How to Solving Solder Remelting and Bridging in Multiple Reflow

In today’s complex electronics manufacturing environment, multiple reflow cycles are increasingly common—especially in double-sided PCB assembly and modules with mixed-technology components. However, this introduces serious challenges: solder remelting, solder bridging, and component misalignment during secondary or tertiary reflows. One of the most effective solutions is the use of high-temperature Pb-free solder pastes.

The Challenge: Remelting and Bridging in Multi-Reflow Assembly

Modern PCBs often require more than one reflow cycle. For example:

• Double-sided SMT boards: Each side is reflowed separately.
• Modular or stacked assemblies: Pre-assembled components (e.g., BGA or PoP) undergo another reflow.
• Through-hole reflow (Pin-in-Paste): Combined with SMT reflow in mixed-technology boards.

In these scenarios, previously soldered joints are exposed to high temperatures a second or third time, which can lead to:

• Remelting of prior solder joints, causing:
  • Tombstoning or skewing of components
  • Formation of solder balls
  • Oxidation-related reliability issues
• Solder bridging, especially between fine-pitch leads or pads
• Solder slump or slippage, particularly in vertically oriented or tight-space applications

These problems result in production defects, yield loss, and costly rework.

The Solution: High-Temperature Pb-Free Solder Paste

To combat these issues, manufacturers can use high-temperature Pb-free solder pastes during the first reflow cycle. These pastes are typically based on high-melting alloys formulation.

The key benefit is that these alloys do not reflow during standard SAC (SnAgCu) soldering in subsequent cycles, which have typical peak reflow temperatures around 245–250°C.

Advantages of High-Temp Pb-Free Solder in Multi-Reflow

1. Thermal Stability

High-temp solder joints remain solid and intact during subsequent reflows, preventing movement, tombstoning, or warping.

2. Reduced Solder Bridging

By avoiding remelting, there is no fresh solder flow during secondary heating, significantly reducing the risk of bridging between pads or fine-pitch leads.

3. Improved Joint Reliability

These alloys often offer superior mechanical and thermal fatigue resistance, critical for high-reliability applications like automotive, aerospace, and industrial electronics.

4. Design Flexibility

High-temp Pb-free solders allow engineers to sequence reflows more freely, optimizing production flow without concern for joint reactivation or misalignment.

Implementation Tips

To get the most benefit from high-temperature solder pastes:

• Use high-temp alloy paste in the first reflow stage, typically on the bottom side or on sub-assemblies.
• Use standard SAC paste for later reflows (e.g., top-side or final assembly).
• Ensure accurate thermal profiling to avoid overheating or underheating joints.
• Select flux systems compatible with high-temp reflow, ensuring proper wetting and minimal residue.
• Verify component temperature ratings, especially when using peak profiles over 250°C.

Applications Where High-Temp Solder Paste Excels

• Double-sided SMT boards
• RF and power modules with heat-sensitive top layers
• Automotive electronics with multiple soldering passes
• Pre-soldered components in selective soldering or wave solder processes
• Stacked die or package-on-package assemblies

Conclusion

As electronics designs become denser and more complex, multi-pass reflow processes are becoming the norm—not the exception. To maintain solder joint integrity and eliminate costly rework from remelting and solder bridging, using a high-temperature Pb-free solder paste during the first reflow stage is a proven and reliable solution.

By integrating this approach into your SMT process, you ensure stronger joints, higher yields, and better long-term performance—even in the most thermally demanding applications.

Please contact GlueRu to provide solutions to your challenges!