Implications of border lines and 3D footprint lines on Mechanical Layer 1
6 min
- Example Scenario:
- Guidelines for Proper Component Placement and Slot Recognition:
- FAQ about Mechanical Layer 1
- Conclusion
Key Takeaways
Mastering Mechanical Layer 1 is essential for accurate PCB manufacturing. Always place the complete board outline, mounting holes, and irregular slots on Mechanical Layer 1 (GM1), as JLCPCB prioritizes the lowest-numbered Mechanical Layer while ignoring KEEP-OUT and higher Mechanical Layers. 3D footprint lines serve only as design references and will not be fabricated. Following these layer priority rules and best practices ensures correct board dimensions, proper slot formation, and avoids costly manufacturing errors.
In the process of PCB (Printed Circuit Board) design, the Mechanical Layer plays a crucial role in defining the physical aspects of the board. It helps in determining the placement of components, recognition of slots, and creation of drilled holes. In this article, we will explore the implications of border lines and 3D footprint lines on Mechanical Layer 1 in PCB manufacturing using an example. Mechanical layers (often denoted as GM1, GM2, etc. in Gerber files) are critical for board outline definition, with JLCPCB prioritizing the lowest-numbered Mechanical layer for routing and panelization.
Example Scenario:
Let's consider a scenario where Mechanical Layer 1 has placed border lines and 3D footprint lines for components. The image provided below showcases the design:
Analyzing the Design:
In the given design, the 3D footprint lines of the components are drawn on Mechanical Layer 1. However, it is important to note that in the final product, these 3D footprint lines will not be included as they serve primarily as assembly and 3D modeling references (e.g., component height and courtyard visualization in CAD tools). Instead, the design includes four circles that are intended to produce drilled holes. Ensure hole diameters meet JLCPCB's minimum drill size (typically 0.15-0.2mm depending on layer count) and maintain proper annular ring (≥0.1mm recommended).
Guidelines for Proper Component Placement and Slot Recognition:
Based on the provided design, we can identify certain aspects that need to be addressed to ensure accurate manufacturing and avoidance of unnecessary slots or missing holes. Here are the key guidelines:
Placement of Edge Lines and Circles:
To optimize design clarity and accuracy, it is generally recommended to use Mechanical Layer 1 (GM1) exclusively for the complete board outline and critical features such as mounting holes. Non-outline elements like 3D footprints should be moved to Mechanical Layer 2 or higher. If four circles are needed for drilled holes, ensure they are properly defined in the Drill layer as well.
Unique Edge Layer for Irregular Slots:
For screw holes or irregular slots (shapes drawn) that need to be formed within the board, it is crucial to draw them correctly on a unique edge layer. This allows for precise recognition and implementation of the desired slot shapes during manufacturing.
| Feature | Recommended Layer | Notes / Capabilities |
|---|---|---|
| Board Outline & V-Cut | Mechanical 1 (GM1) | Lowest number priority; Line width ~0.15mm |
| Irregular Slots/Cutouts | Same as Outline (GM1) | Min plated slot width 0.5mm; Length ≥ 2x width |
| Mounting Holes | Drill + Mechanical 1 | Tolerance ±0.1mm (regular) |
| 3D Footprint / Courtyard | Mechanical 2+ | Not manufactured; Reference only |
Consideration of Layer Priority:
It's important to understand how layers are prioritized for routing purposes. In situations where both the KEEP-OUT layer and Mechanical Layer have complete borders, JLCPCB (the PCB manufacturer) will prioritize the elements from the Mechanical Layer and disregard data from the KEEP-OUT layer.This ensures consistent manufacturing even if multiple outline definitions exist.
In cases where multiple Mechanical Layers exist, all with complete borders, the lowest numbered Mechanical Layer will be considered as the outline layer. For instance, if there are Mechanical 1, Mechanical 2, Mechanical 3, and Mechanical 4 layers, the elements from Mechanical 1 will be used for routing purposes, while data from Mechanical 2, Mechanical 3, and Mechanical 4 will be ignored.
FAQ about Mechanical Layer 1
Q: Will the 3D footprint lines drawn on Mechanical Layer 1 appear on the final manufactured PCB?
No. 3D footprint lines are for design reference and 3D modeling only. They will not be fabricated. JLCPCB treats them as non-manufacturing elements.
Q: Which Mechanical Layer should I use for the board outline?
It is strongly recommended to use Mechanical Layer 1 (GM1) as the primary board outline layer. JLCPCB prioritizes the lowest numbered Mechanical Layer when multiple outlines exist.
Q: What should I do if I have board borders on both KEEP-OUT layer and Mechanical Layer?
JLCPCB will prioritize the Mechanical Layer and ignore the KEEP-OUT layer data. For best results, keep only one complete outline on Mechanical Layer 1.
Q: How do I correctly define irregular slots or cutouts?
Draw the exact slot shape using closed contours on Mechanical Layer 1 (or the same layer as your board outline). For plated slots, also define them properly in the Drill files. Minimum plated slot width is 0.5mm.
Q: Can I place mounting holes (circles) on Mechanical Layer 2 instead of Layer 1?
Yes, but it is better to define all critical holes on Mechanical Layer 1 together with the outline. The actual holes must also be defined in the Drill layer for manufacturing.
Q: What happens if I have complete borders on multiple Mechanical Layers (e.g. Mech1, Mech2, Mech3)?
JLCPCB will only use the lowest numbered Mechanical Layer (Mechanical 1) as the board outline. Data from higher-numbered Mechanical Layers will be ignored for routing and panelization.
Conclusion
Understanding the role of the Mechanical Layer in PCB design is crucial for accurate component placement and slot recognition. By following the guidelines provided in this article and considering the example scenario, you can ensure their designs are optimized for manufacturing processes. Proper utilization of the Mechanical Layer, along with other relevant layers and adherence to JLCPCB's DFM guidelines, will result in high-quality PCBs that meet the desired specifications.
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