Xray Inspection

xray inspection: what this playbook covers (and who it’s for)

This guide is designed for Quality Engineers, NPI Managers, and Procurement Leads who are responsible for validating complex PCB assemblies (PCBA). If your design includes Ball Grid Arrays (BGAs), Quad Flat No-leads (QFNs), or high-density connectors where solder joints are hidden from the naked eye, relying solely on visual checks is a liability. This playbook focuses on xray inspection as a critical gatekeeper for reliability, moving beyond basic "pass/fail" metrics into actionable specifications and risk mitigation.

At APTPCB (APTPCB PCB Factory), we often see inspection strategies fail not because the technology is lacking, but because the requirements were ambiguous. This guide helps you define exactly what you need from your manufacturing partner—from void percentage limits to image resolution requirements—ensuring that your "black box" components are soldered correctly before they leave the factory floor.

You will find a structured approach to defining inspection criteria, a breakdown of hidden risks that standard 2D X-ray might miss, and a validation plan to correlate X-ray data with physical reality. We also provide a buyer-ready checklist to include in your Request for Quote (RFQ) packages, ensuring your suppliers are capable of the level of scrutiny your product demands.

When xray inspection is the right approach (and when it isn’t)

X-ray inspection is not a universal replacement for optical methods; it is a specialized tool for specific geometries. Understanding where it fits in the quality control ecosystem prevents over-spending on unnecessary tests while ensuring critical coverage.

Use X-ray Inspection when:

Hidden Solder Joints: Components like BGAs, CSPs (Chip Scale Packages), LGAs, and QFNs have pads underneath the package body. Optical line-of-sight is blocked.
Multi-layer Barrel Fill: You need to verify through-hole solder penetration (barrel fill) in thick, multi-layer PCBs where visual inspection from the bottom side is inconclusive due to heat sinks or shielding.
Wire Bond Verification: In Chip-on-Board (COB) or complex IC packaging, checking for wire sweep or broken bond wires requires X-ray penetration.
Void Analysis: You must quantify the percentage of gas voids within a large thermal pad (e.g., under a power FET or QFN) to ensure thermal conductivity meets IPC specs.

Do not rely solely on X-ray when:

Surface Defects: For checking component polarity, text markings, or solder bridging on visible leads, aoi inspection (Automated Optical Inspection) is faster, cheaper, and higher resolution.
Solder Paste Volume: To prevent defects before reflow, spi inspection (Solder Paste Inspection) is superior. X-ray is a post-reflow detective; SPI is a pre-reflow preventative.
Electrical Function: X-ray confirms structural integrity, not electrical connectivity. A joint can look good on X-ray (head-in-pillow) but fail electrically. It must be paired with ICT or Functional Test.

Specs & requirements (before quoting)

To get a reliable quote and a reliable product, you must move beyond asking for "X-ray test" and specify the parameters. Ambiguity here leads to suppliers using low-resolution settings to save time.

Void Percentage Limit (IPC Class): Explicitly state the maximum allowable void percentage. For IPC-A-610 Class 2, this is typically <25% area. For Class 3 or high-reliability power applications, you may require <15% or <10%.
Largest Void Diameter: In addition to total percentage, specify if single large voids are prohibited (e.g., "No single void >50% of pad diameter") to prevent stress concentration.
BGA Ball Diameter & Collapse Height: Define the target diameter and collapse height for BGA balls. This helps detect "snowman" defects where the ball hasn't reflowed properly.
Image Resolution (Microns): Specify the required resolution based on your smallest feature. For standard BGAs, 5-10 microns is sufficient. For micro-BGAs or flip-chips, you may need <1 micron capability.
Tilt/Oblique Angle Capability: Require capability for oblique viewing (e.g., 45-70 degrees). Straight-down (2D) X-ray often misses Head-in-Pillow defects.
Sampling Rate (AQL vs. 100%): Define if you need 100% inspection (every board, every BGA) or a statistical sample (e.g., AQL 0.65). 100% inspection significantly increases cycle time and cost.
Image Retention Policy: Dictate how long X-ray images must be stored. For automotive or aerospace, you may need images retained for 5-10 years for traceability.
False Call Rate (False Failure): Set expectations for false calls if using automated X-ray (AXI). High false calls stop the line; you want a process tuned for <500 ppm false calls.
Component Specifics: List exactly which reference designators (e.g., U1, U12) require X-ray. Do not leave it to the operator to guess which parts are critical.
Radiation Sensitivity: If your board contains flash memory or sensitive sensors, specify maximum radiation exposure limits to prevent data corruption or sensor damage.
Report Format: Define the output format. Do you need a PDF summary, or raw DICOM/TIFF images for your own analysis?
Rework Verification: Explicitly state that any reworked BGA must undergo 100% X-ray inspection to verify the repair.

Hidden risks (root causes & prevention)

Scaling from a prototype to mass production introduces variables that can render your initial X-ray validation ineffective. These risks often hide in the process variations.

Head-in-Pillow (HiP) Defects:
- Risk: The BGA ball deforms into the paste but doesn't metallurgically bond (like a head on a pillow).
- Why: Warpage during reflow or insufficient flux activity.
- Detection: Invisible in top-down 2D X-ray. Requires oblique/angled views or 3D Laminography.
- Prevention: Use high-activity flux, control warpage, and mandate angled inspection.
Champagne Voids:
- Risk: Tiny voids gathered at the interface of the ball and the pad, creating a weak fracture point.
- Why: Outgassing from via-in-pad plating chemistry.
- Detection: Very difficult to see due to small size; requires high-magnification X-ray.
- Prevention: Strict PCB fabrication control (plating quality) and baking boards before assembly.
Shadowing from Double-Sided Assembly:
- Risk: Components on the bottom side block the X-ray view of top-side components.
- Why: High-density layouts place large capacitors or inductors directly under BGAs.
- Detection: Images look cluttered and unreadable.
- Prevention: Design for Test (DFT) review to stagger critical components or use 3D X-ray (Laminography) which slices through layers.
Insufficient Barrel Fill (THT):
- Risk: Through-hole pins look soldered from the top and bottom, but the center is empty.
- Why: Insufficient heat or wave height during soldering.
- Detection: X-ray shows a "hourglass" shape in the solder column.
- Prevention: Optimize wave profile and use X-ray to dial in pre-heat settings.
False Confidence in "Pass":
- Risk: Operator passes a marginal board because the image is blurry or settings are loose.
- Why: Lack of "Golden Sample" comparison or operator fatigue.
- Detection: Audit the audit. Re-inspect a sample of "passed" boards.
- Prevention: Implement Automated X-ray Inspection (AXI) to remove operator subjectivity.
Radiation Damage to Memory:
- Risk: Erasure or corruption of pre-programmed firmware in MCUs or Flash.
- Why: High-energy photons can flip bits in floating gate transistors.
- Detection: Functional test fails after X-ray.
- Prevention: Shield sensitive parts or limit exposure time/kV settings.
Throughput Bottlenecks:
- Risk: X-ray becomes the slowest step, reducing line output.
- Why: 100% inspection requirement on a slow machine.
- Detection: WIP (Work in Progress) piles up at the X-ray station.
- Prevention: Switch to sampling (AQL) after process stability is proven, or invest in faster inline AXI.
Interpretation Disagreements:
- Risk: Supplier says "Pass", you say "Fail".
- Why: Subjective interpretation of grayscale images.
- Detection: Arguments over rejected lots.
- Prevention: Establish a "Defect Library" with agreed-upon images of Pass/Fail boundaries before production starts.

Validation plan (what to test, when, and what “pass” means)

A robust validation plan moves you from "hoping it's good" to "proving it's good." This plan correlates X-ray data with physical reality.

Golden Sample Creation:
- Objective: Establish a baseline for a perfect solder joint.
- Method: Assemble 5 boards, verify with X-ray, and confirm 100% functionality.
- Acceptance: Images are clear, sharp, and saved as the reference standard.
Defect Seeding (The "Red Rabbit"):
- Objective: Prove the X-ray machine can actually catch defects.
- Method: Intentionally create defects (missing ball, bridged pads, insufficient paste) on a test board.
- Acceptance: The X-ray operator or machine must correctly identify 100% of the seeded defects.
Cross-Section Correlation (Destructive):
- Objective: Verify X-ray measurements against physical reality.
- Method: Take a board that passed X-ray, cut and polish the BGA interface (microsectioning).
- Acceptance: The physical void percentage and IMC (Intermetallic Compound) layer match the X-ray interpretation.
Dye and Pry Test (Destructive):
- Objective: Detect "Head-in-Pillow" or open joints that X-ray might miss.
- Method: Inject red dye under the BGA, cure, and pry the component off.
- Acceptance: No dye should be present on the solder joint interface (dye indicates a gap/crack).
Gauge R&R (Repeatability):
- Objective: Ensure the measurement system is consistent.
- Method: Have the operator measure the same void percentage on the same board 10 times.
- Acceptance: Variation should be less than 10%.
First Article Inspection (FAI) Report:
- Objective: Formal approval of the first production run.
- Method: 100% X-ray of the first 5-10 boards with detailed void analysis reports.
- Acceptance: All critical components meet IPC Class 2/3 specs; report is signed off by your engineer.
Inline AXI Tuning (If applicable):
- Objective: Optimize speed and false calls.
- Method: Run 50 known-good boards through the machine.
- Acceptance: False call rate < 500ppm; cycle time matches line beat rate.
Heat Sink Interference Check:
- Objective: Ensure final assembly doesn't block inspection.
- Method: X-ray the board after heat sinks or shields are attached.
- Acceptance: Critical joints are still visible, or inspection is moved to an earlier process step.
Data Retention Test:
- Objective: Verify traceability.
- Method: Request X-ray images for a specific serial number from a previous week.
- Acceptance: Supplier retrieves correct images within 4 hours.
Radiation Safety Audit:
- Objective: Ensure component safety.
- Method: Verify machine settings (kV, mA, time) against component datasheets.
- Acceptance: Settings are below damage thresholds for sensitive ICs.

Supplier checklist (lity. We also provide a buyer-ready checklist to include in your Request for Quote (RFQ) + audit questions)

Use this checklist to vet APTPCB or any other manufacturing partner. It separates capable suppliers from those who just "have a machine."

Group 1: RFQ Inputs (What you send)

Critical Component List: Defined list of Ref Des (U1, U2, etc.) requiring inspection.
Acceptance Standard: IPC-A-610 Class 2 or Class 3 specified.
Void Criteria: Specific % limit (e.g., Max 25% total, Max 10% single void).
Reporting Requirement: PDF summary vs. full raw image delivery.
Volume Strategy: 100% inspection vs. AQL sampling plan.
CAD/Gerber Files: Provided to help program automated machines.
Stackup Details: Copper weight and layer count (affects X-ray penetration power needed).
Panelization: Defined panel array (affects machine travel and speed).

Group 2: Capability Proof (What they must show)

Machine Type: 2D (Transmission) vs. 2.5D (Oblique) vs. 3D (CT/Laminography).
Resolution: Minimum feature recognition size (e.g., < 2 microns).
Tube Voltage: Sufficient kV (e.g., 130kV+) to penetrate your thickest board.
Detector Type: Flat panel detector (digital) preferred over image intensifier (analog).
Oblique Viewing: Can the machine tilt >45 degrees to see BGA ball shape?
Automated Analysis: Software capability to auto-calculate void % (removes human error).
Max Board Size: Does the machine fit your panel dimensions?
Defect Library: Examples of defects they have successfully caught recently.

Group 3: Quality System & Traceability

Serialization: Is X-ray data linked to the PCB serial number?
Image Storage: Are images stored on a local drive or a central server? Backup frequency?
Operator Certification: Are inspectors trained to IPC-A-610 standards?
Calibration: Is the machine calibrated annually? (Ask for the sticker/cert).
Non-Conforming Material: Process for segregating boards that fail X-ray (red bin/locked conveyor).
Rework Loop: Does the system force a re-inspection after rework?

Group 4: Change Control & Delivery

Process Change Notification (PCN): Will they notify you if they change X-ray machines?
Capacity Planning: Do they have enough X-ray capacity for your peak volume?
Bottleneck Management: Plan for if the X-ray machine goes down (backup machine?).
Throughput: Estimated inspection time per board vs. production line beat rate.
Reporting Lead Time: How quickly after production are reports available?
Escalation Path: Who decides on "borderline" images?

Decision guidance (trade-offs you can actually choose)

Engineering is about trade-offs. Here is how to navigate the decisions around X-ray inspection.

2D vs. 3D (CT) Inspection:
- If you prioritize Cost and Speed: Choose 2D X-ray. It’s fast and catches gross defects like bridging and large voids.
- If you prioritize Reliability and Complex Geometries: Choose 3D/CT. It is slower and more expensive but essential for HiP detection, double-sided boards, and PoP (Package on Package).
Sampling (AQL) vs. 100% Inspection:
- If you prioritize Throughput: Choose AQL Sampling (e.g., inspect 10% of lot). Use this only after the process is stable (Cpk > 1.33).
- If you prioritize Zero Escapes: Choose 100% Inspection. Mandatory for automotive, medical, and aerospace, but adds cost.
Offline vs. Inline Inspection:
- If you prioritize Flexibility/NPI: Choose Offline (Island) X-ray. Great for debugging and low volume.
- If you prioritize Mass Production Consistency: Choose Inline AXI. It sits in the conveyor line, inspects automatically, and doesn't rely on an operator moving boards manually.
Manual vs. Automated Analysis:
- If you prioritize Low Setup Cost: Choose Manual Analysis. Operator looks at the screen. Good for prototypes.
- If you prioritize Data Integrity: Choose Automated Analysis. Software counts voids. Removes "opinion" from the quality decision.
Destructive vs. Non-Destructive Validation:
- If you prioritize Board Preservation: Stick to X-ray.
- If you prioritize Root Cause Analysis: You must sacrifice a board for Cross-Sectioning or Dye and Pry to prove the X-ray is telling the truth.

FAQ

Q: Can X-ray inspection damage my components? A: Generally, no. However, certain types of memory (EPROM, Flash) and sensitive analog sensors can be corrupted by prolonged exposure.

Check component datasheets for radiation limits.
Limit exposure time and use the lowest effective kV setting.

Q: What is the difference between AOI and X-ray? A: aoi inspection uses cameras and light to check visible features (polarity, text, solder fillets). xray inspection uses radiation to see through the package to hidden joints.

AOI = Surface / Visible.
X-ray = Internal / Hidden.

Q: Why can't X-ray detect all "Head-in-Pillow" defects? A: In a top-down (2D) view, the ball overlaps the pad perfectly, masking the lack of fusion.

You need oblique (angled) views or 3D laminography to see the separation layer.
Dye and Pry is the ultimate arbiter for HiP issues.

Q: How much does X-ray inspection add to the cost? A: It depends on the strategy.

Sampling (AQL) adds negligible cost.
100% manual inspection can add significant labor and time costs.
Inline AXI has a high upfront machine cost but low per-unit labor cost.

Q: Can X-ray check for counterfeit components? A: Yes, it is a primary tool for this.

It reveals the die size and wire bonding pattern inside the chip.
Comparing the X-ray of a received part against a "Golden" datasheet part exposes fakes immediately.

Q: What is "Voiding" and is it always bad? A: Voiding is gas trapped in the solder joint.

Small voids are normal and acceptable (IPC allows up to 25%).
Excessive voiding reduces thermal transfer and mechanical strength.
Location matters: Voids at the interface (planar microvoids) are dangerous; voids in the center are less critical.

Q: Does APTPCB perform X-ray on all boards? A: We perform X-ray on all boards containing BGAs, QFNs, or leadless packages as part of our standard quality process.

For prototypes: We typically inspect 100% of BGAs.
For mass production: We define a sampling plan or inline strategy based on customer requirements.

Q: Can X-ray see inside a multi-layer PCB? A: Yes. It can inspect inner layer registration, drill alignment, and blind/buried vias.

This is often done during PCB fabrication, separate from PCBA assembly inspection.

X-ray Inspection Services – Detailed breakdown of our X-ray capabilities, machine specs, and defect detection range.
BGA & Fine Pitch Assembly – Why fine-pitch components require specialized thermal profiles and inspection strategies.
AOI Inspection – Understand how we pair optical inspection with X-ray for complete coverage.
First Article Inspection (FAI) – How we use X-ray data to validate the very first board off the line before full production.
DFM Guidelines – Design tips to ensure your component placement allows for accurate X-ray analysis (avoiding shadowing).

Request a quote

Get a Quote & DFM Review – Send us your design today; our engineers will review your BGA/QFN layout and propose a tailored inspection plan.

To get the most accurate quote and DFM, please include:

Gerber Files: RS-274X format.
BOM (Bill of Materials): With manufacturer part numbers.
Centroid/Pick & Place File: For automated programming.
Assembly Drawings: Highlighting critical inspection points.
Test Requirements: Specify IPC Class (2 or 3) and any custom voiding limits.

Conclusion

Effective xray inspection is not just about having a machine; it is about defining the right acceptance criteria and validating the process that produces the images. By specifying void limits, requiring oblique views for BGA validation, and correlating X-ray data with physical cross-sections, you transform a standard check into a rigorous quality gate. Whether you are building high-reliability aerospace hardware or consumer IoT devices, a clear inspection strategy ensures that what you can't see won't hurt your product's performance in the field.