SMT Soldering Process & Assembly Techniques
SMT Soldering Process – SMD Soldering & SMT Assembly Techniques.
SMT Soldering Process and techniques is little different from through hole technology.
SMD soldering or Surface Mount Technology assembly needs different SMT Equipment and greater experience and expertise. SMT PCB has flat tin-lead or gold-plated copper pads without any hole. These traces are called “solder pads“.
SMT Soldering Process
- Semi-solid solder called solder paste consisting of very fine solder and flux is dispensed on solder pads. Solder paste can be dispensed using stencil with screen printing process with a SMT screen printer.
- Once the solder paste is dispensed, the circuit board is moved to a conveyor belt of a pick-and-place machine. SMD components are picked up by the pick-and-place machine and placed on the PCB.
- Once all the electronic components are placed on the SMT circuit board, it is then conveyed into the reflow soldering oven. The SMT reflow oven has different chambers. The first chamber or zone is called pre-heat zone, where the temperature of the board and all the components is gradually and uniformly raised to prevent any crack to the PCB due to thermal shock.
- The next zone is high temperature zone where the temperature is high enough to melt solder paste so that the component leads get soldered to the pads on the circuit board. The surface tension of the molten solder keeps the components in place. Surface tension also automatically aligns the components on their pads.
SMT Soldering Techniques
Reflow soldering can be done using different techniques:
- Infrared reflow;
- Hot gas convection; or
- Vapor phase reflow
Each method having its own advantages and disadvantages.
With double-sided PCBs, the reflow soldering process is repeated using either solder paste or glue to hold the components in place. If glue is used then the parts must be soldered later using wave soldering process.
After SMT Soldering Process is Over
Once the soldering process is over, the PCA or the PCB Assembly (PCBA) need to be washed to remove flux residues and any stray solder balls that could short out closely spaced component leads. This can be done with SMT solvent cleaning machine. Rosin flux can be cleaned with fluorocarbon solvents, high flash point hydrocarbon solvents, or low flash. Water soluble fluxes are removed with deionized water and detergent, followed by an air blast to quickly remove residual water. No-Clean fluxes don’t need any cleaning.
Lastly, the Printed Circuit Assembly is visually inspected for any missing or misaligned components or solder bridging. This can be done using SMT Inspection Machine. If any fault is found during inspection, the board is sent for rework. Finally the board is sent for testing to verify that it is working correctly.
PCB Assembly Process Flow Chart (PCBA Process)
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