SMT Process Introduction   SMT is known as surface mount technology that is the combination of electronic components and PCB assembly process in the newest welding technology. The welding assembly method is known simply on the use of Screen Printer to print the solder paste in the PCB's pads, and then install the SMD components by Mount Placement Machine (such as resistors, capacitors, inductors, diodes, transistors and integrated circuits (IC), etc.) on board, then go through the hot air reflow to melt solder paste for electronic components and PCB-PAD with complete assembly and welding technology. The following is the detail description for the SMT process: Solder Paste Printing Process: Solder paste is a solder powder and flux mixture, with a certain viscous paste, Sn-Ag-Cu (SAC) alloy is the first choice of lead-free solder now. Usually they are placed in the refrigerator, it needs to do the temperature recovering before using and prevent moisture remains in the solder paste to affect the solder joint quality, and then do a well mixed between SAC alloy and flux through the stirrer, placed onto the stencil, and do the solder paste printing. Stencil is another important article in the solder paste printing process. Currently, it has been produced by laser cutting to ensure that the hole wall is smoothly and easily to facilitate the solder paste through the hole. Check the tension of stencil before using to ensure the printing quality. We used the automatic solder paste printer that there is an automatic visual position correction system to correct the position accuracy between board and stencil for each PCB, and effective control of squeegee speed, pressure and separation speed/way of stencil to make a stable printing quality. In addition, it has an automatic cleaning system to clean the stencil by frequency setting in machine system to avoid the hole plug appearance in process. To perform the solder paste inspection and thickness measurement operation to monitor the solder paste printing quality and stability. Dispensing process: We used the dispensing process after solder paste printing if need that it's applied to double side SMT components design of the board to prevent the components falling on the bottom side when the board is going through wave soldering machine in DIP process. Solder paste printed on the pads of board and then the board will be transferred by conveyor to the dispenser. Dispensing head drips the glue on the center of component according to the program setting, the glue volume is control point in this process. Since most of the current products are designed of using small parts (eg: 0402), high density and complexity. More components protection for bottom side to use the DIP carriers in DIP process so the dispensing process is less likely to be used slowly. Components Mounted process: After the solder paste printing or dispensing process of the board and transferred to high-speed mounter.  This machine has an automatic visual position correction system to correct and adjust on the center of the components for each PCB. After board position finishing, the pickup head is combination of the revolution axis with vacuum and nozzle that will pick the components up from appointed feeder according to the program setting. Camera recognition system can make sure the components size correctness and correct the center of components. Rotation parts for direction or angle and close the vacuum suction to place the components on the pds of the PCB according to the program setting. Since the pads of PCB have been printed of the solder paste because of solder paste has a viscosity characteristic so components can be securely placed on the pads of board and not falling before reflow.  Based on components size and machine speed difference that the machine is divide into high-speed mounter and multi-function mounter. The high-speed mounter is used in small components mounting such as 0201, 0402 size of chip components. The multi-function mounter is used in IC parts mounting such as QFP and BGA type components. Reflow process: The main function of flux inside of the solder paste in reflow is for the auxiliary heat conduction, removed oxides and reduced the surface tension of welded materials. We used the hot nitrogen reflow oven. There is a good average temperature in inside of reflow and also reduced oxidized phenomenon to increase the spread and reliability on the solder joints. We will do the profile measurement and adjustment at the first production for new product based on the size and thickness of the board, component type and solder paste properties to get a divided into pre-heating zone, melting zone and cooling zone of ramp-soak-spike-cool curve for lead-free profile. To set the reflow temperature for each zone according to temperature profile and keep in SOP for future production standard.