If we are wondering which process has the biggest room for improvement in the automatic component assembly process (SMT), the answer without a doubt is the screen printing or solder paste application process.
It is known that this solder paste printing process produces more defects than any other process within the automatic SMD component assembly process.
If to this we add that components are increasingly small, that the lead spacings of the components are narrower and as a result, the PCBs are more densely populated, we have enough variables to make it increasingly difficult to control the screen printing or solder paste application process.
In addition to all these difficulties, we have to bear in mind that we are dealing with a world of miniature dimensions; a stencil has a thickness of 120 µm, meaning that the expected height of the paste is 120 µm. Standard solder pastes (Type 3) contain 25-45 µm spheres (Type 4 contains 20-38 µm spheres)… In this miniature universe what is good to the human eye in reality has an enormous margin for improvement. And it is here, always with the aim of reducing poor quality costs, where this technology becomes valuable.
An SPI (Solder Paste Inspection) is a machine that we place right after the screen printing machine and it carries out an inspection of the solder paste applied to each one of the PCB pads in three dimensions. It measures and compares the expected volume and alignment with what has actually been applied. And thanks to its database and powerful statistical process control software. It has become an essential tool to control the most delicate process on the SMD assembly line.