4.1 Land Patterns, Spacing & Polarity

Design for Assembly (DFA) optimizes the layout to minimize defect rates such as tombstoning, bridging, and skew. The physical footprint on the PCB must account for the wetting forces of molten solder and the placement accuracy of the machine.

Land Pattern Standards

Footprints must adhere to IPC-7351 Generic Requirements for Surface Mount Design.

• Density Level B (Nominal): The standard for controlled manufacturing.
• Thermal Relief: Ground pads connected to large copper planes must use thermal relief spokes. Direct connection to the plane acts as a heatsink, preventing the pad from reaching reflow temperature and causing "cold solder joints" or tombstoning due to uneven wetting forces.

Component Spacing and Shadowing

• Courtyards: A keep-out zone around each component (e.g., +0.25 mm) ensures the pick-and-place nozzle does not collide with adjacent parts.
• Shadow Effect: Tall components (Electrolytic Caps, Connectors) block convective heat flow to smaller components. A minimum spacing ratio (e.g., 1:1 height-to-distance) must be maintained to ensure small parts behind "walls" receive adequate heat.

Polarity and Orientation

To prevent rotation errors (e.g., placing a diode backwards):

• Zero Orientation: Follow IPC-7351B "Pin 1 Top/Left" convention for 0-degree rotation.
• Indicators: Silkscreen and Assembly Drawing must clearly mark Pin 1 or Cathode. Relying solely on the asymmetrical pad shape is insufficient.

Final Checklist