PCB Pad Basics: Functions & Importance

A Printed Circuit Board (PCB) pad is an exposed area of metal on an electrical circuit board where component leads are soldered. These pads play a crucial role in PCB design and manufacturing, contributing to the overall functionality and reliability of electronic devices. There are two primary types of PCB pads: surface mount pads and through-hole pads. In this comprehensive article, we will delve into the world of PCB pad design, explore the features and characteristics of surface mount and through-hole pads, discuss considerations for via placement, delve into the concept of bond pads, examine the drawbacks of manual pad design, and provide insights into constructing reliable pad designs.

Surface Mount Pads

Surface mount pads, also known as SMD (Surface Mount Device) pads, are used to attach components directly to the surface of a PCB. These pads offer several key features:

1. Composition: Surface mount pads are typically made of copper and serve as the connection points for soldering components.
2. Shape Variety: These pads can come in various shapes, including rectangular, round, elliptical, or square, depending on the specific design requirements.
3. Solder Mask Layer: A solder mask layer is applied to the PCB, covering areas that should not be soldered. This helps prevent solder bridges and ensures precise soldering.
4. Solder Paste: Solder paste is applied to surface mount pads during assembly to facilitate the soldering process.
5. Pad Number: The number of surface mount pads varies based on the component being mounted, ensuring the correct alignment and connection of components.

NSMD BGA Pads  vs. SMD BGA Pads

Ball Grid Array (BGA) components are widely used in modern electronics, and the design of BGA pads is crucial. There are two main types of BGA pads: Non-Solder Mask Defined (NSMD) and Solder Mask Defined (SMD).

1. Non-Solder Mask Defined BGA Pads (NSMD):
   • NSMD pads are designed with a gap between the solder mask and the copper pad.
   • These pads can be smaller in size compared to the solder balls, reducing the overall pad size by approximately 20%.
   • NSMD pads are suitable for high-density and fine-pitch BGA chips but are more susceptible to delamination under thermal and mechanical stress.
2. Solder Mask Defined BGA Pads (SMD):
   • SMD pads have solder mask apertures that define the size of the copper pad.
   • The solder mask covers a portion of the copper pad, ensuring that the solder joint is precisely controlled during soldering.
   • SMD pads are known for their reliability in preventing pad lift-off from the circuit board due to mechanical or thermal stress.

Through-Hole Pads

Through-hole pads are used for components that are mounted by passing their leads through holes in the PCB. There are two main types of through-hole pads: plated through-hole (PTH) pads and non-plated through-hole (NPTH) pads.

1. Plated Through-Hole (PTH) Pads:
   • PTH pads have a hole through which the component lead passes. The entire hole wall is plated with copper, ensuring electrical connectivity between different layers of the PCB.
   • Plating is achieved through electrolysis, providing a strong and reliable connection.
2. Non-Plated Through-Hole (NPTH) Pads:
   • NPTH pads do not have metal plating in the hole. These pads are often used in single-sided circuit boards or for components that require a non-conductive path.
   • Unplated holes have a small area of exposed copper near the hole’s edge to prevent shorts between copper layers and components.

Via-in-Pad Technology

In high-density interconnect (HDI) PCB designs, via-in-pad technology is employed to optimize space usage. This involves placing vias directly on pads, reducing the space required for trace routing. Via-in-pad technology is commonly used for BGA components with pitches of 0.5 mm or smaller.

Bond Pads: Bond pads play a crucial role in connecting the electrical circuit on a semiconductor die to the pins on a packaged chip. These pads are constructed using multiple metallic layers stacked on top of each other and connected through vias. Bond pads are designed to be accessible for wire bonding, ensuring a reliable connection between the die and the chip package.

Drawbacks of Manual Pad Design

Manual pad design, which involves drawing pad shapes using design software tools, has several drawbacks:

1. Through-Hole Breakout: Manual design may result in inadequate pad sizes, leading to through-hole breakout and broken circuits.
2. Insufficient Solder Joints: Small surface mount component pads may not receive enough solder during the soldering process, leading to weak solder joints.
3. Floating Parts: Oversized pads for surface mount components can cause parts to float during the reflow process, potentially causing short circuits.
4. Tombstoning: Components with uneven pad sizes can lead to tombstoning, where one pad heats up faster than the other, resembling a tombstone.
5. Shorts to Other Elements: Smaller than required pads may allow surface traces to come close to soldered components, resulting in short circuits.

Constructing Reliable Pad Designs

Creating PCB pad stack designs that meet manufacturability and reliability requirements involves considering various factors:

• Tolerances: Ensuring adequate insulation between conductors, following specified product standards.
• Robust Connections: Strong connectors between traces and plated through-holes or vias.
• Aspect Ratio: Designing hole surrounds to withstand the stresses of the plating process without failure.
• Maximum Tolerances: Meeting minimum insulation requirements for different product categories.

Resources for Pad Sizing

Designers can refer to industry standards like IPC-7351 for detailed pad sizing specifications. Additionally, PCB designers can access resources such as pad and land design sources, PCB pattern CAD vendor libraries, third-party CAD library suppliers, and pad and land design calculators to determine the appropriate pad sizes for their designs.

Conclusion

PCB pads are fundamental components in electronic circuit design, and their proper design is essential for the functionality and reliability of PCBs. Understanding the features of surface mount and through-hole pads, the differences between NSMD and SMD BGA pads, the significance of bond pads, and the drawbacks of manual pad design is crucial for PCB designers. By following best practices and considering the specific requirements of each design, designers can create fully operational and efficient PCBs that meet the demands of modern electronics.