Learning objectives
- Explain what a three dimensional solder paste inspection system measures on every pad
- Read volume, height, area and offset results for a deposit
- Respond to trend alarms by checking the printer, not by resetting the alarm
- State why catching a print fault at this stage is far cheaper than catching it later
Lesson
Solder paste inspection, called SPI, stands immediately after the printer and checks the work of Stage 3 before a single component is placed. A three dimensional SPI machine uses structured light to measure every paste deposit on the board. For each pad it reports the volume of the deposit, its height, its area and its offset from the centre of the pad, and it compares each value against the limits set in the inspection program. On advanced lines the data does not just pass or fail boards. It feeds back to the printer, correcting alignment offset and warning of volume drift, so the process corrects itself while the fault is still small.
The limits deserve respect and understanding. One published production case works with a pass window of about 60 to 160 percent of the target paste volume, but trainees must learn that limits are set per program and per product, never treated as a universal law. What matters everywhere is the trend. A single board near a limit is information. Five boards drifting the same way is the process speaking, usually about a stencil that needs cleaning, apertures that are clogging, or paste that is drying out. The classic operator mistake at this station is resetting or ignoring an alarm without going to the printer to clean the stencil or check the apertures. The alarm then returns, board after board, until the drift crosses the limit and placement begins on bad deposits.
The economics explain why this station exists. A missing or offset deposit caught at SPI costs almost nothing: the board is cleaned and reprinted, or the printer is adjusted. The same fault found after placement wastes components and machine time, and found after reflow it costs skilled rework or scraps the board entirely. James Onsomu teaches in his lean PCB manufacturing book that teams should analyse performance data and act on it early, building a culture where problems are corrected at their source. SPI is that philosophy built into a machine. The operator who reads trends, cleans the stencil early and keeps the printer centred is quietly protecting the yield of the whole line.
Key numbers
- Inspection coverage: every paste deposit on every board is measured, not sampled
- Example pass window from one published case: about 60 to 160 percent of target volume, with real limits always set per program and product
- Measurements per deposit: volume, height, area and offset from pad centre
- Feedback: SPI data can drive automatic printer offset and volume correction on advanced lines
Defects to spot
- Insufficient deposits: clogged apertures or falling print pressure
- Excess deposits: aperture design, worn stencil, or pressure set too high
- Offset deposits: alignment drift between stencil and board
- Bridging between deposits: excess paste or slump bringing neighbouring deposits together
- Poor deposit shape: paste drying out or separation faults at the printer
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 a three dimensional SPI system measure?
A. The colour of each component
B. The volume, height, area and offset of every paste deposit
C. The temperature of the oven
D. The strength of finished solder joints
Show answer
Answer: B
2. SPI alarms keep reporting falling volume on the same pads. What is the correct response?
A. Reset the alarm and keep running
B. Widen the limits in the program
C. Stop and check the printer, clean the stencil and inspect the apertures
D. Skip SPI for the rest of the lot
Show answer
Answer: C
3. Paste deposits drift a little further in the same direction on five boards in a row, but none has failed yet. What does this mean?
A. Nothing, only failures matter
B. The trend is an early warning that the process is drifting and should be corrected now
C. The SPI machine is broken
D. The boards should be scrapped
Show answer
Answer: B
4. Why is a print fault cheapest to fix at the SPI stage?
A. The board can still be cleaned and reprinted before components are placed
B. SPI operators work for free
C. Faults disappear in the oven
D. Customers never check printed boards
Show answer
Answer: A
5. A trainee says the SPI pass window is the same for every product. What is the correct teaching?
A. The trainee is right, one window fits all
B. Windows are set by the operator each morning
C. There are no windows in SPI
D. Limits are set per program and per product, and published examples are only a guide
Show answer
Answer: D
Previous: Stage 3: Solder paste printing