The components are soldered on the circuit boards to form a completed PCBA, which can perform intended functions. If your PCBA is used for a reliable application, good solder joints are critical. The ideal and well joints are smooth and shiny. However, in the soldering process, there are many soldering defects that can lead to poor solder joints and ultimately destroy the electronic assembly. In this blog, we’ve summarized 5 common soldering defects, analyzed their main causes and effects, and provided corresponding solutions to prevent them.
1. Solder Bridging
With increasing usage of smaller components, solder bridging defects are becoming more and more likely to occur, and these soldering defects cannot be ignored or overlooked.
What Does Solder Bridging Look Like?
Two parts on the circuit board that shouldn’t be linked were accidentally joined together by solder. These solder bridges can sometimes be very small, making them challenging to identify.
Causes
- Apply too much solder, flowing into connecting adjacent pins or pads.
- Small pin spacing.
- Poor PCB stencil design, causing incorrect solder deposition.
- Incorrect soldering technique, such as overheating or insufficient solidification time.
- Component misalignment.
Effects
- If not detected in time, it may cause short circuits or burn out components.
- For more severe consequences, the board may be destroyed and can not be used.
How to Prevent Solder Bridging?
- Avoid applying too much solder and use an appropriate technique.
- Control the temperature (such as optimize reflow profile) to avoid overheating that leads to the solder to flow indiscriminately.
- Choose precise soldering tools, like smaller tips.
- Use a desoldering pump or solder wick to eliminate extra solder.
- Maintain correct THT component lead length, avoiding excess solder to connect them.
- Use pick-and-place equipment with high precision to confirm accurate component placement.
2. Tombstoning
These soldering defects commonly occur in the SMT components, such as capacitors, inductors, and resistors.
Why is It Called Tombstoning? What Does It Look Like?
The solder paste on the component pads does not melt simultaneously during reflow soldering. The combined force generated by surface tension pulls the component toward the pad, where the solder melts faster. Ultimately, one end of the component curls up from the pad, resembling a tombstone.
Causes
- Thermal imbalance between two pads in the reflow process.
- Uneven solder pad size or shape can lead to uneven or insufficient solder wetting.
Effects
- The components can not be soldered to the pad correctly.
- It can lead to open circuits and impact the board’s performance.
How to Prevent Tombstoning?
- Uses symmetrical pads and ensures the pads have the same heat.
- Optimizes reflow profile to ensure an even heating profile.
3. Cold Solder Joints
These are common but dangerous soldering defects, easily leading to cracking of the solder joints and eventual failure. When the solder does not completely melt, cold solder joints will appear.
What Do Cold Solder Joints Look Like?
These solder joints are easily identified, having a rough, dim, and messy appearance. Their structural strength is weak.
Causes
- Low soldering iron temperature, causing insufficient heating requirements.
- Insufficient heating time of the soldering iron on the solder joints.
- Inadequate cleaning allows contaminants (such as dust, metal oxides, and grease) to enter the solder joints, hindering proper soldering.
- Fast cooling process or disturbance with vibration factors.
Effects
- It will cause poor electrical conductivity, which may be intermittent or completely fail.
- Solder joint fatigue is likely to happen due to continuous mechanical stress, such as vibration and movement.
- Solder joint failure may occur during the initial use of the product.
How to Prevent Cold Solder Joints?
- Keep the components stable during the cooling process until the solder has completely cooled (commonly 3 to 5 seconds) and formed good solder joints.
- Properly clean to remove any contamination that affects the formation of good solder joints.
- The temperature must be kept at a moderate range, neither too high nor too low. At least 15°C higher than the melting point of the solder alloy and held for at least 45 seconds.
4. Solder Balls
Solder balls are also common soldering defects that do not directly impact circuits. When the board operates in a high-temperature environment, the solder ball may melt, causing short circuits between components and traces. They not only affect its performance but also impact the electronic product’s appearance.
What Do Solder Balls Look Like?
As its name suggests, they are small solder balls that are sitting around the PCB surface (such as the pads or other metal parts). Solder balls are usually bright, shiny, and silver-colored, varying in size.
Causes
- During reflow soldering, the trapped moisture in the solder paste expands rapidly, causing the solder paste to splatter and form ball shapes.
- Apply too much solder paste, and extra paste overflows to create solder balls.
- Due to an incorrect reflow profile (excessively low temperature or inadequate preheat time), the solder cannot completely fuse, resulting in solder balls. Moreover, when ramping up too fast, solder paste violently boils, which also results in balling.
- PCB contamination, like dirt or other residues, may prevent the solder from flowing smoothly, causing solder balling.
Effects
- Stray solder balls may bridge adjacent pads or fine-pitch traces, leading to intermittent or permanent shorts.
- Visible solder balls mean poor process control and may lead to inspection failures.
- Solder balls make the PCB look messy and unprofessional.
How to Prevent Solder Balls?
- Properly store the solder paste, keep it at 2-10° When using it, place it in a normal environment for four to six hours to allow it to reach 20-25°C (ambient temperature). The purpose is to prevent condensation from forming on the solder paste’s surface.
- Advised to use the SMT stencil to control the solder paste usage. For different designs (fine-pitch components), you can choose the ideal thickness of stencil.
- Reset and optimize the reflow profile. Make sure the peak temperature is higher than the solder’s melting point. Prolong the preheat time to 60 to 120 seconds, and keep the temperature at 150-180°C to remove the moisture, promoting solder flow evenly.
- Clean PCB completely before soldering, preventing any contaminants from affecting solder paste flow properly.
5. Lifted Pads
While lifted pads are common soldering defects that can be fixed, prevention is always preferable to treatment.
What Do Lifted Pads Look Like?
PCB pads are metal parts that connect components to the circuit boards. When solder pads are subjected to mechanical or thermal stress, they may detach from the circuit board surface.
Causes
- The adhesion of copper foil decreases as surface temperature rises. Overheating during soldering, especially during wave soldering or reflow soldering, can make solder pads more prone to detachment.
- When solder pads are located under components, they are in a blind spot. Technicians find it difficult to see the solder joints. If they attempt to move a component while the solder is still molten, the pads may lift due to mechanical stress.
- Poor pad design, like incorrect size, shape, and position, will also lead to lifted pads during the assembly process.
Effects
- The electrical connection between the component and the circuit board will be interrupted.
- If lifted pads contact with nearby components or traces, it’s possible to cause short circuits. This is particularly important for compact, high-density boards with trace spacing as small as 0.1mm.
How to Prevent Lifted Pads?
- Use temperature-controlled soldering irons to avoid overheating.
- Choose high-temperature resistance adhesive materials.
- Employ proper soldering technique and follow the correct procedure to avoid moving the components.
- Optimize pad design to withstand mechanical and thermal stresses during the assembly process.
Final Words
Understanding and avoiding common soldering defects are beneficial for improving soldering quality, decreasing repair requirements, and ensuring more reliable board performance. By analyzing the causes and effects of these soldering defects, corresponding preventative measures can be taken. If defects have already occurred, they can be quickly identified and repaired.
