SOLDERING

Hand Soldering Flow-Through: PTH & SMT Components

 Scope

This SOP outlines the process for hand soldering Plated Through Hole (PTH) and Surface Mount Technology (SMT) components, ensuring consistent quality and compliance with IPC standards.

 Process Flow
1. Preparation

Workstation Setup: ESD-safe mat, wrist strap, fume extraction.
Tools: Temperature-controlled soldering iron, tweezers, flux pen, solder wire (typically Sn63Pb37 or SAC305).
Inspection: Verify PCB cleanliness, component orientation, and correct part numbers.

2. Component Placement

SMT: Use tweezers or vacuum pick-up to place components accurately.
PTH: Insert leads fully through the board, ensuring flush seating.

3. Soldering Technique

SMT:

Apply flux if needed.
Heat pad and lead simultaneously.
Feed solder wire until a concave fillet forms.

PTH:

Heat pad and lead for ~1–2 seconds.
Apply solder to the joint, not the iron tip.
Withdraw solder, then iron.

4. Post-Solder Inspection

Visual Check: Look for cold joints, bridging, insufficient solder, or tombstoning.
Touch-Up: Rework any defects using flux and proper heat control.

5. Cleaning

Use isopropyl alcohol and lint-free wipes to remove flux residues if required.

6. Final QA

Inspect under magnification or AOI.
Verify mechanical strength and electrical continuity.

Temperature Guidelines

Solder Type Iron Temp Notes
Sn63Pb37 315–350°C Lower melting point, easier flow
SAC305 350–375°C Lead-free, requires higher temp
Fine Pitch SMT 300–320°C Prevents overheating delicate parts

Best Practices

Use flux liberally for better wetting.
Keep iron tip clean and tinned.
Avoid excessive heat—can damage pads or components.
Use correct tip size for component type.
Practice good ergonomics to reduce fatigue and errors.

Common Pitfalls

Cold joints: Caused by insufficient heat or dirty surfaces.
Solder bridges: Excess solder or poor technique.
Lifted pads: Overheating or repeated rework.
Misaligned SMT parts: Poor placement or uneven heating.
Insufficient solder: Weak mechanical connection.

Standard Operating Procedures (SOPs)

Follow IPC-A-610 Class 2 or Class 3 standards depending on product criticality.
Document rework and inspection results.
Use calibrated equipment and maintain tip condition logs.
Ensure operators are trained and certified.

Useful Tips

Preheat PCBs for large ground planes.
Use magnification for fine-pitch components.
Label and store components to prevent mix-ups.
Practice on scrap boards to refine technique.