Through-Hole Soldering: Double-Sided Plated-Through-Hole Boards

Through-Hole Soldering on Double-Sided Plated-Through-Hole Boards

Double-sided plated-through-hole (PTH) boards are the backbone of production electronics and telecom equipment. Unlike single-sided boards, a PTH board has a copper barrel electroplated inside each drilled hole, connecting the top copper layer to the bottom copper layer. When you solder a component lead through that barrel, the joint must bond to the barrel wall, the top pad, and the bottom pad to be electrically and mechanically sound.

Why Barrel Fill Matters

IPC-A-610 — the industry-standard Acceptability of Electronic Assemblies document — sets the bar for Class 2 workmanship (general electronics products, including most telecom gear). For Class 2, the solder must fill at least 75% of the barrel by height. This means solder wets the barrel wall from the solder source (bottom) upward, and you should see a visible concave fillet drawing up on the top side of the board. That upward draw is your visual confirmation that the barrel is substantially filled.

• 75% fill or greater — Acceptable for Class 2. The solder meniscus on the top-side pad is visible and concave.
• 50% fill — Defect for Class 2 but acceptable for Class 1 (consumer products only). The top-side fillet may be absent or barely present.
• Less than 50% fill — Defect for all classes. The barrel is mostly empty; the joint will likely fail mechanically and electrically under thermal cycling.

Cross-section photographs published in IPC-A-610 show these conditions clearly. Study them before soldering — your eye needs to be calibrated to what 75% looks like from the outside without cutting the board.

How Solder Climbs the Barrel

Solder rises into the barrel by capillary action — the same physics that pulls liquid up a narrow tube. For capillary action to work, three conditions must be met:

1. Cleanliness. Oxidized copper repels solder. The board must be handled with care; skin oils contaminate pads.

2. Flux activity. Flux chemically reduces surface oxides on the copper and the component lead so solder can wet the metal.

3. Sufficient heat. Both the pad and the component lead must reach soldering temperature before solder is applied. Applying solder to the iron tip rather than the joint produces a cold, unreliable joint.

On a double-sided board, the copper barrel conducts heat away efficiently. You may need a slightly longer dwell time (the time the iron tip stays in contact with the joint) compared with a single-sided board, typically 2–4 seconds, though the exact value depends on component mass and iron wattage. Record your iron tip temperature and dwell time in your lab log — this data lets you repeat success and diagnose failures.

Iron Temperature and Tip Selection

A temperature-controlled iron is essential. A common starting point for standard tin-lead solder (Sn63/Pb37) on PTH work is approximately 650–700 °F (343–371 °C) at the tip. Lead-free alloys (SAC305 is common in modern electronics) require higher temperatures, often 700–750 °F (371–399 °C) or more. Always follow the solder manufacturer's recommendation and your instructor's direction for the alloy in use.

Select a tip with enough mass to transfer heat quickly. A chisel or bevel tip seats against both the lead and the pad at once, maximizing heat transfer. A fine conical tip is better reserved for tight-pitch SMD work.

Soldering Sequence — PTH on a Double-Sided Board

1. Inspect the board and component leads for contamination or damage.

2. Insert the component; bend leads slightly if needed to hold the part in place.

3. Tin the tip — wipe on a wet sponge or brass wool, then apply a thin layer of fresh solder to ensure good thermal contact.

4. Heat the joint — place the iron tip so it contacts both the pad ring and the component lead simultaneously. Allow 1 second for thermal equalization.

5. Apply solder — feed solder to the joint (not the iron tip). Watch solder flow onto the pad and begin to wick upward.

6. Observe draw-up — continue feeding solder until a concave top-side fillet appears. Remove solder feed.

7. Remove the iron — lift cleanly without disturbing the joint. Hold still 2–3 seconds while solder solidifies.

8. Inspect — the finished fillet should be smooth, shiny (for tin-lead), concave, and show the lead outline. A gray, grainy, or peaked surface indicates a cold joint.

Inspection and Continuity Testing

After soldering, use a 10× loupe to inspect a sample of joints for top-side fillet presence and surface quality. Then use a digital multimeter (DMM) in continuity mode to verify every net on the board. Touch probes to the two ends of each net; a tone (or low resistance reading) confirms the path is complete. Check also for solder bridges — unintended connections between adjacent pads — by testing across pads that should be isolated. A bridge reads continuity where there should be none. Any failure triggers a rework cycle: desolder the joint using a desoldering pump or wick, reflow or re-solder, re-inspect, and document every step in your lab log.

Safety

• Burns are the primary hazard. The iron tip and nearby metal reach temperatures that cause immediate tissue damage. Never lay a hot iron on the bench; always return it to its stand.
• Flux fumes are an irritant. Work in a well-ventilated area or use a fume extractor positioned at the source.
• Solder splatter can occur if the workpiece or solder is wet. Ensure all surfaces are dry. Wear safety glasses at all times.
• Tin-lead solder contains lead — wash hands thoroughly before eating or touching your face.