Bench Talk

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Soldering is a foundational skill for makers and engineers alike. It is also just plain fun. As any engineer or seasoned maker knows, the first time you melt some solder to connect an LED or resistor to a circuit board is a transformative experience. It takes you from being a consumer to a creator of electronics. Moving a design from the breadboard and committing it to a printed circuit board (PCB) is a major milestone for any electronics project. And soldering is a crucial part of that process. But what exactly is soldering, and how do you know if you are doing it well?

Soldering is the process of joining metal components by using a filler metal (i.e., solder) with a lower melting point than the base materials. This technique creates mechanically strong and electrically conductive connections (known as joints), which are essential to creating anything from simple LED circuits to advanced embedded systems. From a maker’s perspective, the most common soldering tasks will be affixing components to a PCB using through-hole components or surface mount devices (SMD) and connecting wires for various purposes.

Effective soldering ensures minimal electrical resistance and maximum mechanical durability. Proper soldering techniques ensure reliable and functional projects. Poor soldering can lead to weak joints, unreliable circuits, and troubleshooting headaches. Like any skill, soldering has a learning curve, but starting with the right approach minimizes frustration and maximizes the chances for success. Let’s look at what it takes to become a soldering pro.

Essential Tools for Soldering

Before diving into the techniques of soldering, it’s important to know the tools you should consider acquiring:

• Soldering iron: A 30W–60W adjustable soldering iron is ideal for beginners. Temperature-controlled stations offer better precision. For general soldering, set the iron between 350°C and 400°C (660°F to 750°F). Lead-free solder may require slightly higher temperatures.
• Solder: Solder is a fusible metal alloy used to create a permanent bond between metal surfaces. Solder can come as a wire or paste. Wire solder is used for hand soldering with a soldering iron.  Solder paste, a mixture of powdered solder and flux, is used for reflow soldering surface mount components. Some of the common solder types include:
  • 60/40 tin-lead solder: Easier to use due to its lower melting point, but contains lead and requires proper handling
  • Lead-free solder: Eco-friendly and required for commercial electronics but has a higher melting point
  • Silver-bearing solder: Stronger and more conductive but more expensive
• Soldering stand: This tool holds the iron securely when not in use.
• Tip cleaner: Using brass wool or a damp sponge keeps the iron’s tip clean and effective.
• Flux: Solder used in electronics typically comes in solid-core or flux-core forms, with flux-core solder being the most common for hand soldering. The core type refers to whether the solder has a built-in flux, which is essential for ensuring proper wetting and adhesion to metal surfaces by removing oxidation and contaminants. Flux also improves solder flow and joint quality. Common solder core types include:
  • Rosin-core solder: Contains flux to improve solder flow and prevent oxidation.
  • Water-soluble core solder: Common in high-reliability applications where thorough flux removal is necessary, this contains highly active organic acid flux that is easily cleaned with water.
  • No-clean core solder: Used in consumer electronics and manufacturing where cleaning is impractical, this has a minimal residue flux that does not require post-solder cleaning.
• Tip tinner: A paste or solid compound containing tin and flux designed to clean and re-tin oxidized soldering iron tips. Removing oxidation and applying a fresh tin coating restores heat transfer efficiency, ensuring better soldering performance and extending the tip’s lifespan.
• Helping hands or PCB holder: These tools stabilize components and boards during soldering.
• Desoldering tools: Solder suckers or desoldering braids are necessary to fix mistakes.
• Safety gear: When soldering and trimming leads, use a fume extractor (or otherwise good ventilation) to avoid toxic fumes, and wear safety glasses to protect against smoke and debris.
• Magnification tools: A magnifying glass or microscope will help you see small components.
• Solder tips: Soldering iron tips come in various shapes and sizes, each designed for specific soldering tasks. Choosing the right tip can significantly impact the quality of your solder joints, heat transfer efficiency, and precision. The following are the most common types of soldering tips and their applications:
  • Conical tip:
    • Tapers to a sharp point
    • Primarily used for precision soldering on small components, SMD soldering, and reaching tight spaces on densely populated circuit boards
    • Features a small surface area, which can result in slower heat transfer, making it less efficient for large solder joints
• • Chisel tip:
    • Flat, wide tip with a beveled edge
    • Good general-purpose tip, often used with larger PCB pads, through-hole components, and drag-soldering SMD components
    • Less precise than a conical tip and bulky for tiny components
• • Bevel tip:
    • Angled tip with a wider, slanted surface
    • Best for SMD drag soldering, reworking and soldering larger joints, and soldering multiple pins of flat components in a single pass
    • Not recommended for very fine soldering tasks
• • Hoof or spade tip:
    • Similar to a bevel tip but with a larger, rounded surface
    • Provides good heat transfer efficiency, making it useful for SMD drag soldering and reflow work
    • Not recommended for precision work
  • Knife tip:
    • Flat and sharp tip, resembling a knife edge
    • Best for cutting excess solder or reworking solder bridges
    • Not ideal for general-purpose soldering
• • Blade tip:
    • Wide, flat surface, like a putty knife
    • Helps remove large amounts of solder and spread solder across multiple contacts
    • Not recommended for very fine soldering tasks
• • Pyramid tip:
    • Resembles a three or four-sided pyramid
    • Handy for heavy-duty soldering applications and components with high thermal mass
    • Too large for fine electronics work

Preparing the Circuit Board and Components for Soldering

Before plugging in the soldering iron and getting the flux flowing, properly preparing the circuit board and the components is key to achieving clean, reliable solder joints. Follow these steps to ensure a smooth soldering process:

1. Clean the PCB and components: Use isopropyl alcohol and a lint-free cloth to remove dust, oil, and oxidation from the PCB. Gently rub component leads with fine-grit sandpaper or a fiberglass brush to ensure they are oxidation-free. If necessary, apply a small amount of flux to the pads to improve solder flow.
2. Arrange components before soldering: Follow the circuit diagram to arrange all components in their designated positions on the PCB. Start with the smallest and lowest-profile components (such as resistors and diodes) before moving on to larger components (like capacitors and connectors). Secure components with tape or by slightly bending their leads on the solder side of the PCB.
3. Pre-tin large pads and wires: Apply a small amount of solder to pre-tin large pads, connectors, or stranded wires. This helps with heat transfer and ensures a solid connection when soldering. Hold the iron against the pad or wire, apply the solder, and let it flow evenly before removing the excess.
4. Properly trim leads: Use flush cutters to trim excess component leads after soldering to prevent short circuits. Leave a small portion of the lead intact for structural integrity, but avoid leaving excessively long leads that could cause unintended contact.

Step-by-Step Guide to Basic Soldering

Now, we are ready to get started with soldering. Follow these steps to get started with soldering through-hole components:

1. Prepare your work area: Ensure good ventilation and an organized workspace to prevent mistakes and ensure safety.
2. Set the right temperature: For general soldering, set the iron between 350°C and 400°C (660°F to 750°F). Lead-free solder may require slightly higher temperatures.
3. Tin the soldering iron tip: Apply a small amount of solder to the tip before starting. This is called tinning and improves heat transfer.
4. Heat the joint, not the solder: Place the iron tip against the joint for a second or two, then apply the solder to the joint (not the iron). The heated metal will melt the solder and create a solid connection.
5. Remove the solder, then the iron: Once the solder flows evenly, remove the solder first, then remove the iron. Keep the joint still for a few seconds as it cools.
6. Inspect and test: A good joint should be shiny, smooth, and slightly concave. If it’s dull or grainy, reheat and reflow the solder. Use a multimeter to check continuity.

Common Soldering Mistakes and Fixes

Like all processes, soldering is prone to errors. Here are a few of the common mistakes you will likely encounter during soldering, along with suggestions to repair those errors:

• Cold joints: These failed solder joints are caused by insufficient heat. Reheat and apply more solder if necessary.
• Excess solder: Avoid using too much solder; joints should be smooth and minimal.
• Overheating components: Holding the iron against a pad for too long can damage sensitive parts. If necessary, use a heat sink clip.
• Bridging (short circuits): If solder unintentionally connects two pads, remove excess with a solder sucker or desoldering braid.
• Oxidized tips: Regularly clean and tin the tip to prevent oxidation and maintain performance. Use a brass sponge or damp sponge to remove oxidation and debris. Avoid excessive heat, which can degrade the tip over time. Store tips in a dry place to extend their life.

SMD Soldering

Surface mount soldering is used for modern high-density electronics, eliminating the need for through-hole components. While more challenging, it allows for more compact circuit designs. However, their small size can be a challenge. The following list provides recommended SMD packages to practice soldering:

• 1206 (Imperial)/3216 (Metric): Resistors, capacitors, LEDs
• 0805 (Imperial)/2012 (Metric): Resistors, capacitors
• SOT-23 (small outline transistor): MOSFETs, bipolar junction transistors, voltage regulators
• SOIC (small outline integrated circuit): Operational amplifiers, microcontrollers
• TQFP (thin quad flat package): Microcontrollers, FPGAs
• PLCC (plastic leaded chip carrier): EEPROMs, logic ICs

Essential Tools for SMD Soldering

• Fine-tipped soldering iron: A precision soldering iron with a fine tip is crucial. Tips specifically for passive SMD parts might be available, which can make soldering, and especially removal, much easier.
• Hot air rework station: Helps with reflowing solder and making small adjustments.
• Tweezers and suction tools: Essential for handling tiny SMD components.
• Solder paste: A mixture of solder particles and flux for easy application.
• Flux pen: Improves solder flow and reduces oxidation.
• Reflow oven or hot plate: Used for mass soldering of multiple components.

Essential Steps for SMD Soldering

1. Apply solder paste: Use a stencil or fine applicator to deposit solder paste onto PCB pads.
2. Place components: Carefully position SMD components using tweezers.
3. Solder with an iron or hot air: If using an iron, touch the tip to one pad at a time. For hot air, apply gentle heat until the solder paste reflows and bonds the component.
4. Inspect the joints: A good SMD joint should be smooth and well-adhered to the pad.
5. Clean up: Remove excess flux using isopropyl alcohol and a brush.

Common SMD Soldering Issues and Fixes

• Bridging: If solder connects two pads, remove excess with a desoldering braid.
• Cold joints: If the solder doesn’t adhere properly, apply more flux and reheat.
• Component misalignment: If the component is off-center, reheat and gently reposition it.

Final Thoughts

Whether you’re a hobbyist tinkering with at-home projects or an engineer working on high-precision electronics, good soldering habits will set you up for success.

Remember, soldering is a skill that improves with practice. The more you solder, the more comfortable you’ll become handling components, applying the right amount of heat, and troubleshooting common issues. Start with through-hole components and move on to surface-mount components after you have completed a few hours of practice.

To refine your skills, consider starting with a soldering practice kit. Mouser offers a variety of kits designed to help beginners build confidence and improve their technique. These kits include everything you need—PCBs, resistors, capacitors, LEDs, and more—to practice and perfect your soldering skills.

So, grab that iron, pick up a practice kit, and start making something extraordinary.