PCB X-Ray Inspection for BGA and Hidden Solder Joints

A board can pass AOI and still fail.

That happens more often than people expect.

The reason is simple:

cameras only see what is visible.

Once solder joints move underneath components, optical inspection becomes limited.

That’s where X-ray inspection comes in.

For assemblies using:

• BGA
• QFN
• LGA
• hidden solder pads

X-ray inspection is often the only practical way to verify solder quality.

What Is PCB X-Ray Inspection?

X-ray inspection uses X-ray imaging to look inside PCB assemblies without damaging the board.

Instead of surface images, it shows:

• solder joint quality
• internal connections
• hidden defects

The result is a grayscale image where solder density becomes visible.

This makes it possible to inspect areas that AOI cannot access.

Related reading: AOI Inspection in PCB Assembly: What Can It Detect?

Why X-Ray Inspection Is Important

Modern PCB assemblies continue to become:

• smaller
• denser
• more complex

Many packages now hide solder joints underneath the component body.

Without X-ray:

• solder bridges may go unnoticed
• voids remain invisible
• opens and shorts become harder to identify

For high-reliability products, skipping X-ray can increase field failure risk.

Components That Usually Require X-Ray Inspection

BGA (Ball Grid Array)

The most common use case.

Because solder balls sit under the package:

• optical inspection cannot see them
• electrical testing alone may miss marginal joints

X-ray verifies:

• ball alignment
• solder consistency
• bridging

QFN (Quad Flat No-Lead)

QFN packages often include hidden center pads.

Common problems:

• poor wetting
• insufficient solder
• voiding

LGA Packages

Land Grid Array devices also benefit from X-ray verification.

Especially in:

• RF systems
• automotive electronics

Dense SMT Assemblies

High-density layouts may block visual access, even for visible leads.

Common Defects X-Ray Can Detect

1. Solder Voids

Voids are trapped air pockets inside solder joints.

Small voids are usually acceptable.

Excessive voiding can affect:

• thermal performance
• reliability
• mechanical strength

Especially critical in:

• power electronics
• thermal pads

2. Solder Bridges

Hidden short circuits between pads.

Particularly common in:

• fine-pitch BGA
• dense routing areas

3. Open Solder Joints

Incomplete solder connections.

Can result in:

• intermittent failures
• unstable operation

4. Misalignment

Components may shift slightly during reflow.

X-ray shows whether solder balls align properly.

5. Insufficient or Excessive Solder

Uneven solder distribution becomes visible in X-ray images.

2D vs 3D X-Ray Inspection

2D X-Ray

Most common approach.

Advantages:

• faster
• lower cost
• suitable for production screening

3D CT X-Ray (Computed Tomography)

Provides cross-sectional imaging.

Advantages:

• deeper failure analysis
• internal layer visibility

Typically used for:

• failure investigation
• high-end applications

Related topic: PCB Failure Analysis

X-Ray vs AOI (Quick Comparison)

Feature	AOI	X-Ray
visible solder joints	excellent	good
hidden solder joints	poor	excellent
BGA inspection	limited	strong
speed	faster	slower
cost	lower	higher

In practice:

AOI and X-ray complement each other.

They are not substitutes.

When Is X-Ray Inspection Necessary?

Not every board needs it.

Usually Recommended For

• BGA assemblies
• QFN packages
• automotive electronics
• aerospace systems
• medical devices
• high-density SMT boards

Less Critical For

• simple through-hole boards
• low-density consumer products

How to Improve X-Ray Inspection Results

Common X-Ray Inspection Mistakes

Typical production problems:

• relying only on AOI for BGAs
• ignoring solder void percentages
• skipping X-ray during NPI
• using poor image interpretation standards

X-ray only works when inspection criteria are consistent.

Practical Notes from Real Production

What often happens in factories:

• BGA bridges are usually found with X-ray, not AOI
• many intermittent failures trace back to hidden solder issues
• voiding becomes a major concern in power electronics
• X-ray is most valuable during early production builds

For mature products, sampling inspection is often used instead of 100% inspection.

Conclusion

PCB X-ray inspection plays a critical role in verifying hidden solder joints and ensuring assembly quality.

For BGA, QFN, and high-density PCB assemblies, it helps identify defects that optical inspection cannot detect. While it adds cost, it significantly reduces the risk of hidden failures in high-reliability products.

FAQ