1. Cold Solder Joints

Cold solder joints occur when the solder doesn’t melt completely, resulting in a dull, cracked, or lumpy appearance. Some causes of cold joints include:

• Insufficient heat from the soldering iron
• Dirty or oxidized component leads or pads
• Moving the joint before the solder solidifies

To prevent cold joints:

• Ensure your soldering iron is properly heated (315-343°C or 600-650°F for lead solder; 371-400°C or 700-750°F for lead-free)
• Clean components leads and PCB pads before soldering
• Hold the iron on the joint for 1-2 seconds after applying solder
• Let the joint cool without movement

2. Insufficient Wetting

Insufficient wetting happens when the molten solder doesn’t flow properly onto the component lead and PCB pad, often due to contamination or not heating the joint thoroughly. Signs include solder that forms a ball on the lead or pad instead of a smooth fillet.

Avoid insufficient wetting by:

• Cleaning leads and pads with isopropyl alcohol
• Using a clean, tinned soldering iron tip
• Heating the lead and pad simultaneously for 1-2 seconds before applying solder
• Choosing solder wire with rosin flux core

3. Solder Bridges

Solder bridges are unintended connections between adjacent pins or pads caused by excess solder. They can short out pins and damage components.

Prevent solder bridges with these techniques:

• Apply just enough solder to create a proper joint
• Hold the soldering iron tip at a 45° angle to the joint
• Solder fine-pitch ICs by dragging the iron tip perpendicular to the pins
• Use solder wick to remove excess solder

4. Overheated Components

Applying too much heat for too long can damage heat-sensitive components like ICs, LEDs, and electrolytic capacitors. Signs of overheating include discoloration, melted plastic, and failed components.

To avoid overheating:

• Use the minimum amount of heat needed to solder the joint
• Limit soldering time to 2-3 seconds per joint
• Allow components to cool between soldering pins
• Use a heat sink on sensitive component leads

5. Lifted Pads

Lifted pads happen when a pad detaches from the PCB due to excessive heat or mechanical stress. This often occurs when reworking or desoldering.

Minimize the risk of lifted pads by:

• Avoid overheating pads with the soldering iron
• Don’t pull on components while the solder is still molten
• Use a vacuum desoldering tool instead of solder wick for removal
• Support the PCB opposite the pad when applying force

6. Poor Tip Maintenance

A dirty, oxidized, or deformed soldering iron tip can lead to inconsistent heat transfer, poor solder flow, and difficulty creating good joints.

Maintain your soldering iron tip by:

• Keeping it clean by wiping on a damp sponge frequently
• Re-tinning with solder regularly to prevent oxidation
• Avoiding excessive pressure which can deform the tip
• Using the proper tip size and shape for the job

7. Incorrect Solder Amount

Too little solder results in a weak mechanical and electrical connection, while too much solder can cause bridges, shorts, and an unprofessional appearance.

Apply the right amount of solder by:

• Choosing solder wire diameter appropriate for the job (0.5-0.8mm for most through-hole, 0.25-0.5mm for SMD)
• Touching solder to the iron tip and pad/lead simultaneously
• Feeding in solder steadily until a concave fillet forms between the pad and lead
• Avoiding the temptation to add more solder after removing the iron

8. Improper Joint Cooling

Failing to let solder joints cool undisturbed can cause them to crack, crystallize, or become dull and weakened.

Ensure joints cool properly by:

• Holding components still for 1-2 seconds after soldering
• Not blowing on joints to cool them faster
• Allowing joints to cool naturally in still air
• Using a small fan for gentle cooling if needed

9. Ignoring Solder Type

Using the wrong type of solder can lead to issues like difficult wetting, unreliable joints, and incompatibility with certain components or PCB finishes.

Select the proper solder by considering:

• Alloy composition (tin/lead vs lead-free)
• Solder diameter for your application
• Flux core type (rosin, no-clean, water soluble)
• Compatibility with PCB pad/component plating

10. Skipping Inspection

Not visually inspecting solder joints after assembly can allow defects to go unnoticed until they cause bigger problems later.

Catch soldering issues early by:

• Examining each joint with a magnifying glass or microscope
• Looking for proper wetting, solder amount, and shape
• Checking for bridges, cold joints, contamination, etc.
• Using magnified photos of acceptable joints as a reference

PCB soldering FAQ

Q1: What’s the best way to remove solder bridges?

A1: First, use a magnifying glass to confirm the presence of a solder bridge. Then, apply flux and use a clean soldering iron tip to reheat the solder, allowing the bridge to flow back onto the pads. If excess solder remains, use desoldering braid to wick it away.

Q2: Can you mix leaded and lead-free solder?

A2: While you can technically mix lead and lead-free solder in some situations, it’s generally not recommended. The two solder types have different melting points and characteristics that can lead to unreliable joints. It’s best to choose one type and stick with it for a given project.

Q3: How often should I replace my soldering iron tip?

A3: Soldering iron tip life depends on factors like usage frequency, care and maintenance, solder type, and tip material. With proper care, a high-quality tip can last for months or even years. Replace tips when they become excessively pitted, deformed, or no longer wet easily with solder after cleaning.

Q4: What’s the difference between rosin core and no-clean solder?

A4: Rosin core solder contains a rosin flux that helps the solder flow and wet to the surfaces being joined. Rosin is mildly acidic and should be cleaned off with isopropyl alcohol after soldering. No-clean solder uses a different flux formulation that doesn’t require post-soldering cleaning in most cases.

Q5: Can I solder surface-mount components with a regular soldering iron?

A5: Yes, it’s possible to hand solder many SMD components with a standard soldering iron, but it requires practice, steady hands, and the right tools. Use a fine tip (0.2-0.5mm), thin solder wire (0.25-0.5mm), and techniques like drag soldering or solder paste and hot air for small chips. Tweezers, magnification, and flux are also helpful.

Conclusion

Mastering the art of PCB hand soldering takes time and practice, but being aware of these common pitfalls and how to avoid them will help you achieve more consistent, reliable results. Always use the right tools and materials, maintain proper technique, and inspect your work carefully. With patience and attention to detail, you’ll be creating professional-quality hand-soldered PCBs in no time.