Cold Joints on a Ground Plane
During a practical class a student solders a small LED flasher board. The board has a large copper ground plane connected to the emitter of a TO-92 transistor and to the negative pad of a 100 µF electrolytic capacitor. The student uses a 25 W soldering iron with a fine conical tip, 0.8 mm lead-free solder, and no extra flux. Joints on thin signal traces are shiny and concave. Joints on the ground plane are dull, lumpy and rounded, and the transistor emitter joint looks like a ball sitting on the pad. The student reworks one ground-plane joint three times, holding the iron on the joint for 2 seconds each time. The copper pad begins to lift, and the transistor case feels hot. The flasher works on the bench but becomes intermittent when the board is flexed. The student asks whether to buy 'better solder' or to turn up the iron, which has no temperature control.
1. Diagnose why the ground-plane joints are different from the signal-trace joints and why repeated 2-second heating is not working. Refer to heat transfer, thermal mass and wetting.
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The ground plane is a large copper area that conducts heat away from the joint very quickly. A 25 W iron with a fine conical tip has a small thermal mass and a small contact area, so it cannot deliver enough heat to bring the pad, the hole and the component lead up to soldering temperature. The 2-second dwell is spent heating the copper rather than melting and wetting the joint. Lead-free solder also needs a higher temperature than eutectic tin-lead solder, and no extra flux means surface oxide is not removed, so the molten solder tends to ball up instead of flowing into a fillet.
The shiny joints on thin traces work because the copper there has little thermal mass and the iron can heat the joint quickly. On the ground plane the same iron and time are inadequate. Repeating the joint three times does not add enough heat; it mainly adds thermal stress. The pad begins to lift because the adhesive is being overheated, and the transistor becomes hot because heat is conducted along its lead. The ball shape is a classic cold or insufficiently wetted joint, not proof that the solder itself is poor.
2. Propose a corrected soldering procedure for the ground-plane joint, including iron and tip choice, flux, time and inspection. Explain how to avoid pad lift and transistor damage.
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Use a temperature-controlled iron of about 40–60 W, set for the lead-free solder range, typically around 350–380 °C, with a larger chisel or hoof tip rather than a fine conical tip. The larger tip contacts the pad and lead over a wider area and stores more heat. Clean and tin the tip, apply a small amount of flux to the pad and lead, and heat the pad and lead together. Feed solder into the joint, not onto the iron tip, and hold until the solder flows into a concave fillet. A typical dwell is 2–4 seconds after the joint reaches temperature, but the visible flow is the guide; do not keep heating a joint that is not wetting.
For the transistor, use a heat-sink clip on the lead between the joint and the body, or solder the transistor last and keep its joints short. Do not lever or press on the pad while the solder is hot. For the ground plane, preheating the board slightly or using a larger iron can help, but the main correction is more thermal delivery and proper flux, not longer dwell with the same inadequate tip. Inspect with magnification for a shiny, concave fillet that wets both pad and lead. A dull ball or a joint that sits on top of the pad should be fluxed and reflowed once, not repeatedly overheated.
3. The board is intermittent when flexed. List two likely physical defects and how to inspect or reflow them. Explain why adding more solder may not fix a cold joint.
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Two likely defects are:
- A cracked cold joint on the ground plane or on a component lead. Flexing opens the crack and the circuit cuts out. Inspect with magnification and bright light, and gently wiggle each component lead while watching for movement at the joint. A continuity test can be used while flexing, but it may miss an intermittent joint unless the board is moved carefully.
- A lifted or cracked pad on the ground plane, caused by overheating during the repeated rework. The copper may still touch intermittently. Inspect the pad edge for a dark line or movement; if the pad is lifted, bridge to the nearest copper or repair the pad properly rather than relying on solder alone.
Adding more solder often makes the joint look bigger without improving wetting. A cold joint has a poor metallurgical bond between solder and copper; extra solder may simply form a larger ball on top of the same oxide or insufficiently heated surface. The correct repair is to clean and flux the joint, use a suitable iron and tip, reflow until the solder wets both surfaces, and then inspect. If the pad is damaged or the joint still moves, the board should be repaired or replaced rather than repeatedly reheated.