What is SMT Footprint?

Introduction

Surface mount technology (SMT) has become the predominant method of electronics assembly and component packaging, replacing older through-hole technology. In SMT, components are directly mounted onto the surface of printed circuit boards (PCBs) without passing leads through holes. The land pattern or pads on the PCB that connects the component is known as its “footprint”.

This article provides a comprehensive overview of SMT footprints encompassing pad geometries, sizes, mask openings, orientations, specialty pads and how footprint design accommodates components packaging and joining methods. Guidelines for optimizing and standardizing footprints are also discussed. By understanding SMT footprint design, PCB engineers can layout robust and manufacturable boards.

SMT Footprint Elements

A typical rectangular surface mount component is soldered onto the PCB through metallized terminals or leads on the underside. The corresponding SMT footprint consists of the following elements:

• Pads: Copper pads connect each component terminal to a conductive trace on the board.
• Soldermask openings: Expose the copper pads while covering other traces for solder control.
• Silkscreen outline: Indicates component placement and orientation for assembly.
• Text markings: Identify component designation, polarity etc.
• Fiducials: Alignment markers for pick and place machines.

Pad Geometries

Pads come in different shapes with dimensional attributes tailored to component needs:

• Rectangular: Most common pad shape suited for perimeter leads.
• Rounded rectangular: Rounded pad corners reduce stress concentration.
• ** Oval**: Accommodates pitches down to 0201 sizes while allowing sufficient solder volume.
• Square: Used for area-array packages like BGAs, CSPs etc.
• Donut: Ring pad for shielding cans to allow visual solder inspection.

Key pad dimensions include length, width, corner radii, and finished copper thickness. Typical length/width ratios are 1:1 to 1:1.5. Rounded corners use 20-25 mil radii. Pad thickness aims for 1-2 oz finished copper.

Pad Sizes

Pad sizes primarily depend on three factors:

1. Lead dimensions: Pad size should match component lead width and provide sufficient overlap for wetting and adhesion. Excessive extension beyond the lead is avoided.
2. Solder volume: The pad must accommodate adequate solder to form a reliable joint. IPC-7351 guidelines provide minimum volumes based on lead sizes.
3. Solder mask openings: Pads sizes account for registration tolerances by exceeding mask openings to avoid open circuits. A 25-50 μm annular ring is typical.

High density components may use smaller pad sizes passing minimal solder current to maintain soldering yield across adjoining pads.

Soldermask Openings

The soldermask opening dimensions relative to pads control solder flow and bridging:

• Width/Length: 25-100 μm greater than pad ensures alignment tolerance. Too large increases bridging risk.
• Shape: Match pad shape but enlarged evenly on all sides for uniform wetting.
• Expansion: Can enlarge openings in high vibration areas prone to solder masking separation from pads.
• Clearance: Minimum 50 μm spacing from adjacent pads, or proportional to voltage difference.
• Corners: Right-angled corners simplify masking process capability over rounded.

Orientation Markers

Footprints visually indicate component placement and orientation on the board using:

• Silkscreen outline: Indicates footprint edges and aligns component body.
• Polarity marker: Rectangles or triangles denote orientation of polarized components.
• Text markings: Component designators and values marked adjacent to placement.
• Fiducials: Crosshairs or circles designate pick-and-place locations.

Specialty Pad Types

Unique pad configurations are designed to accommodate different packages:

• Castellated: Edge pad extensions bond to castellated leads of MEMS and LED packages.
• Thermal: Exposed thermal pads provide enhanced thermal dissipation path from packages.
• Metal core: Directly bonds components onto exposed metal core PCBs.
• Compliant interface: Provides stress relief between rigid components like connectors and PCBs.
• Gull wing: Formed pad recesses allow flush bonding of protruding gull wing leads.
• Press fit: Plated through holes accept press fit pins for mounting connectors.

Standardized Footprints

PCB software libraries contain vast collections of manufacturer approved footprints for common components and packages. Standardized footprints enable:

• Correct dimensions: Meets component requirements for reliable assembly.
• Interchangeability: Allows substituting components from different vendors.
• Design reuse: Eliminates reinventing footprints for repeated components.
• Manufacturing compatibility: Provides compatible known-good footprints for fabrication.

However, non-standard custom footprints may still be needed for innovative package designs.

Footprint Design Guidelines

Strategies for optimizing SMT footprints include:

• Match pad sizes to lead dimensions with sufficient tolerances. Avoid overly large pads.
• Incorporate appropriate rounded corners and radii to reduce solder voiding.
• Utilize polarized markers, fiducials and text for clear component orientation.
• Expand solder mask openings beyond pads for solder release and bridging prevention.
• Increase pad spacing in vibration-prone regions.
• Thermally connect large pads to inner plane layers for heat dissipation.
• Allow for rework and repair access in placement and routing.
• Verify footprints against manufacturer recommendations and PCB standards.

Conclusion

Designing suitable SMT footprints requires expertise in combining pad geometries, soldermask openings, thermal considerations and assembly practices into an optimal layout matching the component. Standardization using verified footprints saves time while avoiding field issues. SMT will continue to evolve with components getting smaller, pads becoming denser and higher assembly precision requiring even better understanding of good footprint design principles by engineers.

Frequently Asked Questions (FQA)

Q1: What is the typical copper thickness used for surface mount pads?

A1: 1 oz (35 μm) is suitable for most SMT pads. Higher current pads may use 2 oz (70 μm) thickness. Electroless nickel-immersion gold (ENIG) plating provides solderability.

Q2: How are soldermask openings aligned to pads in actual PCB fabrication?

A2: Soldermasks are photo-imaged using the same copper pad images on the PCB layers to achieve self-aligned openings. This removes need for precise registration of separate soldermask layers.

Q3: Why should thermal pads be connected to inner plane layers?

A3: Connecting exposed thermal pads directly to internal ground or power planes allows heat conduction from components through vias into the planes for effective spreading and cooling.

Q4: What is the typical clearance between copper pads and neighboring soldermask openings?

A4: A minimum 50 μm clearance is typical to avoid solder bridging between adjacent traces. Higher voltage differences require larger spacing proportional to voltage.

Q5: How does footprint design accommodate densely packed fine pitch ICs and components?

A5: Miniaturized pads, tighter spacing and pitch matching, smaller rounded corners, and thin soldermask expansion gaps enable mounting fine-pitch components. Laser direct imaging improves resolution.