PCBA Tech - What is an SMT?

What is an SMT?SMT SMD refers to a series of technical processes on the basis of PCB processing acronym, is the surface mounted technology (Surface Mounted Technology) (referred to as Surface Mount Technology), the electronic assembly industry's most popular technology and process.

Under normal circumstances,the electronic products we use are designed by PCB plus various capacitors,resistors and other electronic components according to the designed circuit diagram,so all kinds of electrical appliances need a variety of smt chip processing techniques to process.

Process

Solder paste printing --> parts placement --> reflow soldering --> AOI optical inspection -->maintenance -->sub-board.

The advantages of smt chip processing: high assembly density,small size and light weight of electronic products.The volume and weight of circuit board components are only about 1/10 of that of traditional plug-in components.Generally,after SMT is adopted,the volume of electronic products is reduced by 40%~60 %,the weight is reduced by 60%~80%.High reliability and strong anti-vibration ability.The defect rate of solder joints is low.Good high frequency characteristics.Reduce electromagnetic and radio frequency interference.It is easy to realize automation and improve production efficiency.Reduce costs by 30%~50%.Save materials,energy,equipment,manpower,time,etc.

It is due to the complexity of the SMD processing process that many SMD processing factories specializing in SMD processing have emerged.In Shenzhen,thanks to the booming electronics industry,SMD processing has achieved an industrial boom.

SMT basic process components include: screen printing (or dispensing), placement (curing), reflow soldering, cleaning, testing, and repair

1.Silk screen:Its function is to leak solder paste or patch glue onto the PCB pads to prepare for the soldering of components. 

2.Dispensing: It is to drip glue onto the fixed position of the PCB board, and its main function is to fix the components on the circuit board. 

3.Mounting: Its function is to accurately mount the surface mount components to the fixed position of the PCB. 

4.Curing: Its function is to melt the patch glue, so that the surface assembly components and the PCB board are firmly bonded together. 

5.Reflow soldering: Its function is to melt the solder paste,so that the surface assembly components and the PCB board are firmly bonded together. 

6.Cleaning: Its function is to remove the solder residues such as flux that are harmful to the human body on the assembled PCB board. 

7.Inspection: Its function is to inspect the welding quality and assembly quality of the assembled PCB board. The equipment used includes magnifying glass, microscope, online tester (ICT), flying probe tester, automatic optical inspection (AOI), X-RAY inspection system, functional tester, etc. 

8.Rework: Its function is to rework the PCB boards that have failed to detect faults. The tools used are soldering irons, rework stations, etc. Configured at any position in the production line.

Single sided assembly

Incoming inspection => silk screen solder paste (point patch glue) => patch => drying (curing) => reflow soldering => cleaning => inspection => repair

Double-sided assembly

A:Incoming inspection => PCB's A-side silk-screen solder paste (point SMD glue) => SMD PCB's B-side silk screen solder paste (point SMD glue) => SMD => Drying => Reflow soldering ( It is best to only apply to side B => cleaning => inspection => repair).

B:Incoming inspection => PCB's A side silk screen solder paste (point patch glue) => SMD => Drying (curing) => A side reflow soldering => Cleaning => Turnover = PCB's B side point Patch glue => patch => curing => B surface wave soldering => cleaning => inspection => repair)

This process is suitable for reflow soldering on the A side of the PCB and wave soldering on the B side. In the SMD assembled on the B side of the PCB, this process should be used when there are only SOT or SOIC (28) pins or less.

Single sided mixed packaging process

Incoming inspection => PCB's A-side silkscreen solder paste (point patch glue) => SMD => drying (curing) => reflow soldering => cleaning => plug-in => wave soldering => cleaning => inspection = > Rework

Double sided mixed packaging process

A:Incoming inspection => PCB's B side point patch glue => SMD => curing => flip board => PCB's A side plug-in => wave soldering => cleaning => inspection => rework

Paste first and insert later, suitable for situations where there are more SMD components than separate components

B:Incoming inspection => PCB's A side plug-in (pin bend) => flip board => PCB's B side patch glue => patch => curing => flip board => wave soldering => cleaning => Inspection => Repair

Insert first, then paste, suitable for the situation where there are more separate components than SMD components

C: Incoming inspection => PCB A side silk screen solder paste => Patch => Drying => Reflow soldering => Plug-in, pin bending => Turnover => PCB side B point patch glue => Patch => curing => flipping => wave soldering => cleaning => inspection => rework A-side mixed assembly, B-side mounting.

D: Incoming inspection => PCB's B side spot patch glue => SMD => curing => flip board => PCB's A side silk screen solder paste => patch => A side reflow soldering => plug-in => Wave soldering on side B => cleaning => inspection => rework for mixed mounting on side A and mounting on side B. First paste on both sides of SMD,reflow soldering, then inserting, wave soldering E: Incoming inspection => PCB's B-side silk screen solder paste (point patch glue) => SMD => drying (curing) => reflow soldering = > Flip board => PCB's A-side silk screen solder paste => SMD => Drying = Reflow soldering 1 (partial soldering can be used) => Plug-in => Wave soldering 2 (If there are few components, manual soldering can be used) => Cleaning => Inspection => Rework A-side mounting and B-side mixed mounting.

Double sided assembly process

A: Incoming inspection,PCB A side silk screen solder paste (point patch glue), patch, drying (curing), A side reflow soldering, cleaning, flipping; PCB B side silk screen solder paste (point patch Glue), patch,drying,reflow soldering (preferably only for side B, cleaning, testing, and repairing)

This process is suitable for picking when large SMDs such as PLCC are attached to both sides of the PCB.

B: Incoming inspection, PCB A side silk screen solder paste (dot patch glue), patch, drying (curing), A side reflow soldering, cleaning, flipping;PCB B side dot patch glue, patch, Curing, B-side wave soldering, cleaning, inspection, rework) This process is suitable for reflow on the A-side of the PCB.