A courtyard is a clear area around component footprints on a printed circuit board (PCB) which serves as a keepout zone safeguarding placement space for parts and preventing encroachment from nearby features during layout.
This article provides a detailed perspective on courtyards encompassing definition, need, sizing considerations, common variations, clearance rules and best practices regarding specification and use of courtyards for robust PCB development.
Definition of Courtyard
A courtyard constitutes a functionally required empty space surrounding the land pattern of a component footprint on the PCB layout. It takes on rectangular, circular or more complex polygon shapes based on part dimensions and pin arrangements.
As illustrated above, the courtyard keeps nearby conductors and markings away providing safe clearances between adjacent features.
Need for Courtyards in PCB Design
Provisioning courtyards zones around component footprints serves several key objectives:
Component Placement Fit
Ensures adequate room for parts being assembled to seat properly over pads without collisions during pick and place processing.
Manufacturing Tolerances Accommodates acceptable variabilities in fabrication (etch factors, alignments) and assembly (position skews) preventing short risks.
Repariability&Rework Gives working room for mechanics and tools to extract/replace parts with ease during modifications or retrofits.
Routing Channels Area clearance helps tunnel routing channels neatly through congested sections in complex layouts.
Thermal Spacing
Air gap allows heat dissipation preventing hotspots during component operation.
Test Probe Access Provides safe probed access between parts with scope tips for validation and troubleshooting without hit.
Board Contours Keeps irregular board shapes from cutting into footprint spaces along sheared panel edges.
Thus courtyard zones crucially support both design and manufacturing needs while minimizing proximity risks.
Standard Courtyard Rules
To uniformly maintain necessary isolation spaces without excessive real estate, industry guidelines have standardized courtyard dimensions.
IPC-7351 generic equations formalize minimum courtyard widths beyond land pattern boundaries considering part geometries.
Where:
- Xmin = Courtyard offset from package X dimension
- Ymin = Offset from package Y dimension
- Cw = Additional clearance
Typical Courtyard Allowances
| Clearance Type | Allowance |
|---|---|
| Default Standard Clearance (Cw) | 0.25mm (SMDs) <br> 0.5mm (Thru-hole) |
| Placement Clearance | 0.1mm |
| Fabrication Clearance | 0.1mm |
However, for complex connectors, tighter clearance demands of specific components override these generic allowances warranting larger courtyard dimensions.
Types of PCB Courtyards
Based on Scope
- Global Courtyards: Apply uniformly to all parts
- Individual Courtyards: Customized per component
Based on Layers
- Assembly Courtyards: On assembly top/bottom layers
- Internal Plane Courtyards: On inner GND/PWR layers
Special Courtyards
- Panel Courtyards: Border individual PCBs in panels
- Shielding Courtyards: Surround emissions sensitive areas
- Stencil Courtyards: Frame solder paste stencil apertures
Right courtyards types facilitate corresponding workflow needs during fabrication, assembly and test.
Courtyard Design Concerns
Despite standard guidelines, practical design issues regarding part allocation congestion and routing channels warrant additional courtyard precautions:
Sparse Placement Regions
Expand courtyards where substantial open areas exist around parts to prevent exploitative trace encroachments.
Crowded Locations
Enforce larger cutouts around tall components placed amid dense neighborhoods to aid assembly and thermal management.
Fine Pitch Packages
Allow extra Construction tolerance margins beyond published standards for complex BGAs/CSPs prone to skew during mounting.
Tall Components
Add elevated vertical allowances suiting socket heights to prevent adjacent pin/part collisions during insertion/mating.
Board Contour Adjacency
Increase keepout where component sites lie next to panel edges or board profile cutouts to account for fabrication tolerances.
High Voltage Circuits For high potential nodes, appropriately scale side clearances to safely satisfy creepage requirements.
Reparability Considerations
Square courtyards preferred over rounded rectangles for better tool clearances and access.
Implementing Courtyards in PCB Layout
Steps for Creating Courtyards in CAD Tool
- Define courtyard template styles for components grouping based on clearance needs
- Attach appropriate courtyard type per placement using properties
- View and adjust courtyard overlaps/spacing violations
- Expand keepouts at congested component sites if needed
- Complete signal route stitching inside courtyard channels
Guidelines for Courtyard Practices
- Attach courtyards to footprints rather than drawing manually
- Maintain consistent courtyard-to-pad clearances across similar components
- Set back key pin pad features from courtyard boundaries
- Ensure placement sites accommodate full courtyard spaces during floorplanning
- Assess complex shape courtyards versus rotated components
- Keep unlabeled logos, lines and copper fills out of courtyard areas
Firm adherence to courtyard design protocols and cautions during layout development stages pays of significantly in smoother manufacturing releases.
FAQs
How are thermal relief connections accommodated?
Dedicated cutout regions are shape-merged into courtyard keepout polygons around pad concavities facilitating placement of thermal spokes without clearance violations.
Can components overhang outside their courtyards?
It is permitted for a few leads to marginally overhang courtyard provided sufficient under-the-body clearance exists for mounting. But assembly risks exist with excessive overhangs.
Is manual cleaning of courtyard violations needed?
Most CAD suites automatically resize courtyard outlines dynamically based on keepouts set in design rules. This prevents overlaps once guidelines are configured appropriately.
Why are slots used in BGA package courtyards?
Slots help in bending rollout flex PCBs post-assembly without tombstoning by reducing stiffness. Long continuum slots parallel to axis enable easier bend curvatures keeping parts anchored.
Can acute corners in courtyards cause issues?
Sharp < 90° courtyard corners are fabrication-unfriendly. They are prone to etch factor deviations causing insufficient actual spacing. Slight rounding or 45° angled corners are recommended.
Thus, courtyards constitute a simple yet effective design technique to enhance manufacturability and guard component sites across the complex progression from prototypes to mass production.
