X-ray inspection is the only reliable way to verify solder joints hidden beneath Ball Grid Array (BGA), Quad Flat No-lead (QFN), and other bottom-terminated component packages. While AOI catches visible defects, X-ray reveals voids, bridging, and cold joints that are completely invisible to cameras. As Indian electronics companies increasingly adopt BGA packages for compact designs, X-ray inspection has become an essential quality assurance tool. This guide covers X-ray inspection technology, interpretation of results, void acceptance criteria, and when you need X-ray vs other inspection methods.
Table of Contents
- Why X-Ray for PCBs
- X-Ray Technology Types
- Defects Visible on X-Ray
- Void Analysis and Acceptance
- BGA Inspection Guidelines
- QFN and Bottom-Terminated Components
- Cost and Availability in India
- Frequently Asked Questions
Why X-Ray for PCBs
X-rays pass through the PCB substrate and component bodies, but are absorbed by dense metals like solder (tin-lead or SAC305). This creates a shadow image showing the solder distribution under components. You can see:
- Individual BGA solder balls — their shape, size, and spacing
- Voids (air bubbles) trapped inside solder joints
- Solder bridging between adjacent BGA balls
- Missing or collapsed solder balls
- Head-in-pillow defects where the ball does not fully merge with the paste
- QFN thermal pad solder coverage and voiding
X-Ray Technology Types
| Type | Resolution | Capability | Cost |
|---|---|---|---|
| 2D X-ray | 5-20µm | Top-down view, overlapping layers | ₹500-2,000/board |
| Oblique angle | 10-30µm | Angled view separates overlapping features | ₹1,000-3,000/board |
| CT (Computed Tomography) | 1-10µm | Full 3D reconstruction, slice-by-slice | ₹5,000-20,000/board |
For routine BGA inspection, 2D X-ray with oblique angle capability is sufficient. CT is used for failure analysis and research where full 3D solder joint geometry is needed.
Defects Visible on X-Ray
| Defect | X-Ray Appearance | Severity |
|---|---|---|
| Void (air bubble) | Bright area within dark solder ball | Depends on size (<25% acceptable) |
| Solder bridge | Dark connection between adjacent balls | Critical — electrical short |
| Missing ball | Bright spot where ball should be dark | Critical — open connection |
| Head-in-pillow | Ball with irregular outline, partial merge | Intermittent failure risk |
| Collapsed ball | Ball with larger diameter, lower height | Potential short if touching neighbour |
| Thermal pad void | Bright areas in QFN centre pad | <50% voiding acceptable for thermal |
Void Analysis and Acceptance
Voids are unavoidable in reflow soldering — the flux outgassing during reflow creates gas bubbles that may get trapped in the solder. IPC-7095 provides void acceptance criteria for BGA joints:
- Individual ball void: No single void larger than 25% of the ball diameter
- Total void area: Total void area less than 25% of the ball cross-section
- Thermal pad void: IPC-7093 allows up to 50% voiding on QFN thermal pads (less restrictive because thermal pads are larger)
- Critical applications: Automotive and medical may require tighter limits (under 10%)
Reducing voids: use nitrogen reflow atmosphere, optimise reflow profile (longer soak zone for outgassing), reduce solder paste volume, and use void-reducing flux formulations.
BGA Inspection Guidelines
- Inspect 100% of BGA joints on the first production batch (first article inspection)
- For ongoing production, inspect a statistical sample (AQL level — typically 10-20% of boards)
- Focus on corner and edge balls — these are most prone to defects due to warpage and thermal stress
- Check ball diameter uniformity — collapsed or enlarged balls indicate reflow profile issues
- Verify ball pitch matches the design — misalignment indicates placement machine offset
QFN and Bottom-Terminated Components
QFN packages have solder pads on the bottom surface, hidden from visual inspection. The centre thermal/ground pad is especially critical:
- Thermal pad solder coverage determines heat dissipation — target above 50% coverage minimum
- Use a windowed stencil pattern (multiple small apertures) for the thermal pad to reduce voiding
- Signal pads around the perimeter are visible from the side with angled inspection, but X-ray provides a clearer view
- DFN (Dual Flat No-lead) packages have the same inspection requirements
Cost and Availability in India
| Service | Cost | Turnaround |
|---|---|---|
| 2D X-ray inspection (per board) | ₹500-2,000 | Same day |
| CT scan (per board) | ₹5,000-20,000 | 2-3 days |
| X-ray failure analysis report | ₹10,000-50,000 | 5-10 days |
X-ray inspection services in India: Tessolve (Bangalore), Tektronix/Keysight labs, SGS India, and several contract assembly houses in Bangalore and Pune offer X-ray as a service. JLCPCB and PCBWay do not offer X-ray as a standard service but may accommodate special requests for large orders.
Frequently Asked Questions
Do I need X-ray for every BGA board?
For prototypes, yes — inspect every board to validate your reflow profile and stencil design. For production, sample inspection (AQL) is standard unless the product is safety-critical (medical, automotive), in which case 100% inspection may be required.
Can X-ray damage the PCB?
Industrial PCB X-ray machines use low-energy X-rays that do not damage components or affect functionality. Flash memory and EEPROM are not affected at the energy levels used for inspection. This is confirmed by IPC-7095D.
What is the alternative to X-ray for BGA inspection?
Boundary scan (JTAG) can verify electrical connectivity of BGA joints without X-ray. However, it cannot detect voids, partial merges, or marginal joints that may fail in the field. For thermal pad assessment, only X-ray or destructive cross-sectioning can verify solder coverage.
How do I find X-ray inspection services near me?
Search for “PCB X-ray inspection India” or contact your assembly house — many PCBA providers in Bangalore, Chennai, Pune, and Delhi-NCR have in-house X-ray machines or partnerships with inspection labs.
Browse PCB prototyping boards, soldering tools, and electronics supplies at Zbotic PCB & Prototyping — fast shipping across India.
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