Solder Paste Inspection: Measuring Volume, Height, Offset
Solder Paste Inspection (SPI) checks every paste deposit in three dimensions right after stencil printing. This guide explains how SPI measures volume, height, area and offset, which limits make sense under IPC-7527, and how to catch print defects before they get expensive.
View SPI systemsHow does an SPI system measure a paste deposit?
An SPI system scans the freshly printed board straight after stencil printing and builds a three-dimensional height profile of every paste deposit. From that profile the software derives four metrics: volume, height, area and offset relative to the pad.
Height is captured by laser triangulation or by projected fringe patterns (Moire phase shifting). Both methods return a point cloud per deposit, from which the true volume in cubic millimetres is calculated - unlike a plain 2D camera that only sees the footprint.
How to set squeegee pressure, snap-off and cleaning for stable deposits.
Read the guideWhich metrics does SPI check and what do they mean?
Every deposit is judged against a target volume derived from pad size and stencil thickness. The software compares the measured value with the target and flags outliers both above and below the set limits.
- Volume too low: clogged aperture, excessive snap-off or squeegee speed too high.
- Volume too high: smeared stencil underside, too much pressure or paste bleeding.
- Offset consistently in one direction: stencil-to-board misalignment, not random noise.
- Bridging between deposits: critical for fine-pitch and BGA components.
How do you set sensible limits?
Limits separate good from bad. Set them too tight and the false-call rate rises while the line stops for no reason; set them too loose and real defects slip through. IPC-7527 provides a proven framework and distinguishes between Class 2 and Class 3.
A common starting point is a volume window of 50 to 150 percent of target and an offset limit of 25 percent of pad width. Fine-pitch, 0201 or micro-BGA get tighter windows, uncritical coarse features can be wider. The key point: define limits per pad type or component family, not globally across the whole board.
- IPC-7527 Class 3 (aerospace, medical) requires tighter windows than Class 2.
- Grade limits by pad type: 01005/0201 tighter, connectors wider.
- Set a warning limit ahead of the fail limit to see trends early.
- Feed SPI data back to the printer as a closed-loop control.
Frequently asked questions
Why is a 2D camera not enough?
A 2D camera only sees the footprint and offset, not the height. Without height you cannot calculate volume, yet too little or too much paste is the most common root cause of solder defects. That is why true 3D measurement is now standard.
What volume window is a good starting value?
A common starting point is 50 to 150 percent of the target volume. For fine-pitch and micro-BGA pads the window is tightened, for example 60 to 140 percent, and defined separately per pad type.
Does SPI replace later AOI inspection?
No. SPI checks the paste before placement, while AOI checks the populated or reflowed assembly. SPI prevents defects at the source, AOI verifies the end result. The two complement each other and cover different defect types.
What does IPC-7527 cover?
IPC-7527 is the standard for inspecting solder paste print. It defines the metrics, acceptance criteria and their mapping to manufacturing Class 2 and Class 3, and serves as the basis for setting inspection limits.
Looking for an SPI system or inspection strategy?
We advise on 3D SPI systems, limit concepts to IPC-7527 and integration with your stencil printer - for stable, low-defect production.
True 3D measurement
Volume, height, area and offset per deposit.
To IPC-7527
Limits and classes set to the standard.
Defects at the source
Print faults stopped before placement.
Expert advice
Soldering specialists support the rollout.