Activity 1.1.8 — Soldering Fundamentals¶
Learning Objectives¶
By the end of this lesson, students will be able to:
- Explain what soldering is and why it creates strong electrical connections.
- Identify the components of a soldering station and describe proper tip care.
- Demonstrate proper soldering technique for through-hole components.
- Recognize and avoid common soldering mistakes.
- Follow safety procedures when working with soldering equipment.
Vocabulary¶
Vocabulary (click to expand)
| Term | Definition |
|---|---|
| Soldering | The process of joining two metal surfaces by melting a filler metal (solder) with a lower melting point than the base metals |
| Solder | A metal alloy (typically tin and lead or tin and silver) that melts at low temperatures and creates an electrically conductive bond |
| Soldering Iron | A hand tool with a heated metal tip used to melt solder and join components |
| Flux | A chemical cleaning agent in solder that removes oxidation and promotes wetting |
| Tinning | Applying a small amount of solder to the tip of an iron to improve heat transfer |
| Wetting | The process of solder flowing and bonding to clean metal surfaces |
| Cold Joint | A defective solder joint caused by insufficient heat; appears dull and grainy |
| Solder Bridge | An unintended connection of solder between two adjacent conductors |
| Lifted Pad | A pad that has been separated from the circuit board due to excessive heat |
Part 1: What is Soldering?¶
Soldering is the process of joining electronic components to a circuit board using heat and a metal alloy called solder. When done correctly, soldering creates both a strong mechanical bond and an excellent electrical connection.
Why Soldering Matters in Electronics¶
- Soldered joints provide reliable electrical connections that can last for decades.
- Unlike wire nuts used in household wiring, solder joints are small and precise.
- Proper soldering is essential for circuit reliability — bad joints cause failures.
The Science Behind Soldering¶
When you solder correctly: 1. The iron heats both the component lead and the circuit board pad to the solder melting point. 2. Flux in the solder removes any oxidation on the metal surfaces. 3. Solder wets (flows onto) the clean metal surfaces, creating a metallurgical bond. 4. When cooled, the solder forms a shiny, strong connection.
Key insight: Soldering is not glue — it is a chemical bond between metals. The connection is only as good as the cleanliness of the surfaces and the amount of heat applied.
Part 2: Solder Types and Composition¶
Not all solder is the same. Different types are designed for different applications.
Lead-Free vs. Leaded Solder¶
| Property | Leaded (63/37 or 60/40) | Lead-Free |
|---|---|---|
| Composition | 63% tin, 37% lead (or 60/40) | 99.3% tin, 0.7% copper |
| Melting Point | 183C (183F) | 217C (423F) |
| Working Temperature | 315-370C (600-700F) | 370-400C (700-750F) |
| Appearance | Bright, shiny | Slightly dull |
| Flow Characteristics | Excellent wetting | Good but requires more heat |
| Safety | Contains lead (toxic) | Safer but requires higher heat |
| Common Use | Hobbyist, repair, older equipment | Commercial/manufacturing (ROHS compliance) |
Solder Forms¶
| Form | Description | Best For |
|---|---|---|
| Rosin Core | Contains flux inside the solder wire | Most general soldering |
| Acid Core | Contains acid flux for plumbing | Plumbing only — NOT for electronics |
| No-Clean | Leaves minimal residue | Clean assemblies, production |
| Flux Paste | Separate flux applied with a syringe | Surface-mount work |
Solder Wire Thickness¶
- 0.8mm — Fine work, surface-mount components
- 1.0mm — General purpose, through-hole components
- 1.5mm — Large joints, heavy gauge wires
Part 3: Soldering Iron Basics¶
Anatomy of a Soldering Station¶
+-------------------+
| Soldering Iron |
| Handle |
| | |
| +----+----+ |
| | Cord | |
| +----+----+ |
| | |
| [Power Cord] |
| | |
| +--------+ |
| |Station | |
| |Control | |
| +--------+ |
+-------------------+
Key Components:¶
- Station Base — Houses the power supply and temperature control
- Iron Handle — Holds the heating element and tip
- Soldering Tip — The business end that transfers heat
- Temperature Control — Adjusts tip temperature
- Iron Stand — Safe resting place for the hot iron
Tip Types and Selection¶
| Tip Shape | Best Use |
|---|---|
| Conical | Precise work, small joints, SMD components |
| Chisel ( screwdriver) | General through-hole soldering, most common |
| Knife | Drag soldering, IC pins |
| Bevel | Large pads, good heat transfer |
| Pyramid | Reaching into tight spaces |
Temperature Settings¶
| Application | Temperature (Celsius) |
|---|---|
| General through-hole | 315-370C (600-700F) |
| Surface-mount | 260-315C (500-600F) |
| Heavy components/connectors | 400-450C (750-850F) |
| Lead-free solder | 370-400C (700-750F) |
Key insight: Higher temperature is NOT always better. Too much heat damages components and circuit boards. Use the lowest temperature that allows good wetting.
Part 4: Workspace Setup¶
A properly organized workspace makes soldering easier, safer, and produces better results.
Essential Workspace Components¶
- Soldering Iron Stand — Always rest the hot iron here, not on the workbench
- Brass Wool or Sponge — For cleaning the tip (wet sponge can crack tips from thermal shock)
- PCB Holder or Vise — Secures the circuit board while you work
- Helping Hands — Alligator clips on a stand for holding wires or boards
- Ventilation — Fume extractor or fan to remove solder fumes
- Safety Glasses — Protect eyes from solder splatter
- Heat-Resistant Mat — Protects work surface from solder and burns
- Rubbing Alcohol — For cleaning flux residue (optional)
Workspace Layout¶
+--------------------------------------------------+
| [Vent/Fume Extractor] |
| |
| [Board in Vise] [Component Tray] |
| |
| [Iron in Stand] [Solder Roll] |
| |
| [Wet Sponge/Brass Wool] [Tools: cutters, |
| tweezers, etc.] |
| |
| [Safety Glasses] |
+--------------------------------------------------+
Tip Care — Tinning and Cleaning¶
Why tinning matters: - A tinned tip has a thin coat of solder that improves heat transfer - Oxidation builds up on bare copper tips and drastically reduces heat transfer - Always tin your tip before putting it away
Daily tip maintenance: 1. Before first use: Clean tip on damp sponge, then tin it immediately. 2. During use: Wipe tip on brass wool or damp sponge every few joints. 3. When not in use: Rest iron in holder; never leave it sitting on the bench. 4. At end of session: Clean tip, apply fresh solder (tin), then turn off or store.
Part 5: Soldering Technique — Step by Step¶
Follow these steps in order for reliable solder joints every time.
The 6-Step Soldering Process¶
Step 1: Prepare the Work Area - Turn on the soldering iron and let it reach temperature (5-10 minutes) - Clean the tip on brass wool or damp sponge - Tin the tip with fresh solder
Step 2: Clean and Position the Component - Ensure the component leads pass cleanly through the holes - Bend leads slightly on the other side to hold the part in place - Clean the board and components if needed (oxidation prevents good joints)
Step 3: Heat Both Surfaces - Place the tip against BOTH the component lead AND the circuit board pad simultaneously - Hold for 2-3 seconds to bring both to temperature - The tip should contact metal on both surfaces
Step 4: Apply Solder to the Joint - Touch solder to the junction of the tip, lead, and pad - DO NOT apply solder to the iron tip — apply it to the joint! - The solder should melt and flow onto the surfaces (wet)
Step 5: Remove Heat and Solder - Remove the solder wire first - Then remove the iron tip from the joint - Keep the iron moving to the stand
Step 6: Inspect the Joint - A good joint is shiny and looks like a small volcano or cone - Allow the joint to cool naturally (do not blow on it or move the component) - A good joint takes about 5 seconds to cool and solidify
Visual Reference — The Five Solder Joint Types¶
| Joint Type | Appearance | Problem | Cause |
|---|---|---|---|
| Ideal | Shiny, concave, volcano-shaped | None | Perfect technique |
| Acceptable | Slightly dull but shaped correctly | None | Minor variation |
| Cold Joint | Dull, grainy, crystalline | High resistance, intermittent | Insufficient heat |
| Insufficient Solder | Small, concave, may expose pad | Weak, high resistance | Too little solder |
| Excess Solder | Large, rounded blob | May cause bridge | Too much solder |
Part 6: Common Soldering Mistakes and How to Avoid Them¶
Mistake 1: Cold Joint¶
What it looks like: Dull, grainy, crystalline appearance; may have cracks.
Cause: Insufficient heat during soldering.
How to fix: Reheat the joint with the iron for 2-3 seconds, adding a tiny bit of fresh solder if needed.
Prevention: Ensure the iron tip contacts both the lead and the pad. Hold long enough for solder to flow freely.
Mistake 2: Solder Bridge¶
What it looks like: Unintended solder connection between two adjacent pins or pads.
Cause: Too much solder or moving the board before the solder solidifies.
How to fix: Use a solder sucker or solder wick to remove excess solder.
Prevention: Use the right amount of solder. Hold the board still until joints cool.
Mistake 3: Insufficient Solder (Ney Joint)¶
What it looks like: The joint is small, barely covering the pad.
Cause: Not enough solder applied.
How to fix: Reheat and add more solder.
Prevention: Apply enough solder to create a proper volcano shape.
Mistake 4: Lifted Pad¶
What it looks like: The copper pad has separated from the circuit board.
Cause: Excessive heat or dragging the iron on the joint.
How to fix: This is a serious problem requiring trace repair — often cannot be fixed without special tools.
Prevention: Use appropriate temperature. Do not drag the iron along the joint. Rest between joints.
Mistake 5: Component Not Fully Inserted¶
What it looks like: Component is crooked or not fully seated against the board.
Cause: Forcing the component through without proper alignment.
Prevention: Insert components straight and fully before soldering.
Key insight: Most soldering problems stem from insufficient heat (cold joints) or excessive heat (damaged components/pads). The right temperature and proper technique are everything.
Part 7: Safety Procedures¶
Thermal Burns¶
- The soldering iron tip reaches 315-450C (600-850F) — enough to cause serious burns.
- Never touch the tip, even if the iron appears to be off.
- Place the iron in its stand when not actively soldering.
- Be aware of where the iron is at all times.
Solder Fume Exposure¶
- Solder fumes contain flux compounds that can irritate lungs with prolonged exposure.
- Always use ventilation — a fume extractor or at minimum a fan directing fumes away from your face.
- Never breathe solder fumes directly.
- Wash hands after soldering, especially before eating.
Lead Exposure¶
- Leaded solder contains toxic lead.
- Wash hands thoroughly after handling solder or touching circuit boards.
- Do not eat or drink while soldering.
- Keep solder away from mouth and eyes.
Eye Protection¶
- Solder can splatter, and flux bubbles can pop unexpectedly.
- Wear safety glasses at all times during soldering.
Electrical Safety¶
- Inspect the iron and cord for damage before use.
- Keep liquids away from the soldering station.
- Turn off the iron when leaving the workstation.
Part 8: Practice Exercises¶
Practice Exercise 1: Wire-to-Wire Soldering¶
Objective: Create a strong mechanical and electrical connection between two pieces of wire.
Materials: 22 AWG solid core wire (two pieces, 10cm each), heat-shrink tubing, solder
Procedure: 1. Strip 1.5 cm of insulation from each wire end. 2. Twist the two wire ends together tightly. 3. Apply heat to the twisted junction and touch solder to the joint. 4. Let solder flow into the twisted strands. 5. Allow to cool naturally. 6. Slide heat-shrink tubing over the joint and shrink it with the iron (or a heat gun).
Evaluation Criteria: - Solder has wicked into the strands (shiny, solid connection) - Strong mechanical bond (wire does not pull apart easily) - No cold joints or exposed strands
Practice Exercise 2: Component-to-Board (Through-Hole)¶
Objective: Solder a through-hole component to a practice circuit board.
Materials: Practice PCB or perfboard, through-hole resistor, capacitor, or LED
Procedure: 1. Insert the component leads through the board holes. 2. Bend leads slightly outward on the back side to hold the part. 3. Position the board so the component is facing you. 4. Heat both the lead and pad simultaneously (2-3 seconds). 5. Apply solder to the junction point — not the iron. 6. Remove solder first, then iron. 7. Allow to cool for 5 seconds. 8. Inspect the joint for shine and proper shape. 9. Clip excess leads flush with the board.
Evaluation Criteria: - Joint is shiny and volcano/cone-shaped - Solder flows completely around the lead and pad - No bridges to adjacent pads - Component is perpendicular to the board - No burned or lifted pads
Practice Problem — Identifying Solder Joint Defects¶
Examine each solder joint below and identify the defect:
Joint A: The joint is very large and rounded, covering both the target pad and an adjacent pad.
Joint B: The joint looks rough and grainy, almost crystalline, with a dull gray appearance.
Joint C: The joint is shiny but very small, covering less than half the pad.
Joint D: The copper pad has lifted completely off the board surface.
Show Solution
Joint A: Solder Bridge (too much solder connecting adjacent pads) or Excess Solder - The large, rounded shape indicates too much solder was applied, and it bridged to the adjacent pad.
Joint B: Cold Joint (insufficient heat) - The dull, grainy, crystalline appearance is characteristic of a cold joint caused by not enough heat or cooling too quickly.
Joint C: Insufficient Solder ( Ney Joint) - While the joint is shiny (good sign), it is too small to create a reliable mechanical and electrical connection.
Joint D: Lifted Pad (thermal damage) - This is severe damage caused by excessive heat or dragging the iron, which separated the copper pad from the circuit board substrate.
Summary¶
| Topic | Key Points |
|---|---|
| Soldering | Joining metals with melted solder to create electrical connections |
| Lead-Free vs. Leaded | Leaded melts at lower temperature; lead-free requires higher heat |
| Tip Types | Conical (precision), Chisel (general), Knife (drag soldering) |
| Tinning | Coating the tip with solder to improve heat transfer |
| 6-Step Process | Prepare, clean, heat both surfaces, apply solder, remove, inspect |
| Cold Joint | Dull, grainy appearance from insufficient heat |
| Solder Bridge | Excess solder connecting adjacent conductors |
| Lifted Pad | Pad separated from board due to excessive heat |
Quick Reference — Ideal Solder Joint Profile¶
Solder
||
//////
////////
<--------> Pad
[========] Circuit Board
Ideal joint: Concave fillet, shiny surface, ~45 degree angle
Soldering Iron Care Checklist¶
- [ ] Iron reaches temperature before first use
- [ ] Tip cleaned and tinned before starting
- [ ] Iron returned to stand when not in use
- [ ] Tip cleaned between joints
- [ ] Iron cleaned and retinned before storage
Key Reminders¶
- Soldering creates a metallurgical bond — it requires clean surfaces and proper heat.
- Apply solder to the joint, not the iron tip.
- Heat both the component lead and the pad before adding solder.
- A good joint is shiny and volcano-shaped; a cold joint is dull and grainy.
- Use adequate ventilation and safety glasses whenever soldering.
- When in doubt, use lower temperature — excessive heat damages components and boards.
- Practice on scrap boards before working on valuable projects.
Custom activity — adapted from PLTW Digital Electronics