1. Solder Paste Stenciling
A solder paste (a sticky mixture of tiny solder particles and flux) is applied to the component pads on the bare PCB.This is done using a machine that forces the paste through a stainless steel stencil that has openings precisely matching the pads. This ensures that solder is only applied to the required locations.
2. Component Placement
The board moves to the Pick-and-Place Machine.This robotic equipment precisely picks up the Surface Mount Devices (SMDs) from feeders and places them onto the solder paste-covered pads according to the design file (Gerber data).The tacky solder paste holds the components in place temporarily.
3. Reflow Soldering
The assembled board passes through a Reflow Oven.The oven has multiple controlled heating zones. It gradually raises the temperature to melt the solder paste (the reflow phase) and then cools it down (the cooling phase).As the molten solder solidifies, it forms a strong, permanent electrical and mechanical bond between the component pins and the PCB pads.
4. Inspection and Quality Control
After reflow, the board undergoes rigorous inspection to check for defects:Automated Optical Inspection (AOI): A camera system scans the board to check for component presence, correct placement, polarity, and solder joint quality (e.g., bridging, insufficient solder).
X-Ray Inspection (AXI): Used for complex components like BGAs (Ball Grid Arrays) where solder joints are hidden beneath the package.
5. Through-Hole Component Insertion
Components that cannot be surface mounted (e.g., large connectors, high-power components) use Through-Hole Technology (THT).
Components are inserted manually or by machine into plated holes.
These components are typically soldered using a Wave Soldering Machine or selective soldering, where the bottom side of the board passes over a wave of molten solder.
6. Final Test and Cleaning
Functional Testing (FCT): The PCBA is powered up and tested using specialized jigs or equipment to ensure it performs its intended function.
Cleaning: The board is cleaned to remove any residual flux from the soldering process.
