How to Prevent Solder Defects During Reflow Soldering

Reflow soldering is a critical step in surface mount technology (SMT) assembly, where solder paste is melted to form strong and reliable joints between components and the PCB. However, defects during this process can lead to assembly failures, rework, and reduced reliability. Understanding the causes of solder defects and implementing preventive measures can significantly improve yield and quality. Know how PCB assembly is done in JLCPCB factory.

Common Solder Defects in Reflow Soldering:

• Solder Bridges
• Cold Solder Joints
• Void Formation
• Solder Ball formation
• Tombstoning

Let's Discuss them One by One in detail.

Solder Bridging Defect:

Solder Bridging is another common defect, which occurs when the solder has formed an abnormal connection between two or more adjacent traces, pads or pins, and forms a conductive path.

The cause of bridging includes:

• The pads are spaced too close to one another.
• There are residues stuck on the PCB surface or pads.
• A dirty stencil with paste sticking on its underside.
• A misalignment during solder paste printing.

Cold Joints and Voids:

How to Prevent Solder Joint Voiding and Cold Solder Defects during the SMT Reflow Process

Solder joint voiding Solder joint voiding is a phenomenon that causes empty spaces or voids to occur within the joint. Solder joint voiding often occurs on BGAs and larger pads. Voids are related to flux that's entrapped in the joint, as well as paste oxidation. A large number of voids will reduce the soldering joint's reliability.

The cause of error might be too much flux is applied in the solder paste. The flux doesn’t have sufficient time to outgas before the solder has transitioned to a solid state. The preheat temperature is too low so any solvent in the flux is difficult to vapour out completely.

The soak zone time is too short during the reflow process. Lead-free solder typically has a 4% shrinkage in volume when it is cooled down to a solid state. It is possible to obtain voids when large pads are cooled down unevenly.

Fixing the Voids:

• Reflow Profile Optimization: Carefully design and optimize the reflow soldering temperature profile to minimize the potential for void formation. Adequate preheat and controlled cooling can help.
• Vacuum Reflow: In some cases, vacuum reflow soldering processes can be used to reduce voiding by removing trapped gases during reflow.

Solder Ball Defect:

A solder ball is the most common type of defect that occurs in the SMT assembly process. Small balls of solder that form on the PCB during soldering. These balls of solder could be unattached to a component or pad, essentially sitting on the board near an SMD pad or other piece of metal. In the worst case, solder balls will be large enough to cause a short across two pads or two component leads. Typically, solder balls could sit on the board and might be trapped in leftover flux residues on the surface layer.

Solder balls located within 0.13mm of traces violate the minimum electrical clearance principle. They can adversely affect the electrical reliability of the assembled PCB. According to the IPC A 610 standard, a PCB is also considered defective when there are 5 solder balls (<=0.13mm) within 600mm^2.

The cause of error are, solder ball is very closely related to air or water (trapped in solder paste) vapour escaping from the paste and turning into liquid. If the vapor in solder paste escapes too fast, a small amount of liquid solder will be taken from the soldering joint, and a solder ball will be formed when it cools down.

Tombstone Defect:

The root cause of the tombstoning phenomenon is that when the solder paste starts to melt, the wetting characteristics (such as wetting speed) of different terminals are uneven, leading to an unbalanced torque. This is often related to the following factors:

• Improper pad design: Due to the wide spacing between the two pads, the component's terminals are not covered by more than 50% of the PCB pads, leading to uneven wetting.

• Uneven solder paste printing: If there are differences in solder paste deposition and distribution, it will lead to inconsistent melting behavior of each solder joint.

• Component placement deviation: Inaccurate component placement can also cause uneven heating of each solder joint.

• Unreasonable reflow temperature profile: For lead-free solder paste, if the heating rate is too fast before reaching the melting point, it will increase the thermal stress and exacerbate the tombstoning defect.

Some Other Unpopular Solder Defects:

1. Graping: Too much heat applied during preheat and soak causing flux to become exhausted before reflow. Reduce time and/or temperature during preheat and soak stage of profile

2. Head-in-Pillow (HIP): Excessive heat during preheat and soak causes flux to become exhausted before entering reflow stage, oxidized solder connections. Reduce time and/or temperature during preheat and soak stage of profile, consider using nitrogen or higher activity solder paste.

3. Cracked Solder Joint: Poor solderability of component, poor quality solder paste, incorrect reflow profile - incorrect rate of cooling. Check component solderability, check quality of solder paste, check reflow profile to solder paste datasheet.

4. Partial Solder Joint: Silk print on component footprint pads stopping complete formation of solder joint. Ensure PCB supplier clips away any silk screen from component footprint pads.

6 Key Causes of Solder Defects:

1. Oxidation:

Oxidation on the surface receiving soldering can often lead to non-wetting. The oxides intervene between the solder and the base metal and prevent proper adhesion. When this happens, non-wetting can occur over every part of the surface that has become oxidized.

2. Insufficient Solder Paste:

A low volume of solder paste frequently causes non-wetting because an adequate spread of solder paste is necessary for ensuring stable solder joints. When PCB assembly uses too little solder paste, proper adhesion between the solder and the base metal surface of the board cannot occur.

3. Poor Choice of Solder Paste:

Some solder pastes perform better to prevent non-wetting than others. Generally, as with fluxes, a high-activity solder paste will be more effective than a low-activity solder-paste. It will enhance solderability and minimize the risk of non-wetting.

4. Expired Solder Paste:

If the solder paste used on the board has passed its expiration date, it will not contain strong enough flux. The flux will not be active enough to remove oxides from the board and allow for good solderability and wetting.

6. Inconsistent Soldering Temperatures:

Inconsistent or fluctuating soldering temperatures can lead to non-wetting on a PCB because they fail to activate the flux and promote poor adhesion of the solder in certain areas. If specific trouble spots on the PCB don't receive heat that reaches the flux's activation temperature, the solder will not stick to those regions as it should.

Preventive Measures for Solder Defects:

• Optimize PCB Design: Use proper pad sizing and spacing to avoid solder bridges and tombstoning. Include thermal reliefs and vias for heat distribution during reflow.

• Design an Accurate Stencil: Use stencil thickness and aperture designs suitable for component and pad sizes. Consider laser-cut stencils for precision.

• Ensure Cleanliness: Clean PCBs and components to remove dust, grease, and oxidation. Use anti-oxidation measures, such as nitrogen inerting, in the reflow process.

• Monitor and Inspect: Use X-ray inspection to detect voids in solder joints. Employ automated optical inspection (AOI) to verify solder paste deposition and joint quality.

Comparison with Wave Soldering:

Reflow soldering offers better precision and suitability for fine-pitch and high-density components compared to wave soldering, which is more common in through-hole assemblies. However, wave soldering is less prone to certain defects like tombstoning due to its simpler process dynamics. To know more about the journey of PCBs, see the origin story through this blog post.

Conclusion:

Preventing solder defects during reflow soldering requires attention to design, materials, and process control. By optimizing PCB design, stencil accuracy, solder paste selection, and reflow profiles, manufacturers can minimize defects and improve overall assembly reliability. We can use some advanced techniques like, Solder Paste Inspection (SPI) and X-ray Inspection.  

Solder Paste Inspection (SPI): Implement solder paste inspection equipment to monitor and detect defects in solder paste deposition. SPI can help identify areas prone to voiding.

X-ray Inspection: Conduct X-ray inspections to assess the presence and distribution of voids within solder joints after reflow soldering. This allows for non-destructive assessment of solder quality.

By implementing these strategies and maintaining strict process controls, manufacturers can reduce the risk of solder voiding underneath SMT components and enhance the overall quality and reliability of their electronic assemblies. Effective inspection and monitoring tools ensure that any issues are quickly identified and corrected, reducing costs and improving product quality in today's high-demand electronics industry.