Through-hole technology (THT) and wave soldering remain indispensable in the production of many electronic products, especially for applications requiring high mechanical strength, reliable connectors and high-current carrying capacity. However, THT soldering poses notable challenges to inspection:

• Solder joints feature variable shapes and reflectivity
• Flux and solder alloys alter joint appearances
• Diverse pin and lead designs make traditional rule-based inspection algorithms ineffective
• Normal process variations easily lead to frequent false rejects

For this reason, Solder AOI (Automated Optical Inspection for solder joints) has become a proven solution to boost production yield, reduce manual inspection workload and standardize quality judgment criteria.

This article elaborates on the key inspection items of Solder AOI, the limitations of conventional algorithms in solder inspection, how AI-powered technologies enhance inspection stability, and how to integrate Solder AOI with SMT AOI and data platforms to accelerate root-cause analysis.

1) What Is Solder AOI?

Solder AOI is an automated optical inspection solution dedicated to solder joint quality inspection, mainly deployed for THT wave soldering and selective soldering processes.

Common deployment positions on production lines:

• Post wave soldering (after DIP processes)
• Post selective soldering
• Final inspection for PCBs with mixed manufacturing technologies

Its core objective is to deliver consistent and objective quality judgment for solder joints while keeping pace with production line speed.

2) Core Solder Inspection Checklist

A standard solder joint inspection covers the following critical defects:

• Insufficient solder (risk of weak mechanical connection)
• Excessive solder (risk of short circuits and electrical faults)
• Poor wetting & cold solder joints (typical visual defects)
• Solder bridging (direct risk of electrical short circuits)
• Abnormal pins and leads associated with solder quality issues

In mass production, priority is given to inspection items closely linked to actual product failures and potential customer quality risks.

3) Why Conventional AOI Struggles with Solder Inspection

THT solder joints have highly complex visual characteristics. Even qualified joints may look different due to multiple factors:

• Solder alloys and flux properties
• Wave height, conveyor angle and soldering dwell time
• Thermal conditions and preheating parameters
• Pin geometry, lead oxidation and component discrepancies

Traditional threshold-based algorithms tend to misclassify normal appearance variations as defects, resulting in high false reject rates.

4) How Solder AOI Optimizes Production Efficiency

A well-calibrated Solder AOI station brings multiple operational benefits:

• Reduces reliance on manual visual inspection
• Detects defects earlier and with greater consistency
• Improves production throughput and cuts labor costs for re-verification
• Records full inspection data to enable complete production traceability

A key operational KPI is the total re-verification working hours saved per shift.

5) AI-Powered Solder AOI: Lower False Rejects, Higher Stability

AI-driven inspection models learn robust feature patterns from massive real production samples to optimize solder inspection performance.

Key practical advantages:

• Minimizes false rejects caused by normal appearance variations of solder joints
• Enhances inspection stability for various solder joint types
• Delivers stronger adaptability for frequent product changeovers

To verify model performance comprehensively, tests shall be conducted across:

• Multiple production batches
• Raw materials from different suppliers
• Typical process deviations within allowable tolerance ranges

6) Recommended Inline Solder AOI Solutions for THT

For inline automated optical inspection of THT solder joints:

AIS30X-HW – Inline THT Solder AOI

https://www.maker-rayaoi.com/en/product/detail/18

For pre-soldering inspection of THT components:

AIS20X-HW – Inline PCBA AOI for THT Components

https://www.maker-rayaoi.com/en/product/detail/16

For complex assemblies requiring comprehensive final inspection:

AIS50X-HW – Inline Double-sided PCBA AOI

https://www.maker-rayaoi.com/en/product/detail/20

7) Data Integration: Accelerate Root-Cause Analysis

Solder defects are often correlated with upstream production factors:

• Component insertion quality
• PCB surface finish and hole conditions
• Process drift in preceding working procedures

By analyzing data from Solder AOI together with SMT AOI and SPI systems, engineering teams can quickly identify root causes and avoid repeated troubleshooting.

Supporting platform:

InsightX – Centralized AOI Data Management Platform

https://www.maker-rayaoi.com/en/product/detail/25

Next Step

If you aim to standardize THT solder inspection and reduce manual checking, we can provide tailored support including:

• Customized solder joint quality standards for your products
• Planning of inspection positions (post wave soldering, final inspection, etc.)
• False reject reduction solutions combining AI algorithms and optimized lighting configurations

To receive targeted product recommendations, please prepare the following information:

1. PCB types (boards with connectors, high-current areas, mixed technologies, etc.)
2. Detailed introduction of wave soldering / selective soldering processes
3. Recurring solder defects and current defect escape situations
4. Required production takt time and staffing for re-verification

Frequently Asked Questions

Q1: What is the difference between Solder AOI and SMT AOI?

SMT AOI mainly inspects surface-mount components, placement errors and SMT solder defects. Solder AOI focuses exclusively on solder joint inspection, with core application in THT and wave soldering scenarios.

Q2: Can Solder AOI fully replace manual inspection?

In most manufacturing plants, Solder AOI greatly reduces manual inspection workload and unifies inspection standards. The adoption of full automation depends on product reliability requirements and overall process maturity.

Q3: Why does Solder AOI generate false rejects?

False rejects are primarily caused by normal differences in the appearance and reflectivity of solder joints. AI algorithms combined with optimized lighting design can effectively improve inspection stability.