Through-Hole Soldering Technique: Single-Sided Boards — Electronics / Telecom Technician

Objective. The learner will produce at least 20 through-hole solder joints on a practice PCB that meet IPC-A-610 Class 2 acceptance criteria for wetting angle, fillet shape, and solder quantity.

Through-Hole Soldering on Single-Sided Boards

Through-hole soldering is the foundation of electronics assembly. A component lead passes through a drilled hole in the PCB, and solder is applied to the pad on the bottom (solder side) of the board to form a reliable mechanical and electrical joint. Mastering this technique is essential for the ETA CET and ETA FOI/FOT exams and for daily work on the bench.

Safety First

Before you pick up an iron, review these non-negotiable rules:

Burns: Soldering irons operate between 600 °F and 750 °F (315 °C–400 °C). Always return the iron to its stand between joints. Never wave or reach across a powered iron.
Fumes: Flux activators produce irritating rosin fumes. Work at a bench with local exhaust ventilation (LEV) or a fume extractor positioned at the source. Never blow fumes toward your face or a neighbor's workspace.
PPE: Wear safety glasses rated for lab work at all times during soldering and board inspection. Thin nitrile gloves protect skin from flux residue.
Fire: Keep the iron stand on a non-combustible surface. Inspect the cord before each session; a damaged cord must be tagged out and replaced (lockout/tagout procedure applies to the station, not just the tool).
ESD: Although single-sided practice boards with passive components are low-risk, develop the habit of using an anti-static wrist strap when handling DIP IC sockets. Static damage to ICs is invisible and cumulative.

The Correct Solder

This lab uses either 63/37 tin-lead (Sn63Pb37) or SAC305 (96.5% tin, 3% silver, 0.5% copper) lead-free solder, both in rosin-core wire form.

63/37 is eutectic: it transitions sharply from liquid to solid at 361 °F (183 °C), giving a bright, shiny fillet and a narrow process window to master.
SAC305 has a slightly higher melting point (around 422 °F / 217 °C) and a small plastic range. Joints may appear slightly duller than 63/37 but are still acceptable if the IPC-A-610 criteria are met.

Always confirm which alloy is loaded in the dispensing roll before you begin.

Setting Up the Iron

Use a temperature-controlled station. A common starting set-point is 650 °F (343 °C) for 63/37 with standard 0.031-inch (0.8 mm) solder wire. SAC305 typically requires a slightly higher set-point, around 700 °F (371 °C). Tin the tip with fresh solder and wipe it on a damp cellulose sponge or brass-wool tip cleaner immediately before each joint. A clean, tinned tip transfers heat efficiently and reduces dwell time.

Component Preparation and Seating

1. Form leads: Bend resistor and diode leads to match the hole pitch using needle-nose pliers or a lead-forming tool. Avoid bending flush at the body; leave a small stress-relief radius.

2. Seat depth: Push the component flat against the component side of the board. For polarized parts (diodes, electrolytic capacitors), confirm polarity before seating — the cathode band on a diode goes to the square pad or marked side of the silkscreen.

3. Lead clinch: Bend each lead outward approximately 30–45 degrees on the solder side to hold the component in place while soldering. Clinching too far (flat to the board) makes rework difficult.

The Soldering Sequence

This four-step sequence produces consistent, IPC-A-610 Class 2 joints every time:

1. Contact: Touch the iron tip simultaneously to the component lead and the pad. Both must be heated together.

2. Heat: Dwell for 1–3 seconds (the exact time depends on mass and board copper weight) until the pad and lead reach soldering temperature.

3. Feed: Introduce solder wire at the junction of lead and pad — not on the iron tip. The solder should flow toward the heat, not away from it.

4. Withdraw: Remove the solder wire first, then the iron. Hold the board still for 2–3 seconds while the joint solidifies. Movement during solidification causes a cold (disturbed) joint.

IPC-A-610 Class 2 Acceptance Criteria

Class 2 applies to most industrial and commercial electronics. A good through-hole joint shows:

Wetting: Solder wets (adheres and spreads) to both the lead and the pad. The contact angle where solder meets the pad is typically less than 90 degrees, forming a concave fillet.
Fillet shape: A smooth, concave meniscus from pad to lead. The lead outline is visible through the solder.
Fill: Solder fills the annular ring and travels at least 75% up the lead height on the solder side.
No bridging between adjacent pads or leads.
No excess solder that obscures the lead outline or forms a ball on top.

Defect codes you will mark on the inspection form:

B – Bridging
I – Insufficient fill / dewetting
C – Cold / disturbed joint
E – Excess solder
L – Lifted pad

Inspection Technique

Use a 10× loupe under good lighting. Tilt the board to catch specular reflections — a dull, grainy surface often indicates a cold joint or an incompatible alloy mix. Check the component side for solder that has wicked all the way through (a sign of good fill). Record results on your inspection form before handing the board to your peer reviewer.

Rework

A joint that fails Class 2 must be reworked before sign-off. Apply fresh flux, reheat the joint, remove solder with a desoldering pump or wick, and re-solder. Repeated heating of the same pad risks pad lifting — a lifted pad is a structural defect that cannot be fully repaired and triggers mandatory instructor review.

Key terms10

Through-hole solderingA soldering method in which component leads are inserted through drilled holes in a PCB and soldered to pads on the opposite side of the board.

IPC-A-610 Class 2An IPC standard acceptance class for electronic assemblies used in most industrial and commercial products, requiring reliable performance in non-harsh environments.

WettingThe spreading and adhesion of molten solder across a metal surface; proper wetting produces a low contact angle and a smooth, shiny fillet.

FilletThe concave, smooth solder formation at the junction of a component lead and a PCB pad that indicates a good mechanical and electrical connection.

Cold jointA defective solder joint caused by insufficient heat, movement during solidification, or contamination; it appears dull or grainy and has poor mechanical strength.

Annular ringThe copper ring surrounding a drilled hole on a PCB pad; solder must wet and fill this ring to meet IPC through-hole acceptance criteria.

Lead clinchThe slight outward bend applied to component leads on the solder side of the board to hold the component in place during soldering.

Eutectic alloyAn alloy, such as 63/37 tin-lead solder, that melts and solidifies at a single temperature rather than over a range, producing predictable, sharp transitions between liquid and solid states.

Lifted padA defect in which the copper pad separates from the PCB laminate, usually caused by excessive heat, mechanical stress during rework, or repeated soldering cycles.

Local exhaust ventilation (LEV)A fume extraction system positioned close to the soldering source to capture flux fumes before they reach the technician's breathing zone.

In class: Instructor demo on document camera: component seating depth, lead bend, iron-to-pad contact sequence, solder feed point, and dwell time; learners mirror technique on first practice board. · Learners solder a resistor, capacitor, diode, and DIP IC socket on a practice PCB (through-hole), then self-inspect each joint against the IPC-A-610 Class 2 pictorial chart posted at each bench. · Peer inspection round: learners exchange boards and mark defect codes (bridging, insufficient fill, cold joint, excess solder, lifted pad) before instructor final inspection.
Materials: Single-sided practice PCBs (pre-drilled), assorted through-hole components (resistors, capacitors, diodes, DIP sockets), 63/37 or SAC305 solder, temperature-controlled irons, IPC-A-610 Class 2 pictorial wall chart, 10× loupes, inspection forms
Check: Instructor inspects each of 20 joints against IPC-A-610 Class 2 criteria; pass requires at least 18 of 20 joints accepted (90%). Any safety defects (bridging causing short, lifted pad from overheating) trigger mandatory rework before sign-off.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.