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Through-Hole Pad Design for PCB Durability

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • Through-hole pads are preferred for mounting heavier components due to their strength and ease of adjustment, offering advantages in prototyping and debugging.

  • The process involves identifying the maximum lead diameter, calculating the minimum hole and pad diameters with specific additions for design density levels.

  • Adherence to IPC-2222 and IPC-2221 guidelines is crucial for calculating pad diameter, ensuring the reliability and durability of the PCB.

Through-hole pad diagram

Through-hole pad diagram

In printed circuit board (PCB) design, copper pads are points for attaching components with soldering. These pads come in two primary types: through-hole and surface-mount (SMD). Through-hole pads are designed for components with leads that are inserted through the pad's holes and soldered on the opposite side of the board. This method ensures a mechanical and electrical bond between the component and the PCB, beneficial for long-term reliability. Read on as we discuss through-hole pad design.

Through-Hole Pad Design Steps

Step

Description

Formula

Notes

1

Identify Maximum Lead Diameter

Check component datasheet

Consider lead shape (round, square, rectangle)

2

Calculate the Minimum Hole Size

Minimum Hole Size = Maximum Lead Diameter + Hole Diameter Factor

Factors: +0.25mm (A), +0.20mm (B), +0.15mm (C)

3

Calculate Pad Diameter

Pad Diameter = Minimum Hole Size + 0.1mm + Minimum Fabrication Allowance

Allowances: +0.40mm (A), +0.25mm (B), +0.20mm (C)


 

Diameter Calculation Guidelines

Based on the steps outlined in the table above and the supplemental notes discussed below, we will walk you through defining your design’s pad size.

  1. Begin by identifying the largest diameter of the component lead, which may be round, square, or, in some instances, rectangular. For square or rectangular leads, utilize the diagonal measurement as the hole diameter. 

  2. For Density Level A, characterized as general design with a preference for a larger footprint, add 0.25mm. Density Level B represents a moderate design approach with a standard level, requiring an addition of 0.20mm. Lastly, Density Level C signifies a high-density design with a minimized footprint, necessitating an additional 0.15mm.

  3. To calculate the diameter of the pad, incorporate both the minimum annular ring measurement and the minimum fabrication allowance as defined by the IPC-2221 standards. The annular ring refers to the conductive (typically copper) area encircling a PCB hole. This feature is critical for ensuring a sturdy connection between the PCB and the leads of components inserted into these holes.

Note on Regulations

Please review IPC-2222 and IPC-2221 for detailed guidance on calculating pad diameter, which is essentially the sum of the minimum hole size, twice the minimum annular ring, plus the minimum fabrication allowance. 

Per IPC-2221 standards, the minimum fabrication allowance varies by design level: 0.6mm for Level A, 0.5mm for Level B, and 0.4mm for Level C. Additionally, depending on the manufacturer, they may also have annular ring requirements —between 0.13mm and 0.2mm as a minimum requirement is common.

Through-Hole Assemblies

Through-hole resistors and capacitors as part of an assembly

Through-hole resistors and capacitors as part of an assembly

Through-hole assemblies attach components with leads to a printed circuit board (PCB) by first inserting them into through-hole pads. This technique requires soldering the components to pads located on the board's opposite side, a task that can be performed manually or with an automated soldering machine. These pads may be plated through holes (PTH) or non-plated through holes (NPTH).

Through-Hole vs. Surface-Mounted Pads

Through-hole pads are the favored mounting technique for heavier components, including electrolytic capacitors, semiconductor packages like TO-220, plug connectors, and relays, which demand extra strength in mounting. Moreover, through-hole components offer the advantage of being more straightforward to adjust and replace than surface-mount components, which is helpful for prototyping. Their relatively larger size makes it easier to probe with debugging equipment such as oscilloscopes. However, using through-hole pads limits the space available for routing on multilayer boards due to the need for holes and the physical size of the component leads.

Compare this with surface-mount pads that facilitate the attachment of components directly onto the board's surface. This approach is ideal for smaller components, enabling a higher density of parts and, thus, more functionality within a compact space. Surface-mount technology (SMT) is advantageous for complex, multi-layer board designs. Despite its benefits, SMT may not be suitable for components that emit significant heat, as surface-mount pads may not adequately dissipate the generated temperatures.

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