The thickness of the solder paste coating directly determines the actual amount of tin in the solder joint. Unqualified thickness will directly cause various fatal welding defects: when the solder paste thickness is lower than the process requirements, the total tin amount is insufficient. During reflow soldering, the solder paste cannot fully wet the pad and component pins, and cannot form a complete and reliable metallurgical bond, which can easily cause problems such as cold solder joint, false solder joint, and open solder joint. For 01005 and 0201 micro chip components, as well as fine-pin QFP and BGA packages with a pitch of less than 0.3mm, a solder paste thickness deviation of more than 10% may cause batch cold solder joints. When the solder paste thickness is higher than the process requirements, the excess molten solder paste can easily overflow the pad range, leading to bridging and short circuits between adjacent pins and pads, directly causing the PCB board to be scrapped. Only by controlling the thickness within the qualified range through accurate inspection can the root cause of defects be blocked from the printing process and the basic welding quality be guaranteed.