Equipment Comparison

Introduction

AOI and X-Ray are two of the most commonly used inspection technologies in SMT manufacturing.
However, many quality issues occur not because inspection is missing — but because the wrong inspection technology is applied to the wrong problem.

This article explains the inspection boundary between AOI and X-Ray, helping engineers and decision-makers understand:

• What AOI can reliably detect

• What AOI cannot see

• Where X-Ray inspection becomes necessary

• How to define a proper inspection strategy

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What AOI Is Designed to Do

Automated Optical Inspection (AOI) relies on cameras and lighting to inspect visible surfaces of electronic assemblies.

AOI is highly effective at detecting:

• Component presence and polarity

• Placement offset and rotation

• Solder bridges on visible pads

• Missing or incorrect components

• Obvious surface defects

AOI is fast, repeatable, and well suited for inline inspection in SMT lines.

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The Natural Boundary of AOI Inspection

Despite its strengths, AOI has clear physical limitations.

AOI cannot reliably inspect:

• Hidden solder joints (BGA, CSP, QFN)

• Internal voids or cracks

• Solder quality beneath components

• Inner-layer defects or internal connections

No amount of software tuning can overcome the fact that AOI cannot see through materials.

This is where many inspection misunderstandings begin.

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What X-Ray Inspection Is Designed to Do

X-Ray inspection is a non-destructive technology that allows engineers to see inside electronic assemblies.

X-Ray inspection excels at detecting:

• BGA / CSP solder joint voids

• Insufficient or excessive solder under components

• Hidden bridges and shorts

• Internal cracks and cold solder joints

• IC package and semiconductor defects

X-Ray does not replace AOI — it complements it.

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X-Ray Inspection Limitations

X-Ray inspection also has boundaries.

It is generally:

• Slower than inline AOI

• Less suitable for 100% inline inspection

• Used mainly for analysis, verification, or sampling

• More dependent on skilled interpretation

X-Ray is best positioned as a quality analysis and confirmation tool, not a high-speed screening tool.

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AOI vs X-Ray – Inspection Boundary Comparison

This boundary defines where each technology should be applied.

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Why Inspection Strategy Matters More Than Equipment

Many factories invest heavily in inspection equipment but still face quality issues because:

• AOI is expected to detect internal defects

• X-Ray is overused for routine screening

• Inspection results are not linked to process control

A proper inspection strategy assigns the right tool to the right inspection task.

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Typical Best-Practice Inspection Strategy

A balanced SMT inspection strategy often includes:

• SPI for solder paste quality control

• AOI for inline surface inspection

• X-Ray for hidden joint analysis and verification

• SPC and process feedback for continuous improvement

This layered approach minimizes false confidence and reduces rework risk.

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When X-Ray Inspection Becomes Necessary

Consider X-Ray inspection when:

• BGA / CSP components are present

• AOI reports pass but failures occur in testing

• Reliability standards are high

• Root-cause analysis is required

• New packages or processes are introduced

In these cases, AOI alone is not sufficient.

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From Inspection Technology to Inspection System

Inspection equipment should not be selected independently.
It should be part of a system-level quality design that considers:

• Production flow

• Inspection frequency

• Line stability

• Engineering resources

👉 Please refer to our Offline SMT Solutions and Factory Upgrade Projects for structured inspection system planning.