1. What is SMT
SMT is surface mount technology (Surface Mounted Technology) (abbreviation of Surface Mounted Technology), which is the most popular technology and process in the electronics assembly industry. Electronic circuit surface mount technology (Surface Mount Technology, SMT), called surface mount or surface mount technology. It is a kind of surface assembly components without leads or short leads (SMC/SMD for short, chip components in Chinese) mounted on the surface of the Printed Circuit Board (PCB) or the surface of other substrates. Circuit assembly technology in which reflow soldering or dip soldering is used for soldering and assembly.
SMT patch refers to the abbreviation of a series of technological processes that are processed on the basis of PCB.
Second, the detailed process of SMT patch processing
1. Material procurement, processing and inspection The 
material purchaser carries out the original purchase of materials according to the BOM list provided by the customer to ensure that the production is basically correct. After the purchase is completed, material inspection and processing are carried out, such as pin header cutting, resistance pin forming and so on. Inspection is to better ensure the quality of production.
2. Silk
screen screen  printing, that is, screen printing, is the first process of SMT processing. Silk screen refers to the solder paste or patch glue being printed on the PCB pads to prepare for the soldering of components. With the aid of a solder paste printer, the solder paste is penetrated through the stainless steel or nickel steel mesh and attached to the pad. If the stencil used for silk screen printing is not provided by the customer, the processor needs to make it according to the stencil file. At the same time, since the solder paste used must be stored frozen, the solder paste needs to be thawed to a suitable temperature in advance. The thickness of the solder paste printing is also related to the squeegee, and the thickness of the solder paste printing should be adjusted according to the PCB processing requirements. 
3. Dispensing is 
generally in SMT processing. The glue used for dispensing is red glue. The red glue is dropped on the PCB position to determine the components to be soldered and prevent electronic components from being weighted or not due to their own weight during the reflow soldering process. It is fixed and other reasons are dropped or weakly welded. Dispensing can be divided into manual dispensing or automatic dispensing, which is confirmed according to process requirements. 
4. Mounting 
The placement machine can quickly and accurately mount the SMC/SMD components to the specified pad positions on the PCB board without damaging the components and the printed circuit board through the functions of suction-displacement-positioning-placement. . Mounting is generally located before reflow soldering. 
5. 
Curing is to melt the patch glue and fix the surface mount components on the PCB pad. Generally, heat curing is used.
6. Reflow soldering 
Reflow soldering realizes the mechanical and electrical connection between the solder ends or pins of surface mount components and the printed board pads by remelting the solder paste pre-distributed on the printed board pads. Soft soldering. It mainly relies on the effect of hot airflow on the solder joints. The colloidal flux undergoes physical reaction under a certain high temperature airflow to achieve SMD welding. 
7. Cleaning 
After the soldering process is completed, the board surface needs to be cleaned to remove the rosin flux and some solder balls to prevent them from causing short circuits between components. Cleaning is to place the soldered PCB board in a cleaning machine to remove the flux residue on the surface of the PCB assembly board, which is harmful to the human body, or the flux residue after reflow soldering and manual soldering, and the contaminants caused during the assembly process. 
8. Inspection and 
inspection are the welding quality inspection and assembly quality inspection of the PCB assembly board after the assembly is completed. Need to use AOI optical inspection, flying probe tester and perform ICT and FCT function test. The QC team conducts random inspections of PCB board quality, inspects substrates, flux residues, assembly failures, etc.; 
9. Rework 
SMT is usually repaired to remove components that have lost functions, damaged pins or are incorrectly arranged, and replace them with new components. The maintenance personnel are required to be familiar with the repair process and technology. The PCB board needs to go through visual inspection to check whether the components are missing, the direction is wrong, the false soldering, short circuit, etc. If necessary, the board in question needs to be sent to a professional rework station for repair.