Using negative layers in Allegro can help you efficiently handle large areas of copper pour for power and ground layers, but there are indeed some concepts and settings that require attention.
The Core Differences Between Positive and Negative Layers
To understand negative layers, it is essential to compare them with positive layers:
Positive Artwork: On the film,what you see is what you get (the black areas represent copper). It directly presents conductive patterns such as traces and pads.
Negative Artwork: On the film,what you see is not what you get (the white areas represent copper). Negative layers use blank spaces to indicate conductive areas, while lines or shapes indicate areas without copper.
To visually compare their application characteristics in Allegro, please refer to the table below:
|
Characteristic Dimension |
Positive |
Negative |
|
DRC Verification |
Comprehensive DRC checks |
DRC checks are incomplete; for example, inner layers interrupted by Anti-Pads do not trigger DRC errors |
|
Data Volume and Efficiency |
Large data volume when containing extensive copper |
Small data volume, fast processing speed |
|
Moving Components/Vias |
Requires copper re-pouring; otherwise, it may cause short circuits or open circuits |
Automatically updates without needing to re-pour copper |
|
Process and View |
What you see is what you get, more intuitive |
What you see is not what you get; understanding the negative concept is necessary |
Application Scenarios and Advantages of Negative Layers
Negative layers are typically used for internalpower plane layers and ground plane layers. On these layers, large areas of copper belong to the same network (such as GND or VCC), and using negative layers can significantlyreduce data processing volume, improving software efficiency, especially in designs with many layers and large scales (such as backplanes).
Settings and Operations for Negative Layers
Defining Negative Layers in Layer Stack Settings
Open the Layer Stack Manager:Setup → Cross-section.
In the layer that needs to be set as negative (usually a layer with Type asPlane), selectNegative or check the box in theFilm Type orNegative Artwork field.
Special Pad Settings for Negative Layers
Negative layers do not connect directly with conventional pads but through the following two special pad structures:
Thermal Pads: Used to connect vias or plug pads of the same network on large copper areas of negative layers. They connect to the copper through several “thermal paths” (spokes), ensuring electrical connection while avoiding excessive heat dissipation during soldering due to full enclosure connections.
Isolation Pads: Used to maintain a safe distance between vias or pads and negative copper of different networks, preventing short circuits.
In Allegro’s pad designer, it is essential to correctly set the sizes of<span>Thermal Relief</span> and<span>Anti Pad</span>, which are usually larger than<span>Regular Pad</span> (standard pads) (for example, some sources suggest that through-hole pads should be 0.5mm larger on the Begin/End Layer and 30mil larger on the DEFAULT INTERNAL layer).
Copper Pouring for Negative Layers
Negative layers typically do not require manual drawing of copper shapes like positive layers. Its “copper” is the default background of the entire layer, and you create isolation areas by setting<span>Thermal Relief</span> and<span>Anti Pad</span> to “dig out” the isolation zones and establish connections through thermal pads.
Considerations and Common Issues When Using Negative Layers
Incomplete DRC Checks: DRC verification for negative layers is not perfect. For example, if the<span>Anti Pad</span> size is set too small, it may lead to insufficient isolation, but DRC may not catch such errors. Therefore,it is crucial to carefully check and confirm the settings of<span>Thermal Relief</span> and<span>Anti Pad</span> in the negative layer.
Avoid Routing Signal Traces on Negative Layers: Negative layers are primarily designed for power planes. If signal traces are routed on them, those traces will automatically connect to the negative copper, potentially causing short circuits (unless the network is the same as the negative layer network). It is recommended to set signal layers as positive.
Output Settings: When outputting photoplot files, ensure that the negative layer is correctly set with negative attributes. Some sources mention that the<span>negative artwork</span> should be unchecked in the stack, and the<span>negative</span> option should also be selected in the output art file.
Summary and Recommendations
When to Use Negative Layers: For power and ground layers with fewer networks that require large copper areas, especially in complex multilayer board designs, using negative layers can enhance efficiency.
When to Use Positive Layers: For signal layers that require fine routing, or when the design has fewer layers and emphasizes intuitive design and comprehensive DRC checks, it is advisable to use positive layers.
Key Checkpoints: When using negative layers,ensure that the definitions and sizes of<span>Thermal Relief</span> and<span>Anti Pad</span> in the pad library are correct and appropriate, as this is key to avoiding short circuits and connection issues.
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