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3.3 Through-Hole (THT) and Manual Assembly

Where automation ends, skilled craftsmanship begins; the "Back End" of the PCB assembly process handles heavy-duty components that require mechanical strength surpassing what surface mount soldering can provide. Connectors, large capacitors, and power transformers typically require Through-Hole Technology (THT), utilizing metal leads that pass through the board to create a reinforced mechanical bond.

Wave Soldering: The Bulk Process

For boards with high counts of THT components, Wave Soldering offers the highest throughput.

  • Fluxing: A spray fluxer coats the bottom of the board to clean oxides.
  • Preheat: Infrared heaters raise the board temperature to prevent thermal shock (100°C - 130°C) and activate the flux.
  • The Wave: The board passes over a standing wave of molten solder. The solder pumps up into the holes via capillary action, filling the barrel and filleting around the lead.
  • Pallets: To protect SMT parts already on the bottom side, the board is often locked into a "selective pallet" made of composite material, which masks off everything except the THT pins.

Selective Soldering: The Precision Robot

For dense boards where wave pallets are not feasible, Selective Soldering is employed.

  • Mini-Wave: A robotic nozzle creates a small fountain of solder (3mm - 10mm diameter).
  • Programmable: The robot moves the board or the nozzle to solder individual points sequentially. This provides the quality of a wave solder joint without the thermal stress or masking requirements of the bulk wave process.

Manual Soldering and Inspection

Human operators perform tasks too complex or accessible for machines.

  • IPC Certification: Technicians must be certified to IPC-J-STD-001 standards.
  • Fillet Inspection: Quality is verified visually. IPC Class 2 requires 50% vertical fill of the hole; IPC Class 3 (Medical/Aero) requires 75% fill.
  • AOI Integration: Post-wave Automated Optical Inspection (AOI) checks for solder bridges, missing pins, and insufficient wetting, acting as a gate before final assembly.

Final Checklist

Parameter

Function

Critical Limit / Standard

Barrel Fill

Mechanical Strength

>75% (Class 3) / >50% (Class 2)

Preheat Temp

Shock Prevention

90°C - 120°C (Topside)

Dwell Time

Solder Contact

2 - 4 seconds per joint

Flux Application

Oxide Removal

Uniform coverage, no skips

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