Inspection is a stack, not a step
A PCB assembly can fail in ways that no single method sees. Paste printed too thin, a component rotated 90°, a BGA ball unsoldered, a barrel with a void, a firmware image that never boots — each is invisible to at least one of the standard tools.
The useful question is therefore not "which inspection do we use" but "which failure modes matter on this board, and which method sees each one". The six below cover the chain from paste to product.
1. Visual and microscope
The cheapest check and the first one. It catches missing parts, wrong polarity, tombstoning, solder balls and damaged components, and it is the only method that reads silkscreen and markings directly. It is also the least consistent, which is why it is used at set-up and at final inspection rather than as the only gate.
2. Solder paste inspection
SPI measures paste height, area and offset before reflow. It is the only method that catches a print problem while the board can still be washed and reprinted: a 30 % thin print becomes an open joint after reflow, and by then the part is already placed.
3. Automated optical inspection
AOI compares the assembled board against a programmed reference: presence, polarity, rotation, offset, fillet shape and bridges. Its limit is physical — it reads surfaces only. Tall components shadow their neighbours, and anything under a package is invisible.
4. X-ray
X-ray looks through the package at the joints underneath: BGA balls, QFN thermal pads, press-fit pins and plated barrels. For area-array parts it is the only non-destructive check of whether the ball actually collapsed onto the pad, which is exactly where reflow problems concentrate.
5. In-circuit test and flying probe
Both check electrical continuity and component values with the board powered down. A fixture with fixed probes is fast and cheap per board once built; a flying probe needs no fixture and suits prototypes and low volume. Neither can see whether the product works.
6. Functional test and burn-in
Functional test runs the assembly as a product — firmware, interfaces, radio link, current draw — and burn-in runs it under temperature and time. This is where design and assembly meet: a marginal joint that passes every optical check fails here, which is why automotive and medical builds add thermal cycling.
Sequencing them
A practical order is: SPI at the printer, AOI after placement and reflow, X-ray on area-array and thermal pads, electrical test on every board, functional test on every board, and burn-in or thermal cycling where the product justifies it. Sampling can drop some optical steps in volume, never the electrical ones.
Workmanship is judged against IPC-A-610, and the process limits behind these gates are on the PCBA manufacturing page, with first-article detail in the FAI note.
