In this article
The modern inspection stack
Modern PCB assembly uses a layered approach: SPI (solder paste inspection), AOI (automated optical inspection), X-ray for hidden joints, and ICT / functional test. Each stage catches different defects — no single tool catches everything.
AOI: fast but not final
AOI is fast, repeatable, and great for obvious defects — opens, shorts, missing components. It struggles with polarity, BGAs, and parts that look similar to humans. Treat it as a screen, not a verdict.
X-ray for BGAs and QFNs
For bottom-terminated parts where you cannot see the joint, 2D X-ray and — for high-reliability work — CT X-ray give you a look at solder voiding, head-in-pillow, and bridging. The cost is real; the data is invaluable.
ICT and functional test
In-circuit test catches manufacturing faults; functional test catches design and integration faults. A robust test plan covers both. We design test fixtures in-house for our customers' assemblies.
Where humans still matter
Despite all the automation, the most reliable assembly lines still have a human reviewing first-article boards, field returns, and outliers flagged by the line. Machines are great at patterns; humans are great at 'this looks weird, why'.
The best quality stack is layered. The second best is whatever the budget can afford — run it consistently.
