Unit
- Bare board preparation: Printed circuit boards arrive as laminated panels with etched copper traces. Operators inspect for warpage, scratches, and contamination before loading into the assembly line.
- Solder paste application: A stainless steel stencil aligns over the board and a squeegee forces solder paste through apertures onto the copper pads. Paste viscosity and metal content determine transfer efficiency.
- Pick-and-place operation: Automated machines retrieve surface mount components from feeders and position them on the paste deposits with micron-level accuracy. Vision systems correct for component orientation.
- Reflow soldering: The populated board travels through a multi-zone oven where paste melts and solidifies to form mechanical and electrical connections. Temperature profile zones include preheat, soak, reflow, and cooling.
- Inspection stages: After reflow, automated optical inspection (AOI) checks solder joint shape, component presence, and polarity. X-ray inspection verifies hidden joints under ball grid arrays (BGAs).
- Through-hole assembly: For components with leads, wave soldering or selective soldering applies molten solder to the bottom of the board after insertion. This step is often performed on a separate line.
- Cleaning and coating: Flux residues are removed with aqueous or solvent cleaners. Conformal coating protects sensitive circuits from moisture and contaminants in final application environments.
- Functional testing: Powered tests verify circuit operation against design specifications. In-circuit test and flying probe test detect opens, shorts, and component value errors before shipment.
Process Flow
- Stage 1 - Receiving: Incoming PCB panels and components undergo visual inspection and moisture sensitivity level verification.
- Stage 2 - Solder Paste Printing: Stencil aligned, paste printed, deposit height measured using laser profilometry. Rework is performed if height falls outside specification.
- Stage 3 - Component Placement: Feeders loaded, program validated with first board, placement accuracy confirmed with glass slide test.
- Stage 4 - Reflow: Oven profile verified using thermocouple board. Nitrogen atmosphere used for fine-pitch assemblies to reduce oxidation.
- Stage 5 - Inspection: AOI scans all joints; failing boards are routed to repair. X-ray used for hidden joints. Statistical data logged for process control.
- Stage 6 - Through-Hole (if needed): Selective solder nozzle programmed for each lead location. Flux applicator minimizes bridging risk.
- Stage 7 - Cleaning and Conformal Coat: Aqueous wash removes flux, then board is dried thoroughly. Spray or dip coating applied, thickness verified with UV light.
- Stage 8 - Final Test: Functional test performed on bed-of-nails fixture. Outgoing boards receive a quality stamp and serial number traceability.
Quick Revision
- The assembly process consists of eight sequential stages from receiving to final test.
- Solder paste printing is the most critical stage for defect prevention.
- Inspection occurs after reflow and after through-hole to catch defects early.
- Functional testing verifies electrical performance before shipping.