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Suitable Temperature Curve of Lead-free Solder

by: Jan 03,2019 106 Views 0 Comments Posted in Engineering Technical

Introduction:

Alloy, image.png, is the best solder in the reflowing process. Compared with the traditional solder, it has two disadvantages in terms of manufacturability as following:

1)     The melting point is high. It has a melting point of about 217 degrees Celsius and a complete melting temperature of 235 degrees Celsius. Therefore, we must raise the manufacturing temperature in the manufacturing process, otherwise it will cause a series of adverse consequences.

2)     Wettability is slightly worse. The eutectic solder with a 93% diffusibility range, and the lead-free solder with a 73%~77% diffusibility range.

Suitable Temperature Curve of Lead-free Solder:

Typical reflow temperature curves using Sn/Pb eutectic solders are usually divided into four sections. That is, “Heating” – “Heat Preservation” – “Reflow” – “Cooling”. The temperature of “Heat Preservation” is around 140~160. And the temperature of reflow process is above 183, with peaks rising 30~40.

However, the melting point of lead-free solder increases obviously, which challenges the optimal process parameter value tested by long-term production practice. Therefore, many studies recommend the following two types of temperature curves for lead-free solders.

1)     Trapezoidal Temperature Curve:

image.png

This curve is called trapezoidal temperature curve, and its main feature is to extend the peak time of reflow area, so that the time of soldering parts above the liquid phase line is extended from the traditional 40s ~ 60s to 60s ~ 90s, and the time of 30s ~ 60s should be maintained at the peak temperature of reflow soldering.

The effect of trapezoidal curve:

Maintain sufficient time at high temperature to achieve thermal balance of components with large thermal capacity differences and reduce the cavitation of BGA solder joints.

As the liquid time of lead-free solder is prolonged, it is beneficial to overcome the wettability of lead-free solder.

2)     Ramp Profile:

image.png

This curve is called a "tent" or "triangle" shape. Its main feature is the elimination of the heat preservation stage, soldering parts from room temperature slowly rose to the peak temperature, thus extending the heating time, so that the heating rate can be reduced to 0.8~2/s. However, the total process time is not extended, but can be appropriately shortened the total process time. Of course, the use of this temperature curve has a prerequisite what is that it must use the reflux furnace with a good heat transfer performance, in order to make the soldering parts in the peak temperature, thus, its surface temperature difference can meet the requirements.

The effect of using the Ramp Profile:

On the premise of not increasing the total time, prolong the heating time, so as to avoid the damage of components caused by excessive heating rate.

As the temperature rises linearly, it is easy to control and the process has good repeatability.

Due to slow heating, the temperature difference of PCB board surface decreases, which is conducive to overcoming the adverse factors of lead-free solder causing the reduction of process window.

The solderability and bright solder joints can be improved by using solder flux activation temperature matching the paste of lead-free alloy.

Compared with the traditional reflux furnace, energy consumption is reduced and maintenance is relatively simple.

Conlusion:

Of course, the selection of this temperature curve, mainly based on the melting point of metal composition of the lead-free solder paste, the activity temperature of the flux, the structure and function of the reflux furnace, the heat capacity of the welding object and other specific circumstances, the ultimate goal is not to damage the PCB and components under the premise of reliable soldering quality.


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