Cold Joints and a Discoloured Track on Hand-Soldered Controller Boards
A batch of 25 identical controller boards is hand-assembled in the school lab. Each board has an SOIC-8 op-amp, 0805 resistors and capacitors, a through-hole relay, and a 0.5 mm wide, 35 µm copper track carrying 1.2 A from the input connector to the relay. After assembly, 7 boards work, 6 fail only when the board is tapped or flexed, and 12 work but the 0.5 mm track darkens and the solder mask blisters near the relay after 10 minutes of operation. Inspection under a magnifier shows dull, grainy, ball-shaped solder on several ground pins of the SOIC, one 0805 capacitor standing on end, and flux residue around the relay pins. The lab iron is a 30 W mains unit with a chisel tip, no temperature control, and the solder is 60/40 with rosin flux. The relay coil draws 80 mA at 5 V; the 1.2 A load is a separate heater. The board is single-sided, 1.6 mm FR-4, with no thermal relief on the ground plane.
1. Diagnose the intermittent boards and the standing capacitor. Outline a rework procedure using the available iron safely and state what inspection check confirms each repair.
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The dull, grainy, ball-shaped joints are cold solder joints. The 30 W iron has no temperature control, and the ground pins connect to a large copper area with no thermal relief, so the pad never reached soldering temperature. The solder merely sat on the surface; flexing opens the joint. The standing capacitor is tombstoned: one pad heated and wetted before the other, and surface tension lifted the part. Rework by adding rosin flux and using the iron at a measured 350–370 °C, checked with a tip thermometer or a temporary controller. Preheat the board if possible and heat the pad and lead together for 2–3 s before feeding 60/40 solder. For the SOIC ground pins, use a larger chisel tip and a little extra solder to improve heat transfer; do not hold the iron on the pad for more than 5 s. Remove bridges with flux and solder wick. For the 0805, tack one end, align the part, then solder the other end while holding it with tweezers. Clean flux with isopropyl alcohol. Confirm with a 10× magnifier that each joint is shiny, concave, and wets both pad and lead, then perform a tap/flex test and continuity check.
2. A colleague wants to widen the 1.2 A track to 2 mm. Use the IPC-2221 external-layer relation I = 0.048 ΔT^0.44 A^0.725 (A in mil², 35 µm = 1.37 mil) to estimate the temperature rise of the existing 0.5 mm track, and decide whether widening is the right fix. Include one check that would distinguish track heating from soldering damage.
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Convert: 0.5 mm = 19.7 mil, so the cross-section is A = 19.7 × 1.37 = 27 mil². Rearranging the relation, ΔT = [I/(0.048 A^0.725)]^(1/0.44). A^0.725 = 27^0.725 ≈ 10.9. Then I/(0.048 × 10.9) = 1.2/0.523 = 2.29, and ΔT = 2.29^2.27 ≈ 6.6 °C. A continuous 1.2 A should therefore raise the track only about 7 °C above ambient, which is not enough to blister solder mask. Widening to 2 mm is not justified by the 1.2 A figure alone, although extra copper is harmless. The blistering is more likely local heat from the uncontrolled iron during relay soldering, a necked track at the pad, or a poor joint with contact resistance. Measure the voltage drop from connector to relay at 1.2 A and compare it with the calculated track resistance; 0.1 V would mean 0.12 W along that length. Also inspect the track at the pad for a narrow neck and check for solder bridges or a loose relay pin. If the board must operate in a hot enclosure, a 1 mm track gives more margin.