SPI is solder paste inspection system. The main function is to detect the quality of tin paste printing, including volume, area, height, XY offset, shape, bridge and so on. The detection principle of how to detect very small solder paste quickly and accurately is generally PMP( phase modulation profile measurement technology) and Laser( laser triangulation technology).
Laser triangulation technology
The detection light source used is laser, the distortion of laser beam in different height plane, the detection head moves continuously in a certain direction, and the camera takes pictures at a set time interval, so as to obtain a set of laser distortion data and calculate it. The way to get the test results (see below).

Advantages: faster detection speed
Disadvantages :
1) low laser resolution, generally only 10-20 um.
2) single sampling, low repeatability.
3) motion sampling, external vibration and transmission vibration have a great impact on the detection.
The monochromatic light of the 4) laser is less adaptable to the color of the PCB plate.
Market conditions: laser technology has gradually withdrawn from the SPI industry. At present, the Korean Parmi is still using laser technology (dual laser technology)
PMP Phase Modulation Profile Measurement Technology
Using a white light source, the solder paste is measured by the phase change of the structural grating (see below)

Measuring the gray-level variation of the structural grating to obtain high-precision height values (see below)

3.The phase change is used to sample each solder paste 8 times to ensure the high repeatability (see figure below)