PCB X-Ray Inspection (AXI): Complete Guide

A solder joint hidden under a ball grid array can be badly voided or bridged, and no optical camera can see it. X-ray inspection looks through the board to find these hidden defects, which is why it is the standard method for verifying BGA, QFN, and other components with hidden connections on high-reliability assemblies.

This guide covers PCB X-ray inspection: what automated X-ray inspection (AXI) detects, how the systems work, the difference between 2D and 3D X-ray and CT, when X-ray is required, and how it complements automated optical inspection.

What Is PCB X-Ray Inspection?

Definition

PCB X-ray inspection, also called automated X-ray inspection (AXI), is a non-destructive method that passes X-rays through a circuit board to reveal its internal structure. The method is used to inspect solder joints, internal traces, and components hidden beneath packages, where no optical camera can reach. X-ray is widely used in medical, aerospace, and automotive electronics, where hidden-joint defects can cause field failures.

X-ray inspection is an inspection method rather than an electrical test. It finds physical defects such as solder bridges, shorts, voids, and poor wetting inside joints, but it does not verify electrical function. In the full test flow, X-ray complements the PCB testing guide sequence: AOI checks visible joints, X-ray checks hidden joints, and ICT plus functional test verify electrical integrity.

Where X-ray fits in production

X-ray inspection is applied selectively, because it is slower and more expensive per board than AOI. Production programs use it on boards with hidden joints, on high-reliability assemblies, and on first articles to validate the assembly process. Once the process is proven, X-ray often runs on a sample of production boards rather than every one.

Boards that benefit most include those with BGA, QFN, chip-scale packages, and other components whose solder connections sit under the body. For these boards, X-ray is the only production inspection method that can verify the hidden joints, and skipping it leaves the highest-risk connections unverified.

Why hidden joints change the test plan

The presence of hidden joints changes what a test plan must verify. On a board with only visible components, AOI plus electrical test covers the risk. On a board with BGAs and QFNs, a portion of the assembly is invisible to optics, so the test plan must add X-ray or accept that those connections ship unverified. For high-reliability products, shipping hidden joints unverified is not an option.

This is why X-ray appears in test plans for modern, dense electronics. As components shrink and packages hide their connections, the share of a board that is optically invisible grows, and X-ray becomes a necessary part of the inspection mix rather than an optional extra.

How X-Ray Inspection Works

Projection and density

An X-ray inspection system works on the principle that different materials absorb X-rays to different degrees. The system generates X-rays from a source, passes them through the board, and captures the result on a detector to produce a grayscale image. Denser materials such as copper and solder absorb more X-rays and appear darker; lighter materials allow more through and appear brighter.

This density difference is what lets X-ray reveal internal structure. A solder joint inside a BGA appears as a defined shape against the surrounding material, and a void inside that joint appears as a lighter area where the solder is missing. The contrast between materials is what makes hidden defects visible.

System components

An X-ray inspection system has three core components: the radiation source that generates X-rays, the sample stage that holds and positions the board, and the detector that captures the transmitted X-rays. Between them, the system controls magnification and viewing angle, letting the operator zoom into specific joints and rotate the board for a better view.

Modern systems can view boards at oblique angles, which reveals solder joints and through-holes from the side, and can inspect both sides of a populated board. The ability to position the board and adjust magnification is what makes X-ray inspection useful for both production screening and detailed failure analysis.

What the operator sees

The X-ray image is interpreted by a trained operator or by automated analysis software. In production screening, automated tools compare the captured image against a reference and flag anomalies, matching the pattern-recognition approach used across electronics inspection. In failure analysis, an operator reads the image in detail, zooming into specific joints and adjusting angle and contrast to characterize a defect.

Interpreting X-ray images takes skill, because the same density signals can have different causes. A dark spot may be excess solder or a foreign object; a light area may be a void or a missing ball. The combination of automated screening and operator analysis gives X-ray both the speed for production and the depth for investigation.

What X-Ray Inspection Detects

Hidden solder defects

X-ray inspection detects defects that optical inspection cannot see. The primary targets are solder joints hidden beneath components: BGA solder balls, QFN pads, and the connections of chip-scale packages. For these joints, X-ray reveals bridging between adjacent balls, voids inside the solder, poor wetting, and open connections that would pass a visual check.

Beyond hidden joints, X-ray sees internal board features. It can inspect inner-layer traces for shorts and opens, verify through-hole barrel integrity, and check the alignment of internal structures in multilayer boards. This makes X-ray useful not only for assembly defects but also for verifying that the fabricated board’s internal layers are sound.

Internal traces and shorts

X-ray inspection finds internal shorts that no surface inspection can catch. A short between two inner layers of a multilayer board is invisible from the outside, but X-ray reveals the unintended connection. Similarly, an open in an inner trace shows as a break in the expected conductive path.

For boards with high layer counts, X-ray is the practical way to verify internal integrity without destroying the board. The alternative is microsectioning, which cuts the board open and destroys the sample. X-ray provides the same visibility non-destructively, which is why it is preferred for screening and for inspecting boards that must remain usable.

Misplaced and damaged components

X-ray also reveals component issues that optics cannot confirm. It can detect a component placed on the wrong pad, a chip with damaged internal die bonds, and a connector with a cracked solder joint hidden beneath its body. These defects may not change the board’s electrical response at first, but they fail under thermal or mechanical stress later.

For warranty-driven industries such as automotive and medical devices, catching these hidden component defects at X-ray inspection is what separates a reliable product from a field-failure-prone one. The inspection adds cost per board, but it is small next to the cost of a recall or a field failure in a safety-critical system.

2D vs 3D X-Ray & CT Scan

2D X-ray

Two-dimensional X-ray produces a single projection image of the board, where all layers are superimposed. It is the fastest and most common form of X-ray inspection, suitable for locating bridging, misplaced components, and gross internal defects. 2D X-ray is effective for most BGA solder-joint screening and for checking component placement on hidden connections.

The limitation of 2D X-ray is that it compresses the board’s depth into one image. A defect on one layer can be hidden behind another structure, and the exact vertical location of a defect is not clear. For many production checks this is acceptable, but for detailed analysis and for multilayer boards where layer separation matters, 3D methods are needed.

3D X-ray and CT scanning

Three-dimensional X-ray and computed tomography (CT) reconstruct the board in slices, separating the layers so each joint can be examined in isolation. CT captures images from multiple angles and reconstructs a volumetric model, letting the operator measure voids inside a BGA ball, inspect a specific layer, and quantify defects against acceptance criteria.

The trade-off is speed and cost. 3D and CT inspection is slower and more expensive per board than 2D, so it is reserved for first articles, high-reliability boards, and failure analysis rather than line-rate screening. The BGA X-ray inspection article covers when 3D analysis is worth the extra cost.

Choosing 2D vs 3D

The choice depends on the board and the risk. For production screening of visible and hidden joints on standard boards, 2D X-ray is usually enough. For void measurement on critical BGAs, for multilayer boards needing layer separation, and for failure analysis, 3D or CT provides the depth that 2D cannot.

A practical approach is to run 2D X-ray for line-rate screening and reserve 3D or CT for first articles, critical components, and failures. This balances cost against the confidence needed, which is the same logic used across the PCB testing guide for choosing inspection depth.

When CT adds value

Computed tomography adds the most value in two situations. The first is void measurement on critical BGAs, where a 3D slice separates each ball and gives an accurate void percentage that a 2D projection cannot isolate. The second is multilayer failure analysis, where CT separates layers to find the exact internal defect responsible for a failure.

CT is also useful for verifying complex assemblies such as connectors, through-hole joints, and castellations where a single projection image cannot resolve the three-dimensional structure. For these cases, the extra cost of CT buys certainty that 2D cannot provide, which is why it is the tool of choice for high-reliability qualification and failure investigation.

X-Ray vs AOI

Automated optical inspection (AOI) and X-ray inspection both find physical defects, but they see different things. AOI photographs the board’s surface with cameras and compares the images to the design; X-ray looks through the board. The two methods divide the inspection work by visibility.

Dimension	AOI	X-ray
What it sees	Visible surface joints, placement	Hidden joints, internal layers
Speed	Line rate, every board	Slower, often sampled
Cost per board	Low	Higher
Hidden joints (BGA/QFN)	Cannot see	Can verify
Internal layers	Cannot see	Can inspect

AOI is the workhorse of surface inspection, running on every board to catch placement errors, missing parts, and visible solder defects. X-ray adds the depth AOI lacks, verifying hidden joints and internal structures. The two are complementary, not competing: AOI covers the surface, X-ray covers what is underneath.

A common production recipe runs AOI on every board and X-ray on high-risk hidden joints, first articles, and boards with strict reliability requirements. This gives surface coverage at line rate and hidden-joint coverage where it matters, without the cost of X-raying every board.

There is also a place for X-ray on boards without BGAs. Through-hole joints, connectors, and castellations can hide defects that optics miss, and high-reliability programs X-ray these features on samples. The balance is set by the same logic as the rest of the test plan: inspect where the risk is, and sample where the process is proven.

BGA X-Ray Inspection & Void Control

Why BGA needs X-ray

A ball grid array hides its solder connections entirely beneath the component body, so no optical method can inspect them. X-ray is the standard tool: it sees the full array of solder balls, detects bridging between adjacent balls, and reveals voids and poor wetting inside each joint. QFN packages present the same problem, with pads hidden under the component, and the same X-ray solution.

BGA inspection matters because a single bad joint can fail intermittently or in the field. A bridged pair of balls shorts two signals. A voided ball has reduced mechanical and thermal connection. Both pass visual inspection and both are invisible until X-ray reveals them.

Void measurement and acceptance

Void control is the critical acceptance criterion for high-reliability BGA assemblies. A void is a gas pocket trapped inside the solder ball during reflow, and excessive voids weaken the joint’s mechanical and thermal path. X-ray inspection quantifies void percentage and flags balls that exceed the threshold for the application.

For assembly X-ray evaluation, J-STD-001J Appendix C provides general X-ray guidance, while IPC-7095E provides BGA-specific design, assembly, and inspection guidance. When a voiding result exceeds the acceptance limit applicable to the package and product requirement, the data should feed back to the reflow process before the next batch.

Feeding data back to the process

X-ray inspection of BGAs produces data, not just pass-fail results. Void percentage, ball shape, and bridging patterns tell the process engineer what went wrong: a reflow profile too fast, a paste volume too low, or a stencil issue. This feedback loop turns X-ray from a quality gate into a process-control tool.

On high-reliability lines, X-ray BGA inspection runs on first articles and on samples from production, with 3D analysis used where void measurement is critical. The data feeds yield analysis and process correction, which is why X-ray is an investment in process control rather than only a screening step.

X-ray inspection connects to the wider reliability program. The hidden-joint defects it finds, such as voids and bridging, are the same defects that later appear in reliability testing under thermal cycling and mechanical stress. Catching them at X-ray inspection prevents them from surfacing in the PCB reliability testing program or, worse, in the field. Cleanliness and contamination checks, covered in the ionic contamination testing article, complete the picture alongside X-ray for assemblies that must survive harsh environments.

When to Use X-Ray Inspection

X-ray inspection is not needed on every board. It earns its cost when hidden joints or internal structures carry real risk. The scenarios below are where X-ray delivers the most value.

• Boards with BGA, QFN, or chip-scale packages. Hidden joints cannot be verified any other way in production.
• High-reliability applications. Medical, aerospace, automotive, and telecommunications boards need void and bridging control that only X-ray provides.
• Multilayer boards. Internal shorts and layer registration can only be inspected non-destructively with X-ray.
• First articles. Validating the assembly process on the first boards before committing to production.
• Failure analysis. When a board fails in the field, X-ray examines hidden joints and internal structure without destroying the evidence.
• Process control. Monitoring void rates and bridging on critical components to keep the reflow process in control.

For boards without hidden joints, AOI plus electrical test is usually sufficient, and X-ray adds little. The decision to use X-ray should follow the risk of hidden connections and the reliability requirements of the application.

A useful rule is to X-ray every board design with hidden joints on the first article, then sample from production at a frequency set by the process history. If the process is stable and void rates are low, sampling can be reduced. If defects appear, sampling increases until the process is corrected. This data-driven approach keeps X-ray cost proportional to the risk it is managing.

X-Ray Equipment & Cost Considerations

Equipment types

X-ray inspection systems range from inline 2D machines that screen boards on the line to offline 3D and CT systems that examine samples in detail. Inline 2D systems suit high-volume boards with known hidden joints, running at line speed to catch bridging and voids. Offline 3D and CT systems give failure-analysis teams the resolution to measure voids and inspect internal layers.

The capability of a system is set by its X-ray source power, magnification range, detector resolution, and the angle at which it can view the board. High-end systems add CT-style volumetric imaging, letting operators slice the board digitally. Choosing between them depends on the board mix and the confidence level the application demands.

Outsourced vs in-house

Owning an X-ray system is a capital decision. A capable system costs a significant investment, plus maintenance and operator training, and it makes sense only when inspection volume justifies it. For many manufacturers, outsourcing X-ray inspection to a contract partner is more economical, paying per board rather than for equipment that sits idle.

The decision follows volume and control. High volume with critical hidden joints may justify in-house equipment. Low and variable volume favors outsourcing, which also gives access to 3D and CT analysis without the capital outlay. A contract manufacturing partner with in-house X-ray offers both options under one roof.

What to check when choosing a partner

When outsourcing X-ray inspection, verify the partner’s equipment can handle your board types and defect criteria. Ask which systems they run, whether they offer 3D and CT in addition to 2D, and how they apply acceptance limits such as void thresholds. Confirm they can inspect both sides of populated boards and the board sizes you produce.

Also confirm the partner feeds inspection data back, not just pass-fail results. The value of X-ray is in the void rates and defect patterns it reveals, which only help if they reach your process team. A partner that reports data and works with you on corrective action turns X-ray from a cost into an improvement driver.

X-Ray Inspection Checklist

Use this checklist to plan X-ray inspection for boards with hidden joints and high-reliability requirements.

• Identify all hidden joints (BGA, QFN, chip-scale) that require X-ray
• Choose 2D for line screening, 3D/CT for void measurement and failure analysis
• Define void and bridging acceptance limits against IPC-A-610
• Inspect first articles to validate the assembly process
• Plan sampling frequency for production boards
• Feed void and defect data back to the reflow process
• Confirm the partner’s X-ray capability for your board types

FAQ

What is PCB X-ray inspection?

PCB X-ray inspection, also called automated X-ray inspection (AXI), is a non-destructive method that passes X-rays through a circuit board to reveal its internal structure. It inspects hidden solder joints, internal traces, and components beneath packages that optical cameras cannot see.

What does X-ray inspection detect?

X-ray detects solder bridging, voids, poor wetting, and opens in hidden joints, plus internal shorts, open traces, and layer issues in multilayer boards. It is the standard method for verifying BGA, QFN, and chip-scale connections.

What is the difference between 2D and 3D X-ray?

2D X-ray produces a single projection image with all layers superimposed, which is fast and suitable for screening. 3D X-ray and CT reconstruct the board in slices, separating layers for void measurement and internal analysis, but are slower and more expensive.

Is X-ray inspection destructive?

No. X-ray inspection is non-destructive, meaning inspected boards continue into production unchanged. This is its advantage over microsectioning, which cuts a board open and destroys the sample.

When is X-ray inspection needed?

X-ray is needed when a board has hidden joints such as BGA or QFN, internal structures that must be verified, or high-reliability requirements for medical, aerospace, automotive, or telecommunications applications. It is also used for first articles and failure analysis.

What is the difference between X-ray and AOI?

AOI photographs the board’s surface and detects visible defects, while X-ray looks through the board to detect hidden joints and internal structures. They are complementary: AOI covers the surface, X-ray covers what is underneath.

How does X-ray inspection detect BGA voids?

X-ray sees the density difference between solder and the void inside a ball. A void appears as a lighter area within the solder. 3D or CT analysis measures the void percentage, which is checked against acceptance limits defined by standards such as IPC-A-610.

Do I need X-ray inspection on every board?

No. X-ray is slower and costlier per board than AOI, so it is applied selectively: on boards with hidden joints, on high-reliability assemblies, on first articles, and on production samples. Boards without hidden joints are usually covered by AOI plus electrical test.

Can X-ray inspect both sides of a populated board?

Yes. Modern X-ray systems can inspect both sides of a populated board, and many can view joints at oblique angles for a clearer look at solder connections and through-holes. This is essential for double-sided assemblies where hidden components sit on both sides.