Soldering Defects and Solutions

Soldering defects can significantly impact the performance of circuits and joints in the short and long term.

In this article, I will discuss the most common issues and how to prevent and solve them.

Let’s get started!

What Are Soldering Defects?

Soldering defects can be caused by different factors, such as poor soldering techniques, inadequate preparation, PCB design issues, defective soldering iron tips, and more.

Soldering by hand can be more challenging than most people think, but it becomes easier and easier with practice. But even the most skilled persons can make mistakes or be affected by external issues, leading to a poor joint. When it comes to circuits, even a minor flaw can cause the whole system to fail, as the strength of a chain is determined by its weakest link.

Understanding soldering defects is crucial to evaluate your work while and after you finish working on a project, troubleshoot circuit problems, and identify potential failures when repairing jobs.

Last but not least, soldering problems can also affect wire-to-wire connections and other electrical tasks, and I will cover those as well.

This article will focus on soldering defects you may find while doing your projects using manual soldering tools. Industrial soldering problems are out of this article’s scope, but I didn’t want to stop mentioning that even the most sophisticated automatic soldering devices aren’t perfect.

Most Common Soldering Defects

Below, you will find a list of the most common soldering defects, their causes and consequences, and how to solve them.

1. Cold Joints
2. Excess Solder
3. Solder Bridging
4. Insufficient Solder
5. Dry Joint
6. Lifted Pads
7. Overheated Joints
8. Webbing and Splashes
9. Component Shift

Let’s examine each of these soldering mistakes, identify the cause, ways to prevent them, and possible methods to repair the defect.

1. Cold Joints

Think of cold joints as “false joints” because they usually look like well-done and effective bonds for the untrained eye. They are caused by insufficient heat during the soldering process, causing the solder to melt partially, creating an inefficient electrical connection and a weak mechanical bond.

Cold joints are dangerous because they may work fine for a while or even years, but the hardened solder is weak and prone to cracking, especially when the circuit is subjected to vibrations.

In my experience in the automotive field, I have seen controllers or parts like instrument clusters fail due to cold joints caused by manufacturing defects or poorly executed repairs.

Cold Joints Issues

• Intermittent Malfunction and/or Unpredictable Behavior: Cold joints have inconsistent electrical conductivity and varying resistance.
  Besides, the mechanical play between the soldered parts often leads to these problems.
• Mechanical Weakness: Cold joints are prone to cracking or unbinding, leading to loose connections or component misplacements.

Cold Joints Causes and Solutions

Causes:

• Soldering iron tip or heat gun too cold.
• Soldering tool was removed before achieving a solid bond.
• Uneven heat distribution
• Contaminated workpieces or dirty tips.

How to Prevent Cold Joints?

1. Make sure that the soldering iron tip or heat gun is hot enough. When using a soldering or rework station, set the temperature slightly above the solder’s melting point and give it enough time to reach it.
2. Give the soldering tool enough time to let the solder melt and flow over the workpieces properly.
3. Always keep the tip clean and tinned before soldering.
4. Clean the workpieces to remove any dirt or debris and prevent the solder from sticking correctly. Use flux when necessary to ensure a proper connection.

How to Repair Cold Joints:

If the joint has just been created or is relatively new, reheat the solder until it flows and settles into place. When you notice excess solder, remove it using a desoldering vacuum pump or soldering tape.

If you are repairing an old circuit, it’s best to remove the old solder completely, clean the area, and resolder it.

2. Excess of Solder

Whether you are joining two wires, soldering a through-hole or SMD component, or conducting other soldering practices, using too much solder can lead to untidy joints and potential problems.

Beginners and inexperienced persons usually use excess solder accidentally or because they don’t know how to calculate the amount of solder to secure a proper connection.

It’s easy to tell when a joint was created using too much solder: You can see a noticeable ball of solder covering the area. Like a teacher of mine said when he didn’t like our job, “Are these joints or golf balls?“.

Even though these joints might do the job, they can fail over time, so it’s essential to prevent them or fix them as soon as possible.

Problems with Excess of Solder

Excess solder can cause short circuits and bridging, as we are about to see, but this defect also brings other consequences.

For instance, when soldering components into through-hole boards, the excess solder can prevent the mechanical connection between the component and the board or even fail to create a proper electrical connection. If the “ball” of solder solidifies over the component’s pin, it won’t probably stick to the circuit.

Besides, excess solder increases electrical resistance. This is extremely important when working with sensitive or high-speed components requiring minimal resistance.

Causes

• Applying too much solder into the soldering iron’s tip or melting too much soldering with the hot air gun when using hot air rework stations.
• Setting the soldering temperature too low, which might lead to applying more solder than needed.
• Applying solder before the joint has been appropriately heated can lead to using excessive solder.
• Uneven heat distribution due to, for example, a damaged tip can require using more solder than needed and tends to create these joints.
• Applying excessive pressure can also create these undesired “balls” when soldering.

Ways to Prevent Excess Solder

• When soldering, less solder is better. Only apply the necessary solder to create a solid and secure joint.
• Consider the components’ size and the solder’s characteristics to determine the right amount.
• Always preheat the workpieces and let the soldering iron or hot air gun reach the desired temperature before applying the solder.
• Always keep your soldering iron’s tip tinned and clean, and replace it if you notice any signs of tear and wear.
• Don’t apply pressure over the components; just barely touch them and let the solder flow over the joint.

How to Repair Joints with Excess of Solder?

Heat the solder over the joint, remove the excess solder using a desoldering vacuum pump or tape, or desolder the piece and resolder it properly using fresh solder.

3. Solder Bridging

Solder bridges are unintentional connections of two or more pins that shouldn’t be connected.

After soldering a component, it’s always important to check for possible bridges.

Common Problems

• Short circuits, which can cause malfunctions, burn or severely damage the bridged components, and even result in a fire or catastrophic failure.
• Solder bridging can also damage the circuit’s traces.

Causes:

• Applying excessive solder to a pin may end up causing a soldering bridge, mostly if the pins are close to each other.
• Soldering two pins that shouldn’t be connected by mistake.
• Soldering small components using an incorrect-sized tip or nozzle.
• On SMT (Surface Mount Technology), soldering bridges can be caused by applying too much soldering paste, using the wrong stencil, or placing the components misaligned with the soldering pads.
• Bent component leads can be easily joined by mistake.
• Using an incorrect type of solder and/or flux.

How to Prevent Soldering Bridges?

1. Apply the correct amount of solder.
2. Work carefully and, when in doubt, double-check before soldering.
3. Always use the right size of tip or nozzle when soldering.
4. Use a stencil to prevent applying too much soldering paste and ensure proper component alignment. Also, ensure to use the right amount of soldering paste.
5. Beware of bent component leads.
6. Use the solder and flux specified by the component’s and board manufacturer.

Ways to Fix Soldering Bridges

When possible, an easy way to repair soldering bridges is to use a soldering iron to melt the solder between the pins and break the connection.

If the components are too small for a soldering iron, you can use a reworking station with a thin nozzle to melt the undesired solder and remove it using desoldering tape.

Sometimes, you will have to desolder both connections or remove the component to re-solder it back in place.

4. Insufficient Solder

This case is the opposite of too much solder.

When a joint lacks the proper amount of solder, the resulting bond won’t be reliable or efficient immediately. However, in some cases, it may fail after a while, depending on factors such as vibration, shocks, level of current it handles, and others.

When working with SMD components, insufficient solder can cause a phenomenon called “Solder Starvation”, resulting in a weak bond that affects the circuit’s performance.

Insufficient Solder Issues

• Weak joints that are prone to cracking when subjected to physical stress.
• Board failure and malfunction.
• Solder starvation in SMT boards resulting in defective joints that affect the circuit’s performance.
• Electrical conductivity problems.
• Open connections.
• In through-hole PCBs, insufficient solder may leave an opening on the sides of the joint, resulting in inadequate contact between the component and the board.

How to Prevent?

• Use a properly heated soldering iron and ensure you apply enough solder to create a solid and reliable connection.
• Apply heat for enough time to let the solder flow and create a nice and stable connection; in other words, don’t remove the soldering tool too early.
• Ensure you use adequate solder for the components and board you work with. When using lead-free solder, remember to use more temperature than you would when using lead-based solder.
• When working in SMT boards using SMD and SMC components, apply enough soldering paste to the circuit.

Fixing Joints with Insufficient Solder

When working with wires and through-hole boards, you can heat the existing solder and then apply more solder to reinforce the joint.

When working with SMDs, you may not have easy access, and you will need to desolder the component and re-solder it to fix the problem.

5. Dry Joint or Insufficient Wetting

This problem is similar to insufficient solder and has almost the same consequences. The difference is that, in this case, the problem is not related to a lack of solder but to solder that didn’t adhere properly or at all to the component and board.

Joints with this defect look similar to the ones without enough solder. For example, in a through-hole board, you will see a gap between the hole and the pin.

Why Dry Joints are Bad?

• Weak and unreliable connections.
• Possible erratic circuit malfunctioning.
• Poor electrical conductivity that creates excessive resistance.
• Mechanical weakness that can lead to joint failures over time.
• Excessive resistance can lead to overheating and reduce the component’s lifespan.

Ways to Prevent

• Always clean the board and the components before soldering to ensure the solder can wet properly.
• Preheat the pin and the pad to make solder easier to flow.
• Don’t stop applying heat until the solder has flowed into place.
• Ensure the soldering iron is heated correctly, and when using hot air or a soldering station, ensure that you have set the right temperature.

How to Repair Dry Joints?

The safest way to fix this problem is to desolder the component and clean the board, the soldering pad, and the pin. Then, reattach it, applying enough heat to the pad and the pin.

In some cases, when you have easy access, you can melt the joint’s existing solder and add some fresh solder, just enough to cover the area. However, before doing this, make sure that the soldering pad is clean.

If in doubt, apply some flux before taking this approach.

6. Lifted Pads

This is a severe problem because it involves severe physical damage. Lifted pads are soldering pads partially or entirely detached from the PBC’s surface, losing contact with the circuit trace.

A circuit with one or more lifting pads is unusable or highly unreliable; however, in expensive boards, it is worth attempting to fix them.

Lifted Pads Issues

Lifted pads can be difficult to repair. Sometimes, it’s best to discard the board, but some boards are expensive, hard to find, or you just want to try fixing them. In that case, there are some approaches you can take:

• If the pad is just lifted from the board, but the electrical connection between it and the circuit isn’t broken, you can carefully re-attach it to the board using glue, contact cement, or a non-corrosive adhesive.
• If the pad has lifted from the board and the electrical connection between it and the circuit is broken, you can start by attaching it to the board using the method described above.
  Then you can try to connect the pad and the circuit using solder, a conductive pen (a pen used to repair circuit traces), or in a desperate attempt, use a thin wire and solder it to the pad and the circuit trace to restore the connection.

7. Overheated Joints

Excessive heat can cause overheated joints. They don’t look as shiny as well-made joints; they have a rough appearance, are darker, and usually have black or burn stains.

Besides, a deposit of flux residue resulting from excessive heat is noticeably visible on the soldering pad.

Overheated joints can cause immediate or long-term problems, depending on their characteristics.

Issues with Overheated Joints

• Excessive heat can affect the solder’s characteristics, preventing it from conducting electricity properly and resulting in excessive resistance or connection issues.
• Excess heat can damage sensitive components, burn them, reduce their lifespan, or affect their functionality.
• In the long run, excess heat can lead to intermittent failures.
• Overheating can make the joint susceptible to temperature fluctuations, leading to cracks and mechanical failure.
• When joining wires, excessive heat or prolonged heat exposure can damage the wires’ insulation, and in the case of thin wires, it can affect their resistance and conductivity.

Prevention

• Always set your soldering iron or reworking station to the appropriate temperature for the components you are working with. When using a standalone soldering iron, ensure not to use one with more than 30 Watts when soldering delicate or SMD components.
• Leave the soldering tool enough time to ensure proper solder flow but not too much to risk burning the solder, flux, or overheating the components.
• Keep your soldering iron tip clean to prevent uneven heat, which can lead to excessively high working temperatures.

How to Repair Overheated Joints?

If the component is undamaged, try to clean the flux burns by applying isopropyl alcohol or a contact cleaner with a stiff brush such as a toothbrush.

If the damage is beyond that, and you don’t see any signs of board damage, you may need to desolder and resolder that pin or remove, clean the PCB, and reinstall the component.

8. Stray Solder, Webbing, and Splashes

Stray solder, webbing, and splashes are deposits of solder or stains that may appear over the circuit board.

These issues result from accidents or problems related to the soldering process or flux application, causing solder to stick in areas where it’s not supposed to be.

Issues

• This problem can cause shorts, bridges, and circuit malfunction.
• It makes the board look untidy and poorly finished.

Ways to Prevent Stray Solder and Splashes

• Solder carefully and use a proper temperature. When solder is excessively overheated, it tends to stray. Besides, excess solder falling from the joint cools down over the board and can stick, creating “splashes”.
• Don’t use flux in excess, as it can make residual solder stick to the PCB, accidentally causing these stains and potentially merging two contacts.
• Keep the PCB clean when soldering; pollutants make the board prone to these defects.
• Avoid using too much solder.

How to Fix?

For problems associated with “bridging” and shorts, please refer to “Bridging” described above in this guide.

To clean the board, it depends on how bad it is. You can try using isopropyl alcohol or a contact cleaner and a soft rag to clean the affected areas. If it’s stuck too hard, you will have to use a rework station to heat and loosen the stuck solder and remove it using a vacuum desoldering pump.

9. Component Shift

When soldering through-hole components, the risk of misaligned or shifted components is much lower than when soldering SMT boards.

However, component shift can be a problem in SMT boards since component misalignment can cause malfunctions, while on through-hole components, the only problem is often aesthetic.

Common Issues

• Poor or unreliable connections.
• Open or short circuits.
• Weak soldering joints.
• Aesthetical/Finishing problems.

How to Prevent Component Shift?

• Don’t apply too much soldering paste when soldering surface mount components, as they may shift as the paste melts.
• Apply small amounts of solder to a few pins to use them as a guide and hold the component in place while you perform the definitive soldering.
• When soldering complex components, use a stencil to apply the soldering paste in the right place. Most stencils bring a fixture to hold the components in place while you solder them.
• Use tweezers or pliers to hold the parts in place.
• Apply heat evenly to ensure the component doesn’t tilt and move while you solder it.
• Use a criss-cross pattern to keep the components properly aligned.

Fixing Component Shifts

Unfortunately, the only way to fix this is to desolder the component, align it, and resolder it back in place. If you notice that the component is misaligned while still soldering it, don’t try to force it since you could twist or damage the pins or board.

Final Words

This guide covers all the soldering defects you may find in your daily projects, whether you are doing a project from scratch or a repairing job.

It’s essential to use the proper temperature, neither too low nor too high, know the parts, solder, and elements you are working with, and be alert to spot any problem before powering the circuit up to prevent a disaster in case of a mistake like a bridge or short.

Soldering is a beautiful task, but it can be tricky sometimes; besides burnt components or significant damage, all problems have a solution. If you do the job carefully, you will minimize the risk of creating soldering defects and have a pleasant soldering experience.