PCB X-Ray Inspection Explained: Non-Destructive Testing for PCB Assembly Quality Control

In the era of miniaturization, modern printed circuit boards (PCBs) face an invisible challenge: verifying the quality of solder joints hidden beneath components. With Ball Grid Array (BGA) packages featuring pitch sizes down to 0.3mm and Quad Flat No-lead (QFN) components concealing critical thermal connections, traditional optical methods are blind to the most critical failure points.

PCB X-ray inspection has emerged as the definitive solution. This guide explores the technical foundations, applications, and acceptance criteria of X-ray inspection, empowering engineers to ensure manufacturing reliability.

PCB X-ray inspection reveals hidden solder joints and voids under a BGA component.

What Is PCB X-Ray Inspection?

PCB X-ray inspection, also known as Automated X-ray Inspection (AXI), is a non-destructive testing (NDT) technique used in PCB assembly to evaluate internal features that are not visible to optical inspection.

AXI uses X-rays to penetrate PCB materials and electronic components, generating transmission images that reveal internal structures such as hidden solder joints beneath BGA, QFN, or LGA packages, solder fill quality in through-hole barrels, and internal wire bonds.

Unlike Automated Optical Inspection (AOI), which inspects only visible surface features using reflected light, X-ray inspection enables the detection of subsurface defects, including voids, insufficient solder, bridging, and misalignment.

As a result, PCB X-ray inspection plays a critical role in ensuring solder joint reliability, especially for high-density, fine-pitch, and advanced packaging technologies.

Why X-Ray Inspection Is Essential for Modern PCBs

As density increases, "line of sight" disappears. Implementing PCB X-Ray Inspection improves manufacturing quality through several key advantages:

● Hidden Joints: BGAs, CSPs, and Flip Chips have solder joints entirely underneath the package, making them invisible to optical inspection.

● High Density: 01005 passives and high-density interconnects (HDI) require precision that manual inspection cannot provide.

● Reliability: For JLCPCB's industrial and automotive clients, verifying void percentages is critical to preventing thermal failure.

● Early Defect Detection and Yield Improvement: Identifying internal defects like voids or bridges before functional testing prevents expensive scrap and rework. Catching issues early significantly boosts First Pass Yield (FPY).

● Feedback Loop for Process Optimization: X-ray data provides quantitative feedback to the SMT line. If void rates rise, engineers can immediately adjust reflow profiles or stencil apertures, preventing defect outbreaks.

How PCB X-Ray Inspection Works

Basic Principle of X-Ray Imaging

X-ray PCB inspection functions based on the differential absorption of X-rays. When photons (30-160 keV) penetrate a PCBA, materials absorb them based on atomic number (Z) and density.

● High Attenuation (Bright White): Lead-free solder alloys (SAC305: Tin-Silver-Copper) with atomic numbers of 50 (Sn), 47 (Ag), and 29 (Cu).

● Moderate Attenuation (Grey): Copper traces (Z = 29).

● Low Attenuation (Dark): The X-ray absorption of the FR-4 substrate material and silicon die (Z = 14) is so low that they are seen as the darkest parts of the image.

This attenuation differential creates gray-scale images where solder joints, component leads, and internal structures become visible. The image contrast follows the Beer-Lambert law:

Where I is the transmitted intensity, I0 is the incident intensity, 𝜇 is the linear attenuation coefficient, and x is the material thickness.

X-ray inspection showing photon attenuation differences between silicon die and tin-lead solder joints.

2D X-Ray vs 3D CT X-Ray Inspection

PCB Components That Require X-Ray Inspection

BGA X-Ray Inspection

Ball Grid Arrays are the primary driver for X-ray adoption. Inspection verifies:

● Ball Shape: Ensuring reflow collapse is uniform.

● Alignment: Checking for offset between the BGA ball and PCB pad.

● Voiding: Quantifying gas pockets within the solder sphere.

QFN / LGA X-Ray Inspection

For Leadless packages, the critical focus is the Thermal Pad.

● Grounding: Ensuring the large central pad is soldered to the PCB ground plane.

● Fillet Inspection: Verifying wetting on the sidewall terminations, though X-ray primarily measures the solder volume underneath.

Through-Hole and Multilayer PCBs

X-ray penetrates the board substrate to inspect Plated Through-Hole (PTH) integrity.

● Barrel Fill: Confirming solder has filled >75% of the barrel vertical height (per IPC Class 2).

● Layer Alignment: In multilayer boards (up to 32 layers at JLCPCB), X-ray ensures drill holes align perfectly with internal copper rings.

Common Defects Detected by PCB X-Ray Inspection

1. Solder Voids and Void Percentage

Voids are air pockets trapped during reflow. Ball Grid Array inspection is the primary driver for X-ray adoption. Modern BGAs with 0.4mm pitch create solder joints completely hidden from optical view.

● Risk: High voiding reduces thermal conductivity and mechanical strength.

● Measurement:

2. Open Joints and Head-in-Pillow (HiP)

Open Joints (Non-Wetting): An open joint occurs when there is a complete physical separation between the component termination and the PCB pad. On an X-ray, this typically appears as a perfectly spherical solder ball (indicating it did not wet and collapse onto the pad) or significantly smaller in diameter than adjacent functional joints.

Head-in-Pillow (HiP): A notorious and subtle defect where the BGA ball rests on the solder paste but fails to coalesce, often due to component warpage or oxidation. Unlike a complete open, HiP creates an intermittent electrical connection. X-ray detects this by identifying a distinct "waist" shape or separation line in the solder profile, which is invisible to optical inspection.

3. Solder Bridging and Shorts

Bridges appear as bright white bands connecting adjacent pads. X-ray is the only way to detect bridges under BGAs or shielding cans.

4. Insufficient or Excessive Solder

By analyzing grayscale density (thickness), algorithms determine if a joint is "starved" (insufficient solder volume) or bloated (excessive), which could lead to shorts

5. QFN Thermal Pad Voiding

Quad Flat No-lead packages concentrate connections on a central ground pad. Excessive voids (>20-30%) compromise thermal dissipation. X-ray imaging measures the actual solder volume and wetting angles along the component edge.

X-ray inspection image displaying typical SMT assembly defects, including BGA voids, solder bridges, and head-in-pillow defects.

PCB X-Ray Inspection vs Other Inspection Methods

X-Ray Inspection vs AOI (Automated Optical Inspection)

● AOI: Uses cameras/light. Best for component presence, polarity, and visible solder fillets (0201s, SOIC). Cannot see under packages.

● X-Ray: Uses photons. Essential for BGA/QFN. Slower than AOI.

X-Ray Inspection vs ICT (In-Circuit Testing)

● ICT: Uses electrical probes. Verifies resistance, capacitance, and logic. Finds "dead" parts or hard shorts.

● X-Ray: Verifies physical structure. Finds "marginal" joints (like HiP or excessive voiding) that might pass ICT electrically but fail later mechanically.

When X-Ray Inspection Is Necessary - and When It Is Not

● Necessary: BGAs, LGAs, CSPs, QFNs with thermal pads, Metal Can Shields.

● Not Necessary: Simple SMT (resistors/caps) and SOIC packages where optical inspection (AOI) provides sufficient coverage faster and cheaper.

IPC Standards and Acceptance Criteria for X-Ray Inspection

Relevant IPC Standards

● IPC-A-610: "Acceptability of Electronic Assemblies."

● IPC-7095: "Design and Assembly Process Implementation for BGAs."

Void Acceptance Guidelines for BGA and QFN (IPC Class 2)

The quality control standards of JLCPCB follow the IPC Class 2 criteria with strict adherence in all assembly classes. Thus, through robotics and consumer applications, we guarantee the quality of their products or services.

Limitations of PCB X-Ray Inspection

What X-Ray Cannot Detect

● Electrical Logic: It cannot tell if a chip is fried internally or blank, only if it is soldered correctly.

● Zero-Contrast Materials: Aluminum wire bonds on aluminum pads (common in some COB tech) are hard to image without an advanced phase-contrast X-ray.

● Cold Solder Joints (Sometimes): If the solder shape is perfect but the metallurgical bond is weak, X-ray might miss it (unlike physical pull testing).

Cost and Throughput Considerations

● Bottleneck: X-ray is significantly slower than AOI. 100% X-ray inspection is often reserved for specific components (BGAs) rather than the entire board to maintain line throughput.

● Interpretation: While AI is improving, complex images (like CT reconstructions) often still require skilled operator review.

Technical Specifications and Selection Criteria for SMT X-Ray Inspection

Key Performance Parameters of SMT X-Ray Inspection Systems

Resolution: It is influenced by the size of the X-ray focal spot (1-10 μm) and the pitch of the detector pixels. The highest useful magnification is around 1000 times the size of the focal spot.

Tube Voltage: Selected based on sample thickness:

○ 30-50 kV: Thin PCB assemblies (<1 mm) with low copper content.

○ 60-90 kV: Standard SMT assemblies (1.6mm FR-4).

○ 100-160 kV: Thick and multilayered (3-5mm), with a high copper percentage.

Detector Technology: Flat-panel detectors (Amorphous silicon or CMOS) have been preferred for their excellent image consistency and linearity over the older image intensifier technology.

Typical values shown below are indicative ranges. Actual performance depends on system architecture, focal spot size, detector resolution, and inspection mode (2D vs. CT).

Choosing a PCB Assembly Manufacturer with X-Ray Capability

When sourcing PCB assembly service, asking the right questions ensures your BGA designs are safe.

What to Ask Your PCB Assembly Supplier About X-Ray Inspection

1. Is X-Ray inspection Inline or Offline?

○ Why ask: Inline allows for 100% inspection of critical parts in mass production without bottlenecks. Offline is typically used for sampling or detailed failure analysis (NPI).

○ At JLCPCB: We integrate automated inspection directly into the quality control workflow, performing verification on BGA and QFN components as a standard inclusion.

2. What type of X-Ray technology do you use (2D vs. 3D/CT)?

○ Why ask: For double-sided boards, basic 2D X-ray struggles with "shadowing" from components on the opposite side. 2.5D or 3D CT is essential for accurate defect detection in complex assemblies.

○ At JLCPCB: Our facilities utilize high-resolution systems capable of handling complex, double-sided layouts to ensure clear defect visualization.

3. What are your Reporting Capabilities?

○ Why ask: A simple "pass" isn't enough for high-reliability projects. You need to verify if the supplier provides actual X-ray images, Void Percentage Heatmaps, or merely a text-based "Pass/Fail" log. Access to raw data is crucial for debugging reflow profiles.

○ At JLCPCB: We believe in transparency. Customers can request sample X-ray images and void analysis data, ensuring that the "Pass" status is backed by objective IPC-A-610 Class 2 data.

4. What is your Standard Sampling Strategy (100% vs. AQL)?

○ Why ask: Some manufacturers only inspect 1 out of 50 boards (AQL sampling) to save time. For BGAs, 100% inspection is often preferred to catch random process anomalies.

○ At JLCPCB: We apply X-ray inspection for all BGA and QFN components to verify solder joint quality, ensuring no board leaves the line with hidden defects.

5. Are your operators following IPC standards?

○ Why ask: Automated systems still flag "marginal" defects that require human judgment. Operators must be trained to distinguish between a harmless artifact and a true defect.

○ At JLCPCB: Our Quality Assurance team is trained on IPC-A-610 and IPC-7095 standards to make accurate, consistent accept/reject decisions.

JLCPCB PCBA quality control workflow showing the integration of SPI, AOI, and X-ray inspection stages.

Future Trends in X-Ray Inspection Technology

The new capabilities that are changing the PCB X-ray inspection world are:

● AI-Powered Defect Classification: Deep learning models that have been trained on millions of defect images can classify them with an accuracy that is more than 99%, thus cutting down the number of false calls and operator fatigue.

● Higher Resolution Micro-Focus: New types of tubes with <1 μm focal spots can give 5000-10000× magnification, which is very important for Fan-Out Wafer-Level Packaging (FOWLP).

● Real-Time 3D Imaging: Inverse geometry X-ray systems that have multiple detectors can get a CT dataset in seconds, thus making true inline 3D inspection possible for mass production.

Summary: Is PCB X-Ray Inspection Necessary for Your Project?

If your design includes BGAs, QFNs, LGAs, or ultra-fine pitch CSPs, PCB X-ray inspection is not optional - it is mandatory. Optical inspection simply cannot verify the reliability of these connections. By understanding the physics of attenuation and the standards, you can better communicate your quality requirements.

From basic 2D void analysis to advanced 3D CT reconstruction, X-ray inspection provides the data needed to guarantee reliability. Whether you are building consumer electronics or industrial controllers, designing with testability in mind - and choosing a manufacturer like JLCPCB that prioritizes transparency - is the key to success.

Ready to leverage professional-grade quality control? Explore JLCPCB's PCB Assembly service today and ensure your BGA and QFN designs are built to last.

FAQs

Q1. Does X-ray inspection damage sensitive semiconductor components?

In most cases, the answer is no. The modern X-ray inspection systems used for SMT take advantage of very low radiation levels, which are far below the limit that causes damage to silicon or trapping of charges in EEPROM/Flash memory, thus facilitating the inspection of active components with no risk.

Q2. What is the IPC standard for allowable voiding in BGA X-ray inspection?

The IPC-A-610 Class 2 (the standard that JLCPCB adheres to) allows for individual voids of 25% of the ball area, which accounts for 30% cumulative voiding. Class 3 applications have a stricter standard (<15% individual voids). However, these applications usually require special manufacturing agreements to be set up.

Q3. How does 3D X-ray (CT) differ from Laminography?

Both techniques provide depth information, but Computed Tomography (CT) usually does a complete rotation of the sample at 360 degrees for volumetric reconstruction. Laminography, on the other hand, does scanning by taking layers (slices) without performing the full rotation, which makes it faster and more suitable for flat PCBA panels in production environments.

Q4. Can X-ray inspection detect microcracks in solder joints?

Microcracks (1-2 μm) can be detected by the high-resolution X-ray CT that reveals dark linear features within the bright solder volume. But for the extremely fine cracks (<1 μm) or delamination, complementary testing methods such as acoustic microscopy (C-SAM) or destructive cross-sectioning may be required.

Q5. How does Automated X-Ray Inspection (AXI) differ from manual inspection?

The AXI systems are capable of processing 60-200 boards per hour using CAD-programmed coordinates along with AI-based defect recognition. Human-powered systems have to rely on the image positioning and interpretation done by the operators, resulting in a throughput of only 10-30 boards per hour. Although manual inspection is less costly for prototypes, AXI is unavoidable for mass production, even with a longer setup time.

Q6. What X-ray tube voltage is optimal for thick multilayer PCBs?

The best range for standard 1.6mm FR-4 boards is 60-80 kV. For thick multilayer boards (3-5mm) with heavy copper, 90-130 kV is needed for good penetration. A common practice is to raise the voltage 10-15 kV for each extra millimeter of thickness to guarantee the desired contrast without penetration going too far.