on defined rules and constraints to complete the routing task. Different signal lines have specific routing requirements, and all signals should be categorized accordingly. Each signal class should be assigned a priority—higher-priority signals require stricter rules. These rules govern factors such as trace width, maximum via count, parallelism, signal interaction, and layer limitations, all of which significantly impact routing tool performance. Properly considering the design requirements is essential for successful routing.

3. Component Layout

To optimize the assembly process, design for manufacturability (DFM) rules impose certain restrictions on component layout. If the assembly department permits component movement, the circuit can be optimized for easier automated routing. Layout design must follow the defined rules and constraints.

When laying out components, routing channels and via areas must be carefully considered. While these areas are obvious to the designer, the automated routing tool can only process one signal at a time. By setting appropriate routing constraints and layer definitions for signal lines, the routing tool can complete the routing according to the designer’s vision.

4. Fan-out Design

During the fan-out design phase, each pin of a surface-mount device (SMD) should have at least one via to enable inner layer connectivity, in-circuit testing (ICT), and circuit rework when additional connections are required.

To maximize the efficiency of the automated routing tool, the largest via size and the widest possible printed traces should be used, with a preferred via interval of 50 mils. A via type that allows for maximum routing path utilization should be chosen. When designing the fan-out, the possibility of performing in-circuit testing should be considered. Test fixtures can be costly and typically must be ordered once production is finalized, making it crucial to consider testability early in the design process.

By considering online test requirements and via types early in the design phase, you can ensure the circuit’s testability is integrated into the production process. The power and ground planes also influence routing and fan-out design. To minimize inductive reactance in capacitor connections, vias should be placed as close as possible to the pins of surface-mount devices. Manual routing may be required at times, but it could impact previously planned routing paths. It is essential to evaluate the relationship between via placement, pin inductance, and via specifications to ensure optimal routing.

5. Manual Routing and Critical Signal Processing

While this article focuses on automated routing, manual routing remains a vital part of PCB design, both now and in the future. Manual routing assists the automated routing tool in completing the design.

Critical signals can be routed manually or in combination with the automated routing tool. These signals often require careful design to meet performance requirements. Once routing is complete, the design can be checked by relevant engineering personnel. This process is relatively straightforward, and once validated, the routing is finalized, and remaining signals are routed automatically.

6. Automatic Routing

Routing critical signals involves controlling electrical parameters such as minimizing distributed inductance and reducing electromagnetic compatibility (EMC) issues. Routing for other signals follows a similar process. All EDA tools offer ways to control these parameters. Understanding the input parameters and how they affect the routing process can significantly improve the outcome of automated routing.

General rules should govern automated routing. By setting constraints and disabling specific routing areas, the routing tool can route signals according to the engineer’s design philosophy. Without restrictions on layer count or via usage, the automated tool will use every available layer and create many vias.

Once constraints and rules are set, automated routing should produce results close to expectations. However, some finishing work may be needed to ensure the proper routing of other signals or networks. After completing part of the design, it is important to lock it in place to prevent it from being altered by later routing processes.

The same process applies when routing the remaining signals. The number of traces depends on circuit complexity and defined design rules. After completing one signal type, reduce the constraints for other networks. However, many routing tasks may still require manual intervention. Today’s automated routing tools are powerful and can usually complete 100% of the routing, but some signals may still require manual routing.

7. Design Tips for Automatic Routing:

• 1) Make slight adjustments to settings and experiment with different routing paths.
• 2) Keep basic rules consistent while experimenting with various routing layers, trace widths, via types (e.g., blind, buried), and spacing widths to observe their effects on the design.
• 3) Allow the routing tool to handle default networks as required.
• 4) The less critical the signal, the more freedom the automatic routing tool has in handling its routing.

8. Routing Finalization

If the EDA tool used can display the routing length of signals, review the data to identify any signal routes with excessive lengths or few constraints. This issue can be addressed by manually editing the routing to shorten the traces and reduce the number of vias. During the finalization process, assess which routes are reasonable and which are not. Just like with manual routing, the automatic routing design can be refined and edited during the review process.

9. PCB Aesthetics

In earlier designs, the visual appearance of the board was given considerable attention. However, modern automated designs prioritize electrical performance over aesthetics. While an automatically routed PCB may not look as visually appealing as a manually routed one, it will meet the required electrical specifications and deliver the desired performance.

If you have any questions about PCB or PCBA, please contact us at info@wellcircuits.com.