Solder paste deposition 100% accuracy is especially important for PCBs that are highly populated with fine-pitch devices like QFNs, BGAs, and tiny passive component packages like 0201 and 00105. Those packages have very little pad area. Therefore, registration must be precise and accurate.
If you’re dealing with a 01005 pad and if registration is off by a mil, this will shift placement and paste deposition by half the size of the pad. Without sufficient paste, a cold solder joint results. If too much paste is dispensed in micro BGA, CSP, or 0105 packages, it creates a short between the two edges of these packages. It’s best to use paste type 4.0 or 4.5, which has small metal particles for better solder adhesion on the smaller BGA balls, Fig. 1.
Laser solder paste height inspection or SPI is especially valuable here. It uses laser technology to detect the amount of paste applied on the pads. But that’s only part of the story. It also has to be fast, accurate, highly reliable, and repeatable.
Basically, SPI shoots a thin slice of laser light through its lens onto the solder paste. That way, it measures the depth of solder paste being deposited on the SMT pads. Paste height or depth can be as low as one mil deposition since it’s the smallest fraction of deposition. Advanced SPI machines can do up to one micron accuracy or in some cases submicron. A typical machine would do a two-micron accuracy.
The laser light inspects the solder paste first before reaching the PCB’s surface. It calculates the amount of paste from the PCB’s surface to the highest place. It can shoot a laser directly at 90° angle perpendicular to the board. That way, when it hits the surface of the board it will reflect from there.