Why don’t the electronic components on the first side of the double-sided PCB fall off due to re-melting of the solder during the second time reflow process in the SMT oven? How much does the melting temperature of the lead-free SAC305 solder alloy increase during the second time reflow process? Perhaps it’s because the components on the first side are more well-behaved and don’t cause trouble! This is just a joke, of course.

During the second time SMT reflow process, why don’t the already-soldered electronic components on the first side of PCB fall off due to solder joint melting and gravity when the second side is processed in the reflow oven? Typically, the peak temperature setting at 1st and second time reflow is consistent, and most SMT engineers set both the top and bottom reflow temperatures the same. So why does the solder paste on the second side of the board melt while the paste on the first side, which has already melted once, does not? Could it be that the solder paste melts at a higher temperature during the second reflow process?

Many people who have worked with Surface Mount Technology (SMT) may have wondered why already-soldered electronic components on the first side of the PCB don’t fall off during the second time reflow process. Perhaps someone has told you that the solder joint on the 1st side needs to melt again during the second time reflow process at a temperature about 10°C higher than fresh solder paste, but why doesn’t the solder joint on the first side, which has already melted one time, melt again during the second reflow process?

To clarify this, Workingbear consulted an expert from a Taiwanese manufacturer who specializes in solder paste. I primarily asked about the melting point of SAC305:

Question:

The melting point of SAC305 during the first reflow is approximately 217°C. Where does the melting point fall during the second reflow? Does the increase in melting point during the second reflow result from a change in composition?

Answer:

After the first reflow process, the melting point of SAC305 may change due to the formation of intermetallic compounds (IMCs) between the solder elements (mainly tin and copper, while silver forms Ag₃Sn and does not participate in other reactions) and the material of the solder pads or component pins. In addition, some of the elements from the solder pads or component pins may also melt into the solder joint, altering the original SAC305 ratio and causing a change in the melting point.

The range of melting point changes is difficult to estimate because each component’s condition is different. Even solder joints in different areas of the same PCB may vary. Also, the same solder joint may have differences due to variations in element diffusion at different locations (such as differences in the distribution of elements under QFN component pins and outside the pins). However, it can be confirmed that SAC305 is a near-eutectic material, so even a slight change in alloy composition will cause the melting point to start melting at around 217°C (eutectic point of SAC305), and the temperature for complete melting will increase. The overall average temperature for complete melting of the solder joints is estimated to increase to around 225~230°C.

In addition, after the first reflow, the solder joints’ surface will contain oxides, and there will be a lack of flux to remove them. This oxide tension on the surface of the first reflowed solder joints can support the shape of the joints, so the joints’ appearance will not collapse due to melting when the temperature reaches the solder melting point during the second reflow.

Here is my personal additional explanation on why components on the first side of the PCB don’t fall off during the second time reflow process:

1. Assuming that the original solder paste is SAC305, the alloy composition ratio of SAC305 may change after the first time reflow due to the formation of intermetallic compounds (IMCs) between the solder elements (mainly tin and copper for copper-based boards such as OSP, or nickel and tin for nickel-based boards such as ENIG). Also, don’t forget that the most important element “silver” also forms Ag₃Sn IMC. These elements that have already formed IMC will not participate in the melting reaction during the second time reflow. Therefore, the composition of the solder joint has changed, and it is no longer in the original ratio of SAC305, especially since almost all of the “silver” has become IMC and disappeared. This change in composition causes the melting temperature of the solder to increase during the second time reflow. The increase in melting temperature is generally within 10°C, although I do not have experimental data to prove this. Therefore, if the temperature during the second time reflow is too high, there is still a risk of re-melting the solder joints on the first side of PCB, which may cause components to fall off.

2. The metal surface of the formed solder joints will form varying degrees of oxide due to exposure to air. This oxide layer increases the surface energy of the metal and indirectly raises the melting point of the formed solder joints.

3. The flux inside the solder joints on the first side of PCB has already evaporated during the first time reflow, so it cannot reduce the surface tension of the melting solder during the second time reflow. Therefore, even if some of the solder joints melt during the second time reflow, the appearance of the joints will not collapse due to the temperature reaching the solder melting point. Of course, if the temperature is much higher than the melting point, the solder will still melt.

These are just my personal opinions.

This article translate from 為什麼雙面SMT第二次回焊時第一面不會掉件？再回焊熔錫溫度會升高? | 電子製造，工作狂人(ResearchMFG)

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