Through-Hole Solder Overflow on PCB Boards: Causes and Prevention Strategies

Through-hole solder overflow is one of the most frequent defects encountered in THT (Through-Hole Technology) PCB assembly. This failure mode occurs when molten solder escapes from the plated through-hole barrel and spreads onto the opposite pad, adjacent traces, or inner board gaps. Besides affecting surface cleanliness, overflow increases the risk of electrical shorts, voids, and unreliable solder joints that undermine long-term product reliability.

From a technical standpoint, overflow happens when the balance of forces acting on molten solder is disrupted. During soldering, solder flow is controlled by a combination of surface tension, gravity, and the adhesion of the metallized hole wall. When these forces fall out of equilibrium, the solder loses stability and leaks beyond the control of the pad geometry.

1. Root Cause: Design-Level Issues in Pad and Hole Size

Errors originating in the PCB layout phase are the most fundamental triggers of solder overflow. Some engineers focus only on matching the pin and hole size while overlooking the proportional relationship between pad diameter and drilled hole.

According to IPC-6012, the pad diameter should be at least 0.6 mm larger than the finished hole size. Undersized pads reduce the wettable area, meaning surface tension cannot adequately retain the liquid solder, which easily slides down the barrel.

Additionally, the copper plating thickness of the PTH wall plays a critical role. Internal process data at PCBGOGO indicates:

• When plating is below 25 μm, the adhesion and surface energy on the barrel wall drop significantly.

• This increases the likelihood of solder slipping and exiting the hole.

In short, both inadequate pad geometry and substandard metallization quality amplify the risk of through-hole solder overflow.

2. Immediate Cause: Improper Wave Soldering Parameters

Wave soldering remains the most common process for THT assembly. Here, parameter synergy directly influences solder behavior. Excessive temperatures or improper conveyor speed often induce overflow.

Key factors include:

• Solder temperature

• Conveyor travel speed

• Wave height and contact angle

PCBGOGO’s empirical process ranges show the following settings reduce overflow occurrences:

For operators, the goal is not merely parameter compliance but ensuring dynamic balance between flow, heat transfer, and solder volume.

3. Material Factor: Moisture in Substrate Driving Internal Gas Pressure

Base material characteristics also influence overflow. FR-4 and other substrates contain microscopic voids capable of absorbing moisture during storage.

If PCBs are used without pre-baking:

• Entrapped moisture vaporizes at high temperature

• Rapid expansion generates internal gas pressure

• Pressure forces molten solder upward through through-holes

To counteract this, PCBGOGO enforces a mandatory pre-bake protocol:

• 120°C for 2 hours before assembly

• Controlled-temperature storage to minimize reabsorption

This reduces internal gas pressure during soldering and promotes stable solder joint formation.

4. A Closed-Loop Prevention Strategy

To effectively prevent through-hole solder overflow, corrective action must cover the full chain rather than a single stage.

Design Stage

• Apply IPC pad-to-hole ratio

• Confirm ≥25 μm plating thickness

• Optimize annular ring size for pin types

Process Stage

• Implement controlled wave solder settings

• Utilize in-line monitoring for temperature and wave height

• Standardize AOI/visual inspection checkpoints

Material Stage

• Enforce pre-bake handling requirements

• Source boards with certified metallization quality

• Manage humidity-controlled storage

By implementing this approach, PCBGOGO has achieved an internal defect rate below 0.1%, providing evidence that solder overflow can be minimized through structural process discipline.

Conclusion

Through-hole solder overflow is not an isolated production mishap, but a multi-factor assembly defect driven by geometry, process control, and material behavior. By aligning PCB design with IPC standards, controlling wave solder variables, and managing substrate moisture, manufacturers can significantly reduce the risk of overflow and enhance finished product reliability.

For engineers and procurement teams seeking consistent THT assembly outcomes, evaluating vendors based on their control of these factors is crucial. PCBGOGO provides PCB fabrication and assembly solutions with end-to-end quality management to support stable mass production.