PCB Testing Methods Explained: ICT vs FCT vs AOI vs X-Ray

The $2 Million Lesson I Learned About Testing

True story: Early in my career, a customer shipped 50,000 units of a medical monitoring device with "100% tested" boards. Three months later, field failure rate hit 8%. The root cause? We tested functionality (FCT) but skipped AOI. A batch of capacitors had hairline cracks invisible to the eye but perfectly detectable with optical inspection.

Total recall cost: over $2 million. The AOI that would've caught it? About $0.15 per board.

Testing isn't a cost—it's insurance. And like insurance, the right combination matters more than any single method.

Quick Reference: All 7 Testing Methods

**Hommer's Rule of Thumb**: Most projects need at least 3 of these. The question is which 3.

1. SPI: Solder Paste Inspection

What Is SPI?

Solder Paste Inspection happens *before* components are placed. A 3D scanner measures the volume, height, and area of solder paste deposits on each pad.

Why It Matters

Here's a stat that surprised me when I first learned it: 70-80% of SMT defects originate from solder paste printing. Not pick-and-place. Not reflow. Paste printing.

When to Use SPI

• Always for high-reliability products ([medical](/industries/medical), [automotive](/industries/automotive))
• Fine-pitch components (0.4mm BGA, 0201 passives)
• New product introduction (NPI)
• When yield is below target

The Reality

SPI catches problems *before* they become expensive. A board caught at SPI costs pennies to rework. The same defect caught at FCT? Could mean scrapping the entire assembly.

2. AOI: Automated Optical Inspection

What Is AOI?

AOI uses high-resolution cameras (often 3D now) to photograph every component and compare it against the expected placement. Think of it as a very fast, very consistent human inspector that never gets tired.

What AOI Catches

Excels at: - Missing components - Wrong components (wrong value, wrong part) - Polarity errors (caps, diodes, ICs) - Placement offset - Solder bridges (visible) - Solder insufficiency (visible) - Tombstoning

Struggles with: - Hidden solder joints (BGA, QFN) - Intermittent connections - Wrong component values (same package) - Functional defects

Real-World Coverage

In our SMT production lines, AOI typically catches 85-90% of assembly defects. That's excellent for surface-visible issues, but it's not 100%.

The Speed Advantage

For high-volume production, AOI isn't optional—it's mandatory.

3. AXI: Automated X-Ray Inspection

What Is AXI?

AXI shoots X-rays through your board to see what cameras can't: the solder joints hidden under BGAs, QFNs, and other bottom-terminated components.

Why X-Ray Exists

Modern components are increasingly hidden. A BGA might have 500 solder balls—none visible from above. If even one fails, the product fails. AOI literally cannot see these joints.

What X-Ray Reveals

When X-Ray Is Essential

• BGAs (especially fine-pitch < 0.8mm)
• QFN/DFN packages
• [Automotive electronics](/industries/automotive) (IATF 16949 often requires it)
• Aerospace and defense
• Medical implants

The Cost Reality

X-ray inspection adds $0.20-0.50 per board for inline AXI. Manual X-ray for sampling is cheaper but slower. For boards with multiple BGAs, it's worth every cent.

**Hommer's Experience**: We once caught a systematic BGA soldering issue using AXI that would've caused 100% field failure. The reflow profile was *just* off enough to create head-in-pillow defects invisible to AOI. X-ray caught all of them.

4. ICT: In-Circuit Testing

What Is ICT?

In-Circuit Testing uses a "bed of nails" fixture—hundreds of spring-loaded probes that contact test points on your board simultaneously. It then measures every component value and checks every net for opens/shorts.

The Power of ICT

ICT is brutally thorough for manufacturing defects:

Defect coverage: Up to 98% for *manufacturing* defects.

ICT Economics

Here's the catch: ICT requires a custom fixture for each board design.

When ICT Makes Sense

✅ Use ICT when: - Production volume > 1,000/year - Design is stable (no frequent revisions) - Board has dedicated test points - High defect cost (automotive, medical)

❌ Skip ICT when: - Prototype or low-volume - Frequent design changes - No test point access - Budget doesn't support fixture

Our testing services include ICT for qualifying production runs.

5. Flying Probe: The Flexible Alternative

What Is Flying Probe?

Instead of a custom fixture with hundreds of probes, flying probe testers use 4-8 motorized probes that move to each test point sequentially. Same electrical tests as ICT, no fixture required.

Flying Probe vs ICT: Head-to-Head

The Sweet Spot

Flying probe wins for: - Prototypes (absolutely) - Production under 500-1,000 units - Boards with frequent revisions - Emergency production runs

ICT wins for: - High-volume (>2,000/year) - Stable designs - When test time matters

My Recommendation

For most of our customers at PCB Portugal, I recommend flying probe for NPI and early production, then transitioning to ICT once the design stabilizes and volumes justify the fixture cost.

6. FCT: Functional Circuit Testing

What Is FCT?

Functional testing powers up the board and verifies it actually *works*—not just that components are present and connected, but that the system performs its intended function.

FCT vs Structural Tests (AOI/ICT)

Types of Functional Tests

1. **Power-on test**: Does it boot?
2. **Communication test**: Can it talk (UART, SPI, I2C)?
3. **Peripheral test**: Do LEDs, buttons, sensors work?
4. **Performance test**: Does it meet specifications?
5. **Calibration**: Adjust for production variation

FCT Reality Check

FCT catches what other tests miss: - Firmware bugs triggered by hardware variation - Timing-sensitive failures - Noise/interference issues - Thermal-related problems

But FCT is only as good as the test coverage. A 30-second FCT that only checks "power LED on" isn't the same as a 5-minute FCT that exercises every function.

**Hommer's Insight**: I always ask customers: "What would you test if you only had 60 seconds with each board?" That answer should be your minimum FCT.

7. Burn-in Testing: The Stress Test

What Is Burn-in?

Burn-in subjects boards to elevated temperature (and sometimes voltage) for extended periods—typically 24-168 hours. The goal: accelerate "infant mortality" failures that would otherwise occur in the field.

The Bathtub Curve

Electronics failures follow a predictable pattern:

1. **Infant mortality** (early failures): Weak components fail quickly
2. **Useful life**: Low, random failure rate
3. **Wear-out**: End-of-life failures increase

Burn-in eliminates category 1 *before* shipping.

When Burn-in Is Required

The Cost of Burn-in

Burn-in is expensive: - Equipment (ovens, monitoring) - Time (days, not minutes) - Energy - Floor space

But for a pacemaker? A satellite? An aircraft system? Absolutely justified.

Building Your Test Strategy

The Minimum Viable Test Plan

For most commercial products, I recommend:

Essential (non-negotiable): 1. AOI after reflow 2. FCT before shipping

Add based on risk: - SPI for fine-pitch assembly - X-ray for BGAs - ICT/Flying Probe for electrical verification

Test Strategy by Industry

Test Strategy by Volume

Cost-Benefit Analysis

Testing ROI Example

Let's do the math for a 10,000-unit production run:

Scenario: Skip AOI to "save money" - AOI cost saved: $0.10 × 10,000 = $1,000 - Typical defect escape rate without AOI: 2% - Defective units: 200 - Field failure cost per unit: $150 (including shipping, repair, reputation) - Total cost of escapes: $30,000

Net "savings": -$29,000

That's why we never skip AOI.

Where to Invest First

If budget is limited, prioritize in this order:

1. **AOI** - Best cost/coverage ratio
2. **FCT** - Catches functional issues
3. **ICT/Flying Probe** - For electrical verification
4. **X-Ray** - If you have BGAs
5. **SPI** - For yield improvement
6. **Burn-in** - For high-reliability applications

Common Testing Mistakes

Mistake 1: Testing Only at the End

"We'll catch everything at FCT."

No, you won't. FCT tells you the board doesn't work, but not *why*. Without earlier tests, debugging is painful and expensive.

Mistake 2: Over-Testing Low-Risk Products

A simple LED driver doesn't need X-ray, ICT, burn-in, and FCT. Match the test strategy to the risk.

Mistake 3: No Test Points in Design

If you want ICT, you need test points. Design them in from the start—adding them later means a board spin.

Mistake 4: Ignoring Test Coverage Reports

Your test coverage isn't 100% just because you have all the equipment. Review coverage reports. Improve test fixtures. Iterate.

FAQ

How much does a complete test suite cost per board?

For a typical industrial board with BGA: - SPI: $0.03 - AOI: $0.10 - ICT: $0.15 - AXI (sample): $0.05 - FCT: $0.50

Total: ~$0.83/board

Compare to a single field failure at $100-500. Testing is cheap.

Can testing guarantee zero defects?

No. Testing reduces defects but can't eliminate them entirely. Even 100% testing catches 95-99% of defects. The goal is an acceptable defect level, not perfection.

What's boundary scan testing?

Boundary scan (JTAG) tests digital ICs through their debug interface. Useful for dense digital boards where physical probe access is limited. We consider it a supplement to ICT, not a replacement.

How do I choose between ICT and Flying Probe?

Rule of thumb: If you'll test more than 1,000 identical boards per year, ICT. Otherwise, flying probe.

Conclusion: My Testing Philosophy

After 15 years in this industry, here's what I believe:

1. **No single test catches everything**. Use multiple methods.
2. **Catch defects early**. SPI catches what becomes unfixable later.
3. **Match testing to risk**. Not every product needs aerospace-level QA.
4. **Design for test**. Test points aren't optional.
5. **Trust but verify**. Review coverage data regularly.

Need help designing a test strategy for your product? Contact our engineering team or explore our testing capabilities.

Related Reading

Explore more technical guides that complement your testing knowledge:

• **[Turnkey vs Consignment Assembly](/blog/turnkey-vs-consignment-pcba)** - Testing is included differently depending on your assembly model. Understand the implications.
• **[PCB Materials Comparison](/blog/pcb-materials-comparison)** - Material choice affects test accessibility. Aluminum boards need X-ray for thermal via inspection.
• **[Wire Harness & Cable Assembly Guide](/blog/wire-harness-cable-assembly-guide)** - Cable assemblies have their own testing requirements (continuity, hi-pot, pull testing).

References

1. [IPC-A-610](https://www.ipc.org/) - Acceptability of Electronic Assemblies
2. [IPC-9252](https://www.ipc.org/) - Requirements for Electrical Testing of Unpopulated Printed Boards
3. [JEDEC JESD22-A108](https://www.jedec.org/) - Temperature Cycling

*Written by Hommer Zhao, founder of PCB Portugal. Based on testing strategies developed over 15+ years of PCBA production experience. Last updated: December 2024.*