Process of solder joint formation in QFN packages

Firstly, due to the smaller size and closer proximity of the peripheral solder joints (i.e. I/O solder joints) to the heat source, they will start to melt earlier than the heat sink pads.This process may cause the QFN package to be slightly lifted locally, but this lifting will gradually disappear as the solder continues to melt.

As the temperature rises further, the solder paste on the hot-sink pads begins to melt.Due to the large size and high heat capacity of the heat sink pads, the melting process is relatively slow.During the melting process, the solder starts to wet the surface of the QFN pad and gradually expands.This step is critical because it determines whether the solder joint can form a tight electrical connection with the QFN pad.To facilitate the wetting and expansion of the solder, the stencil window is usually designed to be slightly larger than the pads to ensure that the solder paste is evenly applied to the pads.

After the solder has sufficiently wetted the QFN pads, due to the pulling force of the solder, the QFN pads are gradually pulled down to form a tight electrical connection with the pads on the PCB.This process marks the formal formation of the solder joint.However, it takes some time for this process to complete to ensure that the solder can fully melt, wet and pull down the QFN pads.

If the reflow soldering time is too short, it may result in the solder paste on the heat sink pads not being able to fully melt and wet the QFN pads.In this case, there is a risk that the solder joints may develop a false solder, i.e. the solder fails to form a tight electrical connection with the QFN pads.False soldering can seriously affect the electrical performance between the device and the PCB, or even cause the entire circuit to fail.Therefore, factors such as soldering time, temperature profile and solder paste selection need to be strictly controlled during the reflow soldering process to ensure the quality of the solder joints.

Finally, after the solder joint is formed, the solder begins to cool and solidify as the temperature gradually decreases.The cured solder joints form a stable electrical connection, ensuring a reliable connection between the QFN package and the PCB.At this point, the solder joints need to be inspected for quality, including appearance, wettability, and connection strength, to ensure that the solder joints meet the design requirements.

In summary, the formation of solder joints during reflow soldering of QFN packaged BTC devices is a complex and delicate process that requires strict control of various factors to ensure the quality and reliability of the solder joints.