3.3 Manual and Through-Hole Assembly

While SMT is a triumph of robotic consistency, Through-Hole Technology (THT) and Manual Assembly introduce the most unpredictable variable in manufacturing: the human operator. Despite the dominance of SMT, manual work remains unavoidable for components requiring extreme mechanical strength (power connectors, heavy transformers) or specific legacy form factors. This stage shifts the risk profile from "machine calibration" to "workmanship discipline." Here, quality is not programmed; it is trained.

Why Through-Hole Still Exists

You cannot software-update physics. SMT components rely on a surface bond, which is mechanically weak against shear forces.

• Mechanical Strength: A connector plugged and unplugged daily needs anchors. THT leads pass through the fiberglass board, leveraging the structural integrity of the substrate.
• High Power: Large capacitors and inductors often require leads to dissipate heat and handle high current without melting the solder joint.
• Engineering Reality: If you design a heavy I/O port as SMT to save money, it will likely rip the copper pads off the board in the user's hands.

The Slide Line (Manual Insertion)

Action: Humans insert components into holes.

Operators sit along a conveyor belt, each assigned specific components to insert as the board passes by.

• The Risk: Cognitive fatigue. An operator doing a repetitive task 2,000 times a shift will eventually make a mistake.
• If a polarized capacitor is inserted backwards → Then the error is often missed until the smoke test, as machines rarely verify THT polarity post-insertion.
• Control: Use "Golden Sample" boards at every station for visual reference. Use "Clinching" (bending leads) to prevent parts from falling out before soldering.

Wave Soldering (Mass Production THT)

Action: The board passes over a fountain of molten solder.

Instead of soldering one pin at a time, the entire bottom of the board is washed in a solder wave.

• The Engineering Reality: This is a violent thermal event. The entire assembly is subjected to high heat.
• If SMT parts are on the bottom side → Then they must be glued down, or they will wash away into the solder pot.
• If a large component "shadows" a smaller one → Then the wave will not reach the small pin, resulting in an "Open" (missing solder).

Selective Soldering (The Precision Robot)

Action: A mini-solder fountain moves to specific coordinates.

This is the automated alternative to hand soldering. A nozzle rises up and solders only the specific THT pins, leaving the rest of the board untouched.

• Benefit: Repeatable quality without thermally shocking the whole board.
• Cost: Slower than wave soldering, but faster and more reliable than humans.

Hand Soldering (The Last Resort)

Action: A technician with a soldering iron.

Used for post-wave touch-up, wires, or heat-sensitive parts that cannot survive the wave oven.

• The Physics: A good solder joint requires the pad and the lead to be heated simultaneously to melt the wire.
• If the iron is too hot or held too long (> 3 seconds) → Then the adhesive holding the copper pad destroys, and the pad lifts off the board ("Pad Lift").
• If the iron is removed too soon → Then the solder does not wet the barrel, creating a "Cold Joint" (gray, lumpy, and brittle).

Final Checklist