X-Ray Inspection for Electronics: BGA & AXI Reference
Technical reference for X-ray inspection of electronics assemblies — covering BGA void analysis per IPC-7095C, 2D vs 3D AXI, equipment types, sampling strategy, and how to specify X-ray requirements when sourcing PCBAs from China.
X-ray inspection reveals solder joint defects that are physically inaccessible to optical inspection: BGA balls hidden under the package body, QFN thermal pad joints, solder connections in shielded cans, and via-fill quality. For any design with a BGA package, X-ray is not optional — it is the only way to verify joint formation before the product ships. This capability should be confirmed during quality inspection services and when evaluating any PCB assembly supplier.
Overview
Conventional automated optical inspection (AOI) works by photographing solder joints from above after reflow. BGAs, QFNs, LGAs, and similar bottom-terminated packages have their joints on the underside of the package, completely hidden from any optical system. X-ray transmits through the board and package body, projecting a shadow image of the solder balls or joints below. Voids (unfilled regions within a solder ball), bridges (solder connecting adjacent balls), and missing balls are all visible.
X-ray inspection fits into the manufacturing flow after reflow soldering and before functional test. In the sequence: paste printing → SPI → placement → reflow → X-ray → AOI → functional test. Running X-ray before AOI catches BGA defects before functional test wastes time on boards with obvious joint problems.
Two levels of X-ray capability are relevant to electronics buyers: spot-check manual X-ray (a technician positions the board, takes images, and evaluates) and AXI — Automated X-ray Inspection (in-line or off-line system that automatically images and evaluates all specified packages).
Key Parameters
| Parameter | Value | Notes |
|---|---|---|
| BGA void limit — Class 2 | <25% per joint | Per IPC-7095C; commercial electronics |
| BGA void limit — Class 3 | <10% per joint | High reliability: aerospace, automotive, medical |
| QFN thermal pad void limit (typical) | <25% | Affects thermal resistance; not in IPC-7095C directly |
| Typical 2D X-ray resolution | 10–25 µm | Adequate for 0.8 mm and larger BGA pitch |
| Required resolution for 0.4 mm CSP | <5 µm | Requires high-magnification system or CT |
| Typical AXI throughput | 1–4 boards/minute | Depends on board complexity and package count |
| Manual spot check time | 5–15 min/board | Including repositioning and image capture |
| Manual X-ray cost | $15–50/board | For BGA-heavy designs |
| Full AXI scan cost | $50–150/board | 3D CT reconstruction per board |
2D X-Ray vs 3D AXI
2D X-ray (single-angle transmission) The X-ray source and detector are fixed; the board is imaged from directly above. The image is a 2D projection — a “shadow” of all layers superimposed. For BGA balls, a 2D image shows the ball outline and any large voids (>20% of ball area). Bridging between adjacent balls is visible as a connected shadow between two ball images.
Limitations: 2D cannot distinguish whether a void is in the top, middle, or bottom of the joint. It cannot image joints on a second-side component when a first-side component is directly above it (shadows overlap). It cannot provide quantitative 3D void volume.
2D X-ray is adequate for: prototype and pre-production inspection, AQL spot checks, and identifying gross defects (missing balls, obvious bridging). It is the standard capability in most Chinese PCBA factories.
3D AXI — Computed Tomography Multiple X-ray images are taken from different angles; a computer reconstructs a 3D volume (CT scan). Cross-sections at any height are available. 3D void analysis provides volumetric void percentage, not just 2D area. Bridges, opens, pillow defects (ball doesn’t collapse to pad), and non-wet opens are reliably distinguished.
3D AXI equipment: Saki BF-3Di, YXLON FF35, Nordson DAGE XD7600NT. These systems cost $250,000–500,000; not all Chinese factories have them. Ask before assuming. For critical designs (1.0 mm BGA pitch below on a Class 2 board, or any Class 3 requirement), confirm 3D AXI capability before qualifying the factory.
IPC-7095C BGA Reliability Standard
IPC-7095C (Design and Assembly Process Implementation for BGAs) defines:
Void classification:
- Class 2 (Commercial): Individual void <25% of ball cross-section area. Aggregate (multiple voids in one ball) total <25%.
- Class 3 (High Reliability): Individual void <10%. Used for products where field failure is unacceptable.
Pillow defect (head-in-pillow, HiP): BGA ball partially wets to PCB pad but then separates during cooling, leaving a “pillow” indentation. Appears as a dark ring in 2D X-ray around an unreflowed ball. Cause: BGA package warpage during reflow (BGA body flexes, lifting the ball from the paste before it wets). IPC-7095C requires HiP detection in X-ray inspection; this is a reject condition regardless of void percentage.
Non-wet open (NWO): Ball does not wet to pad at all — looks like a circle floating above the pad shadow in 2D X-ray. Causes: oxidized pad, insufficient flux activation, ball contamination.
Ball bridging: Two or more balls connected by solder — a short circuit. Visible as a smear or connection between ball shadows. Cause: excessive paste, misregistration, or BGA-to-board misalignment.
What to Inspect by X-Ray
Specify which packages require X-ray inspection in your quality plan:
| Package Type | X-ray Required? | Why |
|---|---|---|
| BGA (all ball counts) | Yes, always | Joints completely hidden |
| QFN with thermal pad >10 mm² | Yes | Thermal pad voids affect thermal resistance |
| LGA (land grid array) | Yes | Similar to BGA; no balls, flat lands |
| Shielded modules | Yes | Cannot access joints under shield |
| QFN <10 mm² thermal pad | Sampling | Typically low-risk if profile is qualified |
| Standard SMD (resistors, caps, QFP) | No | AOI covers these |
Sampling Strategy
Prototype / NPI run (first 5–10 boards): X-ray 100% of BGA packages. This qualifies the reflow profile for your specific board. If voids exceed spec, adjust the profile before production.
Production qualification run (first 50–100 boards): X-ray all boards. Establish a baseline void distribution. Calculate Cpk for void percentage if AXI data supports it.
Steady-state production: AQL sampling per ANSI/ASQ Z1.4. For BGA-heavy designs, Code L (200 units from 5,000 lot) with Major AQL 1.0 is a reasonable minimum. Some buyers specify X-ray on every board for high-value products (IoT gateways, industrial controllers) where field rework cost exceeds inspection cost.
What to Specify When Ordering from China
- X-ray requirement: “100% X-ray inspection of all BGA packages per IPC-7095C, Class 2 void limits” — state this explicitly in the quality plan attached to your PO
- Inspection system capability: confirm factory has 2D or 3D AXI before production, not after; ask for equipment make and model
- First article X-ray report: require a full X-ray inspection report on first-article boards showing void percentages for all BGA joints
- Sampling plan for production: define AQL level and lot size; require X-ray reports be retained for each production lot
- Pillow defect inspection: explicitly specify that HiP (head-in-pillow) defects are a reject condition — some factories don’t flag these without being asked
Quality Checks
When reviewing X-ray images from your factory:
- Ask for calibrated void measurement, not just “looks okay”
- Request images from the same BGA corner balls (typically highest warpage stress location) consistently across boards
- Void measurement should be reported as percentage of ball cross-sectional area (2D method) or volumetric percentage (3D CT method) — not subjective terms like “small void”
Correlation with functional test: A board that passes X-ray and fails functional test indicates an issue beyond solder joints (component failure, firmware, power sequencing). A board that fails X-ray should not proceed to functional test — triage the joint defect first.
Common Issues
Voids ≥25% on QFN thermal pads: Even with accepted BGA void limits, thermal pad voids increase junction-to-board thermal resistance (θJB). A 50% void on a 10 × 10 mm thermal pad doubles effective thermal resistance. For power devices (switching regulators, power FETs), this accelerates thermal fatigue and reduces output current capability. Specify thermal pad void <25% even if IPC-7095C only formally addresses BGAs.
HiP (head-in-pillow) on large BGAs: Occurs on BGAs >20 × 20 mm where package warpage during reflow lifts corner balls before they fully wet. Appears electrically open at ICT. Prevention: nitrogen atmosphere reflow, slow ramp rate through liquidus, controlled board support in oven. Detection: mandatory X-ray on all corner balls.
False passes from 2D X-ray on dense boards: Dense double-sided boards with BGAs on both sides have overlapping shadows in 2D X-ray. The bottom-side BGA image is obscured by top-side component shadows. Solution: oblique-angle X-ray (tilt the board 15–30°) to separate the layers, or specify 3D AXI for double-sided BGA designs.
Related Resources
- SMT Assembly Process — where X-ray fits in the production flow
- IPC-A-610 Acceptance Criteria — general workmanship standards
- AQL Sampling Plans — how to set sampling frequency for X-ray inspection
- Reflow Soldering Profiles — profile parameters that affect BGA void formation
- Factory Audit Checklist
- Quality Inspection Services
- PCB Manufacturing & SMT Sourcing