Capstok — learn by doing

Why this matters

Soldering joins the prototype world to permanent circuits. The skill is overrated as 'difficult' — five focused practice runs and most people are decent at it. The skill is underrated as 'will determine 90% of your reliability problems if you skip it'. Cold joints look fine and pass inspection; they fail under thermal cycling six months later. Learning to make a real solder joint is a one-day investment.

Demo

The five steps to a good through-hole solder joint.

Try it yourself

Buy a $5 soldering practice kit (or salvage an old PCB).
Solder 20 joints, the first 5 deliberately slow.
Inspect each. Identify cold joints.
Re-do the cold joints. Notice the difference.

Prompt your AI

Use these three in order. Each builds on the one before.

1. Basics & terminology

Explain in one paragraph the difference between a good and a bad solder joint.

2. Why it works (the mechanism)

Walk me through soldering a single through-hole resistor onto a PCB.

3. Advanced — application & what's next

Given a board where 30% of joints look cold, what's the systematic fix vs starting over?

References

EQUIPMENT (Tier 2 from M1.1):
  - Iron heated to 350°C (lead-tin) or 380°C (lead-free).
  - Tip cleaner (brass wool > damp sponge).
  - Solder wire 0.6-0.8mm diameter.
  - Flux pen for stubborn joints.
  - PCB or perfboard with a component leg sticking through.

THE FIVE STEPS:

  1. CLEAN THE TIP.
     Brass-wool wipe before every joint.
     Tip should be shiny silver, not dull black/gray.
     Dull = oxidized = bad heat transfer.

  2. TIN THE TIP.
     Touch a small bit of solder to the tip.
     A small bead of fresh solder helps heat flow.

  3. HEAT THE JOINT.
     Touch tip to BOTH the pad AND the component lead.
     The tip heats the metal, not the solder.
     1-2 seconds of contact.

  4. ADD SOLDER TO THE JOINT.
     Feed solder wire to the OPPOSITE side of the lead from the tip.
     Solder flows toward heat — it should melt and wick around the lead.
     About 1 second of solder feeding.

  5. REMOVE SOLDER, then REMOVE IRON.
     Order matters: take the wire away FIRST, then the iron.
     Hold still while it cools (2-3 seconds).
     Don't blow on it. Don't wiggle the part.

WHAT GOOD LOOKS LIKE:
  - Shiny finish (lead-tin) or matte gray (lead-free).
  - Concave fillet (volcano-shaped) on both sides of the pad.
  - No "ball" of solder sitting on top (cold joint).
  - No "blob" extending past the pad.
  - Component sits firmly in place.

WHAT BAD LOOKS LIKE:
  - "Cold joint": dull, grainy, ball-shaped. Re-do.
  - "Bridge": solder spanning two adjacent pads. Use desoldering braid
    or a clean iron to wick excess away.
  - "Tombstoned": SMT cap stood up. Mechanical/process error.
  - "Pad lift": pad came off the PCB. Be gentle next time.

PRACTICE PROTOCOL (one evening):
  1. Find an old PCB or buy a $5 soldering practice kit.
  2. Solder 20 joints. The first 5 will be ugly. Look at #20.
  3. Compare yours to YouTube reference shots.

LEAD-FREE vs LEAD-TIN:
  Lead-free is RoHS-compliant (Europe + most consumer products).
  Higher melting point (217°C vs 183°C). Harder to work with.
  For learning: lead-tin is more forgiving. Use lead-free for production.

SAFETY:
  - Ventilation. Soldering fumes contain rosin flux — irritating.
  - Hot iron is HOT (350°C). Iron rests prevent fires.
  - Wash hands after handling lead solder.

DESOLDERING:
  1. Apply flux to the joint.
  2. Hold desoldering braid against joint with iron.
  3. Braid wicks the solder up.
  4. Repeat with fresh braid section.

  Or use a solder sucker: heat solder, then press the sucker button to vacuum.

THIS LESSON'S FOCUS:
  Solder 20 joints today. Just 20. Look back at #1 vs #20.

Soldering: First Joints