Designing Reliable Through-Hole Component Pads: A Comprehensive Guide to Standards and Best Practices

Through-hole technology remains a cornerstone of electronics manufacturing, valued for its mechanical robustness and ability to handle high power in demanding applications—from automotive systems to industrial machinery. At the heart of reliable through-hole assembly lies the design of pads: the copper interfaces that secure components, facilitate soldering, and ensure electrical connectivity across PCB layers. Poor pad design can lead to costly failures, including weak solder joints, pad lift-off, and intermittent connections. This guide outlines the critical principles, standards, and step-by-step techniques for designing through-hole pads that meet industry requirements and deliver long-term performance.

Fundamentals of Through-Hole Components and Pads

Types of Through-Hole Components

Through-hole components are distinguished by their leads, which pass through drilled holes in the PCB and are soldered to pads on both sides. Common examples include:

• Axial-leaded components: Leads extend from opposite ends (e.g., carbon resistors, electrolytic capacitors).
• Radial-leaded components: Leads emerge from the same side (e.g., ceramic capacitors, inductors).
• Connectors, transformers, and power resistors: Bulkier components requiring robust mechanical support for vibration and thermal stress.

These components are preferred in applications demanding durability, such as aerospace, automotive, and industrial controls, where reliability under extreme conditions is non-negotiable.

The Critical Role of Through-Hole Pads

Through-hole pads serve three primary functions:

1. Mechanical Anchoring: Secure component leads to resist physical stress from vibration, thermal cycling, or handling.
2. Electrical Connectivity: Provide a conductive path between component leads and PCB traces, including internal layers via plated through-holes (PTH).
3. Solder Management: Offer sufficient copper area to ensure proper solder flow and joint formation during assembly and rework.

A well-designed pad balances these roles, preventing issues like solder starvation, pad cracking, or layer-to-layer connection failures.

Plated Through-Hole (PTH) Reliability

PTHs are essential for multi-layer PCBs, as they create conductive pathways through the board thickness. The copper plating inside the hole walls must meet strict thickness standards to ensure conductivity and mechanical strength:

• IPC-600J Class 2: Minimum 18µm copper plating (common in consumer electronics).
• IPC-600J Class 3: 25µm minimum (required for aerospace, medical, and high-reliability systems).

Manufacturers often exceed these standards (e.g., 20µm for Class 2) to account for plating variations. Non-plated through-holes (NPTH) are only used for mechanical alignment, not electrical connections, as they lack conductive plating.

Key IPC Standards for Through-Hole Pad Design

Adhering to industry standards is critical for consistency and reliability. The following IPC specifications govern through-hole pad design:

Standard	Focus Area
IPC-2221	General PCB design rules, including pad/hole sizing and clearance guidelines.
IPC-2222	Detailed requirements for rigid PCBs, with specific formulas for pad dimensions.
IPC-6012E	PCB fabrication standards, including plating thickness and annular ring rules.
IPC-A-610	Acceptability criteria for solder joints, defining quality levels (Class 1-3).

Critical Parameters Defined by IPC Standards

1. Lead-to-Hole Clearance:

IPC-2221 recommends a gap of 0.2–0.25mm between the component lead and hole wall. This clearance allows solder to flow evenly around the lead during assembly, preventing voids.

• Annular Ring:

The annular ring is the copper area surrounding the hole. IPC-6012E specifies:

• Class 2: Minimum 0.10mm (external layers) and 0.05mm (internal layers).
  • Class 3: Minimum 0.125mm (external layers) and 0.075mm (internal layers).

A sufficient annular ring prevents “pad breakout” (copper separation due to drill misalignment) and ensures mechanical strength.

1. Plating and Voids:

Class 3 PCBs (e.g., aerospace) prohibit voids in PTH plating, while Class 2 allows minor voids (<5% of hole area). Plating thickness directly impacts current-carrying capacity and thermal resistance.

Designing PTH Holes and Pads: Step-by-Step Guidelines

1. Determine Hole Size

The finished hole size (FHS) depends on the component’s maximum lead diameter plus clearance. IPC-2222 defines three clearance levels:

Clearance Level	Formula	Use Case
Level A	FHS = Max Lead Diameter + 0.25mm	High-reliability, easy assembly
Level B	FHS = Max Lead Diameter + 0.20mm	Standard electronics
Level C	FHS = Max Lead Diameter + 0.15mm	Space-constrained designs

Example: For a component with a 0.8mm lead (max diameter), Level A clearance gives FHS = 0.8 + 0.25 = 1.05mm. Round up to the nearest standard drill size (typically 1.1mm) for manufacturability.

2. Calculate Pad Diameter

Pad diameter is determined by the hole size plus twice the minimum annular ring (to account for both sides of the hole) and a fabrication tolerance (typically 0.1mm).

Formula:

Pad Diameter = Finished Hole Size + (2 × Minimum Annular Ring) + Fabrication Tolerance

Example: For a 1.1mm hole, Class 2 annular ring (0.10mm), and 0.1mm tolerance:

Pad Diameter = 1.1 + (2 × 0.10) + 0.1 = 1.4mm. Most designers use 2.1mm for added robustness.

3. Manage Aspect Ratio

The aspect ratio (PCB thickness ÷ hole diameter) must be ≤10:1 to ensure uniform plating. For a 1.6mm thick PCB, the minimum hole diameter is 0.16mm. Larger holes (e.g., 1.1mm) easily meet this (1.6 ÷ 1.1 ≈ 1.45:1).

4. Optimize Pad Shape and Connections

• Round pads: The most common shape, providing even copper distribution for axial/radial leads.
• Teardrop pads: Add copper at trace-to-pad junctions to reinforce against drill misalignment and mechanical stress.
• Thermal relief: When connecting pads to large copper planes, use thin copper “spokes” to limit heat transfer during soldering, preventing cold joints.

Common Design Mistakes and Solutions

Mistake	Consequence	Prevention
Inadequate annular ring	Pad breakout, weak mechanical support	Follow IPC-6012E minimums; use teardrop pads.
Excessive hole clearance	Loose leads, solder wicking, weak joints	Adhere to 0.2–0.25mm clearance (IPC-2221).
Poor PTH plating	High resistance, layer disconnects	Specify Class 2/3 plating per IPC-600J.
Missing thermal relief	Cold solder joints, difficult rework	Add 2–4 spokes (0.2–0.3mm wide) to plane connections.
Insufficient pad spacing	Solder bridging, assembly/rework challenges	Maintain ≥0.5mm between adjacent pads.

Verification and Manufacturing Collaboration

Even the best designs require validation:

• Check datasheets: Confirm lead diameters and recommended land patterns from component manufacturers.
• Engage PCB fabricators early: Validate drill sizes, plating capabilities, and tolerance limits (e.g., ±0.05mm for hole diameter).
• Use design tools: Leverage PCB software with IPC-7351-compliant footprint libraries to automate calculations.

FAQ

Q: What causes through-hole pad failures most often?

A: Inadequate annular rings (due to undersized pads or drill misalignment) and thin PTH plating are the primary culprits, leading to mechanical weakness and electrical discontinuity.

Q: Can the same pad size work for all through-hole components?

A: No. Pad size must match the lead diameter and component type. For example, a 0.8mm resistor lead requires a smaller pad than a 2.0mm connector pin.

Q: When should thermal relief be used?

A: Thermal relief is critical when pads connect to large copper planes (e.g., power/ground planes) to prevent heat from being siphoned away during soldering, ensuring proper joint formation.

Conclusion

Designing reliable through-hole pads demands a balance of IPC standards, component specifications, and manufacturing realities. By prioritizing annular ring integrity, proper clearance, and robust PTH plating, engineers can create pads that withstand mechanical stress, thermal cycling, and high power. Early collaboration with fabricators and adherence to industry guidelines—such as IPC-2221 and IPC-6012E—are key to minimizing defects and ensuring long-term performance in critical applications. As through-hole technology continues to coexist with SMT, mastering these design principles remains essential for building durable, high-quality electronics.