Learning objectives
- Choose tip size and temperature for a joint and control dwell time
- Solder chip, gull wing, J lead and through hole joints to accept criteria
- Rework components following IPC 7711 procedures and repair boards following IPC 7721
- Decide between rework, repair and scrap, and respect authorisation levels and rework limits
Lesson
### Lesson 1: Hand soldering and touch up
This is the most hands on stage of the course, and the one where personal skill carries the product. The tools are temperature controlled irons from makers such as PACE, Weller and Hakko, hot air stations and preheaters for larger work, microscopes for honest inspection, and wick and solder suckers for removing solder cleanly. The governing idea is heat management. The tip is chosen for the size of the joint, the temperature is set for the work, and dwell on the joint is measured in seconds, not tens of seconds. A small iron held longer and longer on a joint connected to a ground plane does not solder better. It cooks the pad, the barrel and the component while the joint still refuses to flow. The correct answer to a hungry joint is a bigger tip, proper flux and preheat, not more time and more temperature.
Touch up corrects the faults the inspection stages find: a bridge divided, a dry joint reflowed with fresh flux, a joint short of solder brought up to criteria. Every corrected joint is cleaned and inspected under magnification against the accept criteria of IPC A 610, taught here in our own words, before the board moves on.
### Lesson 2: Rework, repair and the limits of both
Rework removes and replaces a component, following the procedures of IPC 7711. Repair restores the board itself, pads, traces and laminate, following IPC 7721. Both are skilled, authorised work. A BGA rework station profiles the removal and replacement like a small reflow oven, often with board preheat, and the result is verified by X ray, because the joints cannot be seen. Moisture and ESD rules still apply in full: a moisture sensitive part may need baking before rework, exactly as it did before its first reflow. Every site on a board also carries a maximum number of rework cycles, because pads and barrels weaken each time they are heated and worked. Good operators track that count instead of discovering the limit when a pad lifts.
The final skill is judgement. Some faults are reworked, some boards are repaired, and some boards are scrapped, and the decision follows authorisation levels, not optimism. James Onsomu wrote his book Ensuring Quality in PCB Assembly as a technician’s toolkit, and this stage is its heart: operators and technicians who own quality on the line, with graded practice on real joints, microscope self inspection, and the discipline to stop when the correct answer is to escalate.
Key numbers
- Dwell on a hand soldered joint: measured in seconds, not tens of seconds
- Rework cycles per site: capped by procedure, and the count is tracked for every board
- BGA rework verification: X ray inspection of the finished joints, every time
- Tip selection: sized to the joint, with preheat for boards carrying large ground planes
Defects to spot
- Cold or disturbed joints: too little heat, or the joint moved while solidifying
- Lifted pads and barrels: excess dwell or force on a weakened site
- Bridges and icicles: too much solder or poor tip technique
- Excess or insufficient solder: wrong tip, wrong feed, or a rushed joint
- Component heat damage: dwell far beyond what the joint needed
- Measling and delamination: overheating of the board laminate during rework
- Hidden BGA defects: rework released without X ray verification
Stage check, self test
Answer each question, then open the answer to check yourself. Passing standard: 80 percent, at least 4 of 5 correct.
1. A joint on a large ground plane refuses to flow. What is the correct response?
A. Hold a small iron on it for a full minute
B. Turn the iron to its maximum and press harder
C. Use a correctly sized tip with proper flux and preheat, keeping dwell in seconds
D. Add more solder until something melts
Show answer
Answer: C
2. What governs rework procedures and repair procedures respectively?
A. IPC 7711 for rework and IPC 7721 for repair
B. The packing specification for both
C. The feeder program for both
D. No documents govern them
Show answer
Answer: A
3. A BGA has just been reworked. What must happen before the board is released?
A. Nothing, BGAs always rework perfectly
B. The joints are verified by X ray
C. The board is washed twice
D. The board is packed immediately
Show answer
Answer: B
4. Why is the number of rework cycles per site tracked?
A. To bill the customer per cycle
B. To slow operators down
C. To fill in the log book neatly
D. Pads and barrels weaken with each heating cycle, so every site has a maximum the procedure allows
Show answer
Answer: D
5. Which fault is caused by dwell held far too long on a joint?
A. A missing feeder label
B. Overheated, lifted pads and possible component heat damage
C. A warped panel at the loader
D. An offset paste deposit
Show answer
Answer: B
Previous: Stage 11: Cleaning