Learning objectives
- Explain what two dimensional and three dimensional AOI systems check on a board
- State the difference between inspection before reflow and inspection after reflow
- Disposition flagged calls with microscope verification instead of blind confirmation
- Explain the balance between false calls and escapes, and why thresholds are never loosened to pass boards
Lesson
Automated optical inspection, called AOI, is the tireless camera system of the surface mount line. Two dimensional systems work from camera images, and three dimensional systems add structured light so they can measure height as well as appearance. The machine compares every board against a programmed library built from golden board data. It checks that each component is present, that the part and its printed marking look right, that polarity is correct, that placement sits within its allowed offset, and it judges solder fillet shape by appearance and, on three dimensional systems, by measured height. Bridging between joints is flagged for review.
AOI works at two points. Inspection before reflow is optional and valuable, because placement and polarity faults are caught while components can still be lifted off and corrected in seconds. Inspection after reflow is the standard gate, examining the joints that reflow actually produced. In both cases the machine does not finish the job alone. Flagged calls travel to a repair or verification station, where an operator looks at the real board, often under a microscope, and dispositions each call as a true defect or a false call.
That human judgement is where this stage succeeds or fails, and it carries two classic traps. The first is blind confirmation, accepting the machine verdict without ever looking at the board, which lets real escapes walk through inside a pile of false calls. The measured height data of a three dimensional system earns its keep here, catching lifted leads, parts sitting tilted on one termination, and coplanarity problems that a flat camera image can argue about but a height map settles. When the operator does confirm a true defect, the loop is closed properly: the board is routed to touch up or rework with the call information attached, so the repair technician sees exactly which joint the machine flagged and why. The second is worse: loosening inspection thresholds so the line appears to pass and the shift numbers look good. Every loosened threshold is an escape route purchased with the customer’s reliability. Program quality decides both the false call rate and the escape rate, so good AOI work means maintaining libraries and lighting and height data, and feeding first pass yield trends back to the printing and placement stages where the faults begin. James Onsomu teaches teams to analyse performance data and correct problems at their source. AOI data, honestly dispositioned, is one of the richest sources of that truth on the line.
Key numbers
- Inspection coverage: every board is compared against the programmed library, not sampled
- False call rate and escape rate: balanced together through program quality, never by loosening thresholds
- Verification: every flagged call dispositioned against the real board, with magnification where needed
- Yield feedback: first pass yield trends reported back to printing and placement
Defects to spot
- Missing or wrong components: placement faults or feeder errors upstream
- Reversed polarity: parts placed turned around, caught by marking and polarity checks
- Offset and skewed parts: placement drift or paste problems
- Tombstones: one end lifted during reflow, flagged by presence and height checks
- Solder bridges: excess paste or slump joining neighbouring joints
- Lifted leads and coplanarity flags: bent leads or parts not sitting flat
Stage check, self test
Answer each question, then open the answer to check yourself. Passing standard: 80 percent, at least 4 of 5 correct.
1. What does AOI compare each board against?
A. The previous shift’s scrap bin
B. A programmed library built from golden board data
C. A photograph of a different product
D. The operator’s memory
Show answer
Answer: B
2. Why is inspection before reflow valuable when it is used?
A. Placement and polarity faults can still be corrected in seconds, before the parts are soldered
B. It replaces the need for any later inspection
C. It makes the oven run faster
D. It removes the need for feeder verification
Show answer
Answer: A
3. An operator loosens AOI thresholds so fewer boards flag. What has actually happened?
A. The process improved
B. The paste improved
C. Fewer boards need packing
D. Escapes have been given a route to the customer in exchange for prettier numbers
Show answer
Answer: D
4. What is the correct way to disposition a flagged AOI call?
A. Confirm every call without looking
B. Delete calls that repeat often
C. Verify against the real board, with magnification where needed, then disposition
D. Restart the machine until the call clears
Show answer
Answer: C
5. What does a rising false call rate usually point to?
A. Program, library, lighting or height data that needs maintenance
B. Operators working too carefully
C. Paste that is too fresh
D. Boards that are too clean
Show answer
Answer: A
Previous: Stage 6: Reflow soldering