SPI systems measure height, volume, area on paste deposits, compute stencil offsets and detect defects such as bridges, missing, shape 2D, shape 3D, … on PCBs. Because of this, ideal SPI systems are extremely different from a system that would be designed to measure height on perfect cylindrical targets placed on a smooth surface.
Then the main challenge of an SPI lies in the ability to reference correctly in Z and adapt to PCBs’ specific characteristics.
SPI system’s ability to measure small pads accurately depends first & foremost on striking the right balance between:
the Z-referencing method & associated algorithms
the information generated for each point in the scene by the optical configuration (number of cameras, number of projectors, triangulations angles, optics resolution (MTF), pixel size, …)
the warpage compensation
the local X,Y referencing
the bareboard teaching
On large pads, transfer efficiency is 100% and paste deposits are perfectly shaped. On small pads, transfer efficiency can be around 60% only and paste deposits are no longer perfectly shaped.
Traditional SPI systems use a threshold under which they do not measure, they extrapolate!
In production (not during demonstrations…), this threshold is generally set around 40 µm in height to limit false calls.
But this threshold has a dramatic influence on the systems’ ability to measure small deposits accurately.
This drawback has been evidenced by Shea Engineering (2013):
“None of the machines seemed to capture as much volume as weighing; they all came in 10-65% below the weighed amounts, depending on how they were programmed.”
Results on production PCB for small pad:
This comparison test was performed by Vi TECHNOLOGY on a leading traditional SPI system and on revolutionary 3D SPI, called PI.
The pad size is 250 µm diameter (327 pixels for the pad surface @ 10 µm pixel size) and the real paste deposit volume is between 35 to 40%, measured with microscope.
This leading traditional system measured 4% volume around with a standard deviation of 1% !
The issue is not repeatability but accuracy … nor is it pixel size but rather z-referencing.
Vi TECHNOLOGY believes that accuracy is the key criteria for SPI system and then developed PI accordingly.
With its ultra-large Field Of View, together with patented Z-referencing, PI is the system able to measure paste volume with needed accuracy, keeping necessary speed for PCB assembly.