Designing large multi-layer boards in Allegro PCB can be both a rewarding and complex process. With an increase in component density, density performance requirements and number of layers, experienced engineers encounter a complexity that slows down the development process. At Sunstream, we offer advanced PCB Services, which can help your design teams navigate these processes and get better value from their design tools.
This blog offers some practical ways to improve Allegro based workflows for designing large multi-layer boards that can lead to cleaner layouts, effective iterations, and production-ready designs.
1. Start with a Well-Defined Stack-Up
The success of a multilayer design begins with the stack-up. It is critical to define the stack-up early in the design process to prevent expensive rework at the end. In Allegro, the Constraint Manager is where you define the dielectric thickness, copper weights, and impedance rules. It is essential to know the signal integrity of your high speed boards before routing.
Simulate the signal integrity within the stack-up first. Having a clear stack-up will also assist in collaboration with manufacturing, to ensure your fabricator builds the board as you intended. At Sunstream, our engineers will often create supporting documentation at the same time as the layout so there are fewer surprises for our downstream manufacturing team.
2. Leverage Design Rules Effectively
With the increasing complexity of boards, using manual checks will become very inefficient. Allegro’s rule-driven environment enables the user to specify spacing, trace width, via types, and clearance constraints, at the start of the design process. The rules used in the design will prompt Allegro to automatically flag any violations, as you lay out the board.
For example, if you are laying out a congested BGA, you will be enforcing the rules for proper escape routing or fan-out vias. This will reduce human error and accelerate the review cycle’s speed.
3. Optimize Placement for Performance and Manufacturability
Component placement is the most valuable part of the multi-layer design process. Allegro has advanced placement capabilities (like cluster placement, symmetry placement, and schematic cross-probing) that help you strike a balance on performance objectives such as reducing signal delay and manufacturability requirements such as keeping components within assembly tolerances..
For boards with hefty power requirements, locate regulators to be near their loads and use planes which can be assigned specifically to provide clean distribution. For high-speed signals, you should keep routing paths as short and direct as possible. Keep in mind that the placement stage is setting drivers for how well you are able to route the boards in the next stages.
4. Improve Routing Efficiency with Automation
The longest part of routing large multi-layer designs occurs when you are actually routing, and although it is nice to have the automation features like Auto Interactive Convert, the glossing capabilities and tuning functions with differential pairs, engineers still use some trial and error to ensure consistent impedance and lengths from all the routing automations.
While automation is great, it is not as powerful of a tool if used without designer oversight. For critical signals like DDR interfaces, ensure you review each routing path and confirm you comply with the timing and impedances limitations.
5. Manage Power and Ground Planes Carefully
Power distribution can be neglected in programming, but it can be the determining factor in how well a board functions. Multi-layer boards will often have planes for power and ground to maintain voltage levels and reduce noise (EMI). There are some tools in Allegro, such as Plane Assignment and Shape Editing that allow designers to control and define copper pours or splits with precision.
A poorly defined plane will create EMI issues, or worse, voltage drops will occur. Sunstream’s PCB Layout Services use power integrity analysis to ensure the design functions as expected, even with heavy loads applied.
6. Use Hierarchical and Modular Design Approaches
For major projects, breaking the design down into modules can certainly help manageability. A modular design can also help when designing large systems. With hierarchical block design, subsystems can be developed as individual design blocks and integrated into the final board.
Modular designs may also reduce complexity and will allow you to reuse design blocks for future projects. Companies that are developing product families have a systematic approach to design and standardised modules will facilitate time savings across multiple designs.
7. Validate with Built-In Analysis Tools
Allegro offers robust analysis capabilities, from signal integrity checks to thermal simulations. Conducting these early in the design cycle can help to identify possible failure modes prior to manufacturing.
For example, crosstalk analysis can reveal problem areas within dense routed designs, while thermal checks can also deliver hot spots around processors or power devices. By carrying out all design validation in Allegro before the design will be released, you will reduce the cost of re-spins.
8. Collaborate with a Trusted Design Partner
Even if you have the ideal design tools, there is value in working with a partner like Sunstream to manage a multi-layer board with many components, in every possible optimized configuration. Our team brings a combination of tool proficiency and significant hands-on experience to ensure that your designs are optimal in all aspects, including production.
Many considerations go into the manufacturability of multi-layer boards and our engineers managed every step of the way from stack-up design to manufacturing DFM checks, we take pride in making sure your Allegro projects are in the best of hands.
Whether you need a little bandwidth to support a project, or you need to outsource all of your design for a project, our PCB Services can help you expedite your development cycle.
Optimizing Allegro for larger multi-layers boards takes a combination of strategic tool usage, prudent planning, and experience in the context of the domain. From defining the stack-up to confirming power distribution to performing simulations and every step in between, all the phases contribute to a reliable design that is ready for production. By leveraging the capabilities of Allegro with Sunstream, an organization can execute efficient workflows, reduce the occurrence of design errors, and more quickly move complex products into the marketplace.
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