3.6 Hand Soldering Foundations

Hand soldering remains essential for rework, repair, and specialized low-volume assembly, managing tasks that automated processes cannot safely touch. The reliability of hand soldering comes from treating it as a controlled, repeatable process rather than an art. By standardizing tool selection, temperature settings, and the heat application sequence, manufacturers ensure joints are not only electrically sound but also meet mechanical durability standards across all operators.

3.6.1 The Quality Standard

A successful hand-soldered joint must achieve the following characteristics within strict time constraints to prevent thermal damage to components and PCBs:

• Wetting and Fillet: The solder must form a shiny, concave fillet that smoothly wets both the pad and the component lead, showing a clean wetting angle of less than 60° (per IPC standards).
• Time Limit (Dwell): The entire process—heating, feeding solder, and withdrawing the iron—must be completed within 2–4 seconds for SMT pads and 3–6 seconds for THT pins.
• Repeatability: The result must be consistent regardless of the operator, achieved through documented Standard Work Instructions (SWI).

3.6.2 Tool Selection: Power and Recovery

The soldering station's thermal recovery rate is more critical than its static temperature. A high-power station (60–120 W) is required to maintain the set temperature when heat is rapidly transferred into large thermal masses (e.g., ground planes).

3.6.3 Tip Geometry and Application

Tip selection is based on the pad and lead geometry to maximize the contact area for efficient heat transfer. Using a tip that is too small leads to long dwell times and potential pad lifting.

Tip Maintenance: The tip must be kept tinned (coated with a thin, shiny layer of solder) at all times to maximize thermal transfer efficiency. Use tip tinner to revive oxidized or dull tips.

3.6.4 Temperature Control and Thermal Safety

Temperature selection is a balance between the solder's melting point and the need for a rapid wetting action, while avoiding damage to the PCB or component body.

Rule: If a joint does not wet cleanly within 4 seconds (SMD) or 6 seconds (THT) at the set temperature, stop. The problem is not temperature, but insufficient contact area, oxide contamination, or lack of preheat. Raising the temperature excessively leads to flux burnout and pad damage.

3.6.5 Hand Soldering Technique

The technique must follow a clear sequence to ensure proper intermetallic bond formation: clean, apply heat, add solder, remove heat, remove solder.

1. Preparation: Clean the pad/lead area if using liquid flux.
2. Heat Application: Place the clean, tinned tip to simultaneously touch the pad and the lead. Wait a beat (1–2 seconds) for heat to soak.
3. Solder Feed: Feed wire solder to the joint interface (the opposite side of the iron), not directly onto the iron tip. The solder should flow instantly toward the heat.
4. Wetting and Withdrawal: Once the joint is complete (concave fillet formed), withdraw the solder wire, and then instantly withdraw the iron tip. Total time must be contained within the dwell limits.

3.6.6 Common Defects and Reliable Fixes

3.6.7 Proficiency Check and Training Drill

Hand soldering proficiency requires a clear standard that operators must meet before working on live product. This is achieved through a standardized, measurable drill that verifies controlled application of heat and proper technique.

• The Mandate: Every new operator must pass a timed drill on a designated training coupon (a small PCB board with test pads). This coupon must include high-risk features like fine-pitch SMT, standard chip passives, and heavy THT pins.
• The Metric: The operator is measured on two criteria:
  1. Time Compliance: Completing the sequence (heating, soldering, cleaning) within the specified dwell limits (e.g., 4 seconds for SMT, 6 seconds for THT).
  2. Quality Compliance: Achieving IPC-A-610 Class 2 or 3 acceptance criteria for fillet formation, wetting angle, and cleanliness (no cold joints, no excessive flux residue).
• Verification: The soldered coupon is submitted for a formal Visual Inspection and logged as the operator's proficiency record. Failure requires immediate, focused re-training on the specific technique or defect type.

Final Checklist: Hand Soldering Quality Control