Analysis of SMT

SMT is the abbreviation of surface mounted technology, which is the most popular technology and process in the electronic assembly industry. What are the characteristics of SMT

1. High assembly density, small size and light weight of electronic products. The volume and weight of patch components are only about 1/10 of those of traditional plug-in components. Generally, after SMT is adopted, the volume of electronic products will be reduced by 40%~60% and the weight will be reduced by 60%~80%.

2. High reliability and strong anti vibration ability. Low solder joint defect rate.

3. Good high frequency characteristics. Reduced electromagnetic and RF interference.

4. Easy to realize automation and improve production efficiency. Reduce the cost by 30%~50%. Save materials, energy, equipment, manpower, time, etc.

SMT complete equipment includes:

Printing machine

Mounter

Reflow soldering

SPI

AOI

The basic process elements of SMT include: screen printing (or dispensing), mounting (curing), reflow welding, cleaning, testing and repair.

1. Silk screen printing: it is used to leak solder paste or patch adhesive onto the PCB pad to prepare for the welding of components. The equipment used is a screen printing machine (screen printing machine), which is located at the front end of the SMT production line.

2. Dispensing: it drips the glue onto the fixed position of the PCB. Its main function is to fix the components to the PCB. The equipment used is a dispensing machine, which is located at the front end of the SMT production line or behind the testing equipment.

3. Mounting: its function is to accurately install the surface assembled components to the fixed position of PCB. The equipment used is a chip mounter, which is located behind the screen printing machine in the SMT production line.

4. Curing: its function is to melt the patch adhesive, so that the surface assembled components and PCB boards are firmly bonded together. The equipment used is a curing furnace, which is located behind the mounter in the SMT production line.

5. Reflow soldering: its function is to melt the solder paste, so that the surface assembled components and PCB boards are firmly bonded together. The equipment used is a reflow soldering furnace, which is located behind the SMT production line.

6. Cleaning: it is used to remove the welding residues such as flux harmful to human body on the assembled PCB. The equipment used is a cleaning machine, and the position can be either on-line or off-line.

7. Inspection: it is used to inspect the welding quality and assembly quality of the assembled PCB. The equipment used includes magnifying glass, microscope, on-line tester (ICT), flying needle tester and automatic optical inspection

8. (AOI), X-ray detection system, function tester, etc. The position can be configured in the appropriate place of the production line according to the needs of detection.

9. Repair: it is used to rework the PCB board that is detected to be faulty. The tools used are soldering iron, repair workstation, etc. Configure anywhere in the production line.

1、 SMT process flow ----- single side assembly process

Incoming material detection -- > screen printing solder paste (spot patch adhesive) - > patch -- > drying (curing) - > reflow soldering -- > cleaning -- > detection -- > repair

2、 SMT process flow ----- single side mixed loading process

Incoming material detection -- > screen printing solder paste on side a of PCB (spot patch adhesive) - > patch -- > drying (curing) - > reflow soldering -- > cleaning -- > plug-in -- > wave soldering -- > cleaning -- > detection -- > repair

3、 SMT process flow ----- double sided assembly process

A: Incoming material detection -- > PCB side a silk screen solder paste (spot patch adhesive) -- patch -- > drying (curing) -- side a reflow soldering -- > cleaning -- > turnover -- > PCB side B silk screen solder paste (spot patch adhesive) -- patch -- > drying -- > reflow soldering (preferably only for side B -- > cleaning -- > detection -- > repair). This process is applicable to SMD with PLCC and other large SMD pasted on both sides of PCB.

B: Incoming material inspection -- > screen printing solder paste on side a of PCB (spot solder) -- patch -- > drying (curing) -- reflow soldering on side a -- > cleaning -- > turnover -- > spot solder on side B of PCB -- > patch -- > curing -- > wave soldering on side B -- > cleaning -- > inspection -- > repair) this process is applicable to reflow soldering on side a and wave soldering on side B of PCB. In the SMD assembled on the B-side of PCB, this process should be adopted only when the SOT or SOIC (28) pin is below.

4、 SMT process flow ----- double side mixed loading process

A: Incoming inspection -- > PCB B-side point patch adhesive -- > patch -- > curing -- > turnover -- > PCB A-side plug-in -- > wave soldering -- > cleaning -- > inspection -- > repair first paste then insert, applicable to the case where SMD components are more than separated components

B: Incoming material detection -- > plug-in on side a of PCB (pin bending) - > turnover -- > patch adhesive on side B of PCB -- > patch -- > curing -- > turnover -- > wave soldering -- > cleaning -- > detection -- > repair