Four Stages: Inspection and Testing

　　After soldering, the PCBA undergoes the following quality checks:

　 　a. Visual Inspection

　　Verify by visually inspecting the board, solder joints, and component placement:

　　No solder bridges or shorts between joints or traces

　　No visible cracks or cold solder joints

　　Component alignment/movement with pads

　　Missing or misaligned components

　　Solder paste quality and coverage

　 　b. X-ray Inspection

　　X-ray imaging can inspect the quality of solder joints hidden under components. Common checks include:

　　Solder voids

　　BGA/CSP ball alloy and shape

　　QFN/QFP hidden leads

　 　c. ICT Testing

　　In-circuit testers check for electrical faults on the assembled PCB by probing test points and measuring reference values. Tests performed include:

　　Continuity - Detecting open or short circuits

　　Voltage - Measuring voltage drops and levels

　　Capacitance - Capacitance values

　　Resistance - Resistance values

　　Diodes - Polarity and voltage drop

　 　d. Functional Testing

　　Functional testing of the fully assembled PCB is done by operating it within specified temperature, voltage, and frequency ranges and checking if the output meets product specifications.

　　Fifth Stage: Repair and Rework

　　If needed, faulty solder joints and component placement can be repaired by:

　　Hand soldering - Using soldering iron, solder wick, and flux for small solder bridges or dry solder.

　　Hot air rework - Using a hot air gun to remelt and reshape faulty BGA/CSP solder balls.

　　Soldering station - Powerful programmable station for component removal and precise rework.

　　Panel separation - Cutting and routing individual PCBs from larger panel assemblies.

　 　Sixth Stage: Conformal Coating

　　Assembled PCBs are usually coated with a thin polymer film such as acrylic, silicone, or polyurethane. This conformal coating protects the circuitry from moisture, dust, chemicals, and temperature damage. It is applied by:

　　Dip coating - PCB passed through a coating liquid bath

　　Manual or selective spray coating

　　Automated spray coating process

　　Key criteria for conformal coating assessment are:

　　Coating thickness - Typical 25-75 μm

　　Uniformity of coverage - No bubbles or pinholes

　　Adhesion testing

　　Insulation resistance - Preventing leakage current and arcing

　 　Seventh Stage: Final Testing and Quality Inspection

　　Final back-end production line testing ensures the PCBA is reliable and meets all product specifications:

　　Visual quality control check - Inspecting for any coating defects, markings, labels.

　　Dimensional check - Dimensions and clearances

　　Electrical testing - Repeating ICT, boundary scan, flying probe testers.

　　Burn-in test - Operating the PCBA under load conditions for an extended period to eliminate early failures.

　　Final assembly integration - Assembling the tested PCBA into the final product enclosure.

　 　PCBA Key Equipment

　　Some of the major equipment used in printed circuit board assembly includes:

　　Solder paste printer - For precise deposition of solder paste onto the PCB. Uses stencils and automated squeegees.

　　Pick and place machine - Automatically and quickly places components onto the PCB accurately. Uses feeders to place components.

　　Reflow oven - Heats the PCB assembly to soldering temperature to connect components. Uses convection, infrared heating.

　　AOI machine - Automated optical inspection system that scans the board and detects defects in real-time.

　　In-circuit tester - Tests the assembled PCB for shorts, opens, and measures parameters.

　　Conformal coating machine - Automatically applies protective coating to the assembled PCB via spraying or dipping.

　　Flying probe - Checks electrical continuity and performs analog measurements of the circuit.

　 　Key Considerations for PCBA Process Control

　　To ensure PCBA process quality, some of the key parameters that need to be controlled and monitored include:

　　a. Solder Paste Printing

　　Solder paste tackiness and viscosity

　　Stencil and PCB alignment

　　Printing speed, pressure, and tool gap

　　Solder paste height and coverage

　　b. Component Placement

　　Pick and place machine accuracy

　　Component packaging - Tape, reel, tray

　　Feeder settings - Pitch, gap, force

　　Placement speed, accuracy, and sequencing

　　Reflow soldering

　　Preheat time and temperature

　　Heating zone temperature uniformity

　　Peak reflow temperature and time

　　Cooling rate after reflow

　　d. Cleaning process

　　DI water quality – pH value, conductivity, ionic contamination

　　Water-based cleaning and solvent cleaning

　　Drying – dehumidification, electrostatic control

　　e. Conformal coating

　　Coating material – viscosity, density, shelf life

　　Film thickness, uniformity and coverage

　　Curing method – temperature, humidity, ultraviolet

　　By closely monitoring and controlling these parameters, potential defects can be prevented and process yield can be improved. Statistical process control techniques (such as process capability analysis) help assess performance.

　 　Main advantages of PCBA automation

　　Using machines to automate different stages of PCBA offers the following advantages compared to manual assembly:

　　Speed – Pick-and-place machines can accurately place thousands of components per hour, significantly reducing assembly time.

　　Repeatability – Programmed machines execute precisely and repeatedly, improving process consistency.

　　Quality – Automated optical inspection can accurately detect and flag defects for correction.

　　Scalability – Production volume can be flexibly adjusted with minimal additional time or cost.

　　Reliability – Process control eliminates errors due to human factors.

　　Programmability – Changing programs allows for quick changeover between different PCB components.

　　Traceability – All process data, defects, and parameters can be monitored in real time.

　　Flexibility – Can handle a variety of components, such as 01005 chips, 0.4 mm pitch QFN.

　　For medium-to-high volume production requirements, automated assembly offers significant quality and yield advantages over manual PCBA. New Industry 4.0 machines provide complete data-driven optimization for PCBA processes.