3.4 Inspection and Defect Handling: AOI, X-Ray, Rework

Inspection does not add value to a product; it only adds cost. A board that passes inspection is not "better" than one that was built correctly in the first place—it is merely verified. In a healthy manufacturing environment, inspection is a feedback loop used to tune the machines, not a police force used to catch criminals. If your inspection stations are busy, your process is out of control.

AOI (Automated Optical Inspection): The First Filter

Action:

High-speed cameras scan the PCBA, using algorithms to compare every component against a "Golden Board" or a digital model.

AOI is the primary gatekeeper after Reflow. It is fast, consistent, and does not get tired.

The Engineering Reality

AOI checks for light reflection and geometry. It looks for the "meniscus" (the curved shape) of a solder fillet.

• If the solder joint is shiny and curved → Then light reflects at a specific angle, registering as "Pass."
• If the joint is dry or missing → Then light scatters differently, triggering a "Fail."
• Risk (False Calls): If the threshold is too strict, the machine flags good boards as bad. Operators eventually stop trusting the machine and may wave through actual defects.

X-Ray (AXI): Seeing the Invisible

Action:

X-rays penetrate the package body to image the metal underneath.

This is mandatory for "Hidden Joints" like BGAs (Ball Grid Arrays), QFNs, and LGAs. You cannot inspect these with a microscope; looking from the side reveals nothing about the center balls.

The Voiding Criteria

Solder paste contains flux (solvent). When it boils, it can leave gas bubbles trapped inside the joint. These are called Voids.

• Standard: IPC-A-610 allows up to 25% voiding by area.
• If voids exceed 25% → Then the joint is mechanically weak and will crack under thermal stress.
• If two balls merge (Bridging) → Then the chip is shorted and must be removed.

Rework: The "Hidden Factory"

Definition:

Manual repair of a manufacturing defect to save the unit.

Rework is often celebrated as "saving the shipment." In reality, it is a failure of the process. Every rework station represents a "Hidden Factory" that consumes labor, energy, and time without producing new inventory.

The Reliability Cost

Electronics hate heat. A standard reflow cycle takes the board to 250˚C. Rework heats it again.

• Thermal Stress: Every heating cycle degrades the epoxy bond between the copper pad and the fiberglass (FR-4).
• Intermetallic Growth: Excessive heat thickens the "intermetallic layer" (the alloy mix between copper and tin), making the joint brittle.
• Rule: A board should never see more than two rework cycles. If it fails a third time, scrap it.

Pro-Tip: Never allow "Cosmetic Touch-up" just to make solder joints look prettier. Every time a soldering iron touches a joint, you risk overheating the component or lifting the pad. If it meets IPC Class 2, leave it alone.

The "Bone Pile" (Scrap Criteria)

Some defects are terminal. Attempting to fix them costs more than the board is worth or introduces unacceptable risk.

• Delamination: If the layers of the PCB separate (blistering), the board is structural garbage.
• Pad Lift: If the copper pad rips off the fiberglass, there is nothing to solder to. Glues and jumper wires are not acceptable for new products.
• Internal Shorts: If the defect is inside the inner layers of the PCB, it cannot be reached.

Final Checklist