Introduction:

Alloy


is one of the best solders used in the reflow process. However, compared to traditional solders, it comes with two disadvantages related to manufacturability:

• The melting point is high. With a melting point around 217°C and a complete melting temperature of 235°C, it requires a higher manufacturing temperature during production. Failure to do so could lead to a range of undesirable outcomes.
• The wettability is slightly worse. The eutectic solder has a diffusibility range of 93%, while lead-free solder ranges between 73% and 77% diffusibility.

Suitable Temperature Curve for Lead-Free Solder:

Typical reflow temperature curves for Sn/Pb eutectic solders are divided into four stages: “Heating” – “Heat Preservation” – “Reflow” – “Cooling.” The “Heat Preservation” stage typically occurs at a temperature between 140°C and 160°C, and the reflow temperature exceeds 183°C, with peaks reaching 30°C to 40°C.

However, due to the significantly higher melting point of lead-free solder, optimizing process parameters becomes a challenge based on long-term production experience. Many studies recommend the following two temperature curves for lead-free soldering.

1) Trapezoidal Temperature Curve:

The trapezoidal curve is characterized by extending the peak time in the reflow stage. This extends the time the soldering components are above the liquidus temperature from the traditional 40s-60s to 60s-90s, with a recommended 30s-60s at peak reflow temperature.

The effects of the trapezoidal curve:

• Maintains sufficient time at high temperature to ensure thermal balance for components with large thermal capacity differences, reducing the risk of BGA solder joint voiding.
• Extended liquid phase time helps improve the wettability of lead-free solder.

2) Ramp Profile:

The ramp profile, also known as the “tent” or “triangle” shape, eliminates the heat preservation phase. The components are heated slowly from room temperature to the peak temperature, extending the heating time and allowing the heating rate to be reduced to 0.8°C to 2°C per second. While the overall process time remains the same, this approach allows for a more gradual rise in temperature.

This profile requires a reflow furnace with excellent heat transfer capabilities to ensure that the components reach the peak temperature with minimal surface temperature variation.

The effects of using the Ramp Profile:

• Allows for extended heating time without increasing overall process time, reducing the risk of damage due to excessive heating rates.
• Linear temperature increase makes the process easier to control and enhances repeatability.
• Slower heating reduces the temperature differential across the PCB surface, which helps overcome the adverse effects of lead-free solder and improves the process window.
• Improves solderability and ensures better solder joints by using flux activation temperatures that match the lead-free solder paste.
• Energy consumption is reduced compared to traditional reflow furnaces, and maintenance is simpler.

Conclusion:

The selection of an appropriate temperature curve depends on several factors, including the melting point of the lead-free solder paste, flux activation temperature, reflow furnace characteristics, and the thermal capacity of the components being soldered. The primary goal is to achieve reliable soldering quality without damaging the PCB or components.

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