X-Ray Inspection for Electronics: BGA & AXI Reference
X-ray inspection for PCBAs from China: BGA void analysis per IPC-7095C, 2D vs 3D AXI, equipment types, sampling strategy, and specifying requirements.
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 — particularly for complex PCBAs with BGAs or QFNs.
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 during a factory audit and verification before qualifying the supplier.
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
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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. X-ray sits inside a broader electronics quality control program alongside SPI, AOI, and functional test.
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 multilayer PCBs 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.
How this shows up in our work
When we audited a PCBA factory for X-ray capability, we confirmed the equipment make, model, and operator training before the first article. A common issue we see on the floor is a 2D system used for double-sided BGA boards, where overlapping shadows hide pillow defects. We require 3D AXI for those 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
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