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 accessibleinaccessible for machines.
- IPC Certification: Technicians must be certified to IPC-J-STD-001 standards. This is not casual labor; it is skilled trade work.
- 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.
The "Bed of Nails" Test (ICT)
Once all soldering (SMT and THT) is complete, the electrical integrity is verified.
- In-Circuit Test (ICT): The board is pressed onto a fixture containing hundreds of spring-loaded probes ("bed of nails").
- Parametric Test: The system measures resistance, capacitance, and voltage on every net. It detects "opens" (bad solder joints) and "shorts" (solder bridges) instantly. Unlike functional testing, ICT tells the repair technician exactly which part failed (e.g., "Resistor R45 measures 10k instead of 1k").
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 |
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