Reworking Intermittent Header Joints on a Batch of Boards
During a batch of 25 identical boards, six fail final test because header J1 (4 pins) is intermittent when the cable is wiggled. Visual inspection shows dull, grainy joints on J1, some cone-shaped with the solder sitting on top of the pad, and the iron tip is black. The iron is set to 280 °C and the solder is lead-free SAC305, 0.8 mm. A trainee says the fix is to add more solder to each joint and turn the iron up to 420 °C. The board has a ground plane on the solder side and the J1 pads are connected to it. The other 19 boards pass, but their J1 joints look similar. Delivery is in two days. You have flux pen, braid, a 1.0 mm chisel tip, and a calibrated iron.
1. Diagnose the root cause of the intermittent joints. Why is adding more solder not the correct fix?
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The iron at 280 °C is too cold for SAC305 lead-free solder. Through-hole joints generally need about 350–370 °C at the joint so the pad and lead reach soldering temperature. The black tip cannot transfer heat, so the solder melts on the iron but does not wet the pad and lead properly. It sits as a blob or a cone on top of the pad.
Adding more solder makes a larger cold joint; it does not create a metallurgical bond. Turning the iron to 420 °C oxidizes the tip further, damages the pad, and can lift copper. The ground plane also sinks heat away from the joint, making the low temperature worse. This is a process fault caused by low heat and a contaminated tip, not a missing-solder fault.
2. Describe the rework sequence and the soldering parameters you would use, including the ground-plane pins.
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Power off and disconnect the board. Clean the tip on brass wool and tin it. Set the iron to 350–370 °C; for the ground-plane pins, use up to about 380 °C and a 1.0 mm chisel tip for better heat transfer.
Apply flux to J1, then touch the tip to both pad and lead. Feed a small amount of 0.8 mm SAC305, hold for 2–3 s until a fillet forms, and remove the solder before the iron. Do not dwell more than about 3–4 s per joint. For ground-plane pins, preheat the board or use a larger tip rather than raising the iron to 420 °C. Clean flux residue, inspect, and do a gentle wiggle and continuity test.
3. State the acceptance criteria and decide what to do with the whole batch before delivery.
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Acceptance: a smooth concave fillet, solder wetted to both pad and lead, no bridging, no cracks or voids, and the lead outline visible. Slightly dull is acceptable for lead-free solder; grainy or cold-looking joints are not.
Because 19 passing boards have similar-looking J1 joints, the risk is not limited to the six failures. Rework all 25 J1 headers, then retest every board. Record the iron temperature and tip condition, and replace the tip if it cannot be cleaned. Do not ship known intermittent joints to meet the two-day delivery.