PCB SMT: Analyze The Common Welding Defects

Introduction

Faulty soldering are the most common defects encountered in pcb Surface Mount Technology (SMT) processing. Although visually the solder joint may appear to connect the two surfaces securely, in reality, it lacks the necessary fusion and strength, leading to significant problems during production. This article delves into the causes and effects of Faulty soldering, along with other common PCB surface mount soldering defects, offering detailed analysis and solutions.

Understanding Faulty soldering

Faulty soldering occurs when the temperature at the bonding surface is too low during the soldering process. This results in an insufficiently formed molten core, which may not reach the necessary melting point. The surfaces are merely pressed together, creating a weak bond that appears soldered but is not fully integrated.

Analyzing the Causes and Steps to Prevent Faulty Soldering

Surface Contamination and Contact Issues:

– Inspect the bonding surfaces for rust, oil, or other contaminants. Irregular or poor contact can increase contact resistance, reducing the current and failing to reach the required bonding temperature.

Lack of Proper Overlap:

– Ensure that the overlap of the welded seam is appropriate. Insufficient overlap, particularly on the drive side, can reduce the bonding area, weakening the joint and increasing the likelihood of fracture under high tension.

Inadequate Current Settings:

– Verify that the current settings align with the thickness of the material. If the current does not increase accordingly when the material thickness changes, it may result in insufficient current and poor welding quality.

Inconsistent Welding Pressure:

– Check if the welding pressure is appropriately set. Insufficient pressure can lead to increased contact resistance and decreased actual current, which may not reach the set value, especially if the system’s current compensation limit is exceeded.

Common PCB Surface Mount Soldering Defects

Bridging

Bridging is often caused by excessive solder, solder paste slumping, or PCB pad size discrepancies. This defect can lead to electrical short circuits, particularly the fine-pitch components like SOPs and QFPs, severely affecting product functionality.

Poor Wetting

Poor wetting occurs when the solder does not adequately bond with the PCB pad, resulting in insufficient solder joints. This issue can arise from surface contamination, improper flux application, or forming an oxide layer on metals like tin or sulfides on silver. However, ensuring clean surfaces, proper flux usage, and correct soldering temperatures and times can mitigate this problem.

Conclusion

Faulty soldering and other soldering defects can significantly impact the integrity and functionality of PCB SMT assemblies. Many of these issues can be avoided by understanding the causes and implementing preventive measures, such as proper surface preparation, appropriate overlap, correct current settings, and consistent welding pressure. In cases where defects are detected, reworking with increased overlap, current, and pressure while closely monitoring the weld seam formation can often resolve the issue effectively.