Reworking Cold Joints on a CMOS Kit Board
A student is reworking a single-sided through-hole PCB from a kit. The board has 1 oz copper and no ground plane. The student uses a 40 W soldering iron with a conical tip, lead-free solder, and no flux pen. After soldering a 16-pin DIP socket for a CMOS 4000-series IC, two joints look dull and balled; one joint has a visible crack around the pin. A continuity test shows the cracked joint is intermittent when the board is flexed gently. The student has a desoldering pump, 0.8 mm solder, a wet sponge, and isopropyl alcohol. The kit instructions say the board should be powered from 9 V DC and that the IC must be inserted after soldering. The student wants to fix the joints quickly and power up to test.
1. Are the dull, balled joints and the cracked joint likely cold solder joints? Explain the visual and mechanical evidence, and why simply adding more solder is a poor fix.
Read the worked answer
Dull, balled joints on lead-free solder are consistent with insufficient heat or contaminated/oxidized surfaces; the crack that changes continuity when flexed is a mechanical cold joint/fillet fracture. Adding solder alone is a poor fix because it can trap flux and oxides, build a larger irregular joint, and put more stress on the pad without restoring a proper intermetallic bond. The joint must be cleaned, fluxed and reflowed, or old solder removed and replaced.
2. Give a step-by-step rework procedure for the cracked joint, including iron temperature, tip choice, and ESD precautions for the CMOS IC.
Read the worked answer
Use a chisel tip rather than the conical tip for through-hole pads. Set 350–380 °C for lead-free, clean and tin the tip, and work with a grounded wrist strap because the CMOS 4000-series IC is static-sensitive and will be inserted later. Power off and discharge the board. Apply flux, heat the pad and pin together for 2–3 s, feed 0.8 mm solder, remove solder then iron, and inspect the fillet. If the joint is badly oxidized, remove old solder with the pump, clean with isopropyl alcohol, and reflow. Do not hold heat for more than about 4 s per joint. After cooling, check continuity and gently flex the board to confirm the intermittent is gone.
3. Why is the conical tip a poor choice here, and why must the IC be inserted only after soldering?
Read the worked answer
A conical tip has a small contact area and transfers little heat to a through-hole pad, encouraging long dwell times and cold joints. A chisel tip contacts the pad and lead simultaneously. The CMOS IC is inserted after soldering because soldering heat and static can damage it; sockets allow safe insertion after the board is cleaned and inspected.