Description of reflow soldering process

The basic principle of reflow soldering is relatively simple. It first prints solder paste on the surface mount components (SMD) solder pads of the PCB board, and then uses an automatic SMD mounting machine to attach the SMD onto the pre printed solder pads. Finally, by using a reflow soldering furnace, the solder paste is gradually heated to melt, known as reflow. Then, the PCB board is cooled, the solder solidifies, and the components and pads are firmly soldered together. In reflow soldering, both the solder pads and component pins do not melt. This is the difference between Reflow Soldering and Metal Fusion Welding.

To gain a deeper understanding of reflow soldering, one must start with the working principle of solder paste and the physical and chemical changes that occur during the soldering process. The composition of solder paste is mainly a mixture of tin lead alloy powder and soldering flux. Under heating conditions, the tin atoms in the melted solder material diffuse with the contact interface atoms of the solder pad or solder component (mainly composed of copper atoms), forming intermetallic compounds (IMCs). The first formed layer is Cu6Sn5, called n-phase, which is the key connecting layer for forming welding force. Only when n-phase is formed can it indicate true reliable welding. Over time, Cu3Sn will continue to be generated between the n-phase and copper layers, known as ∈ - phase, which will weaken welding force and reduce long-term reliability.

Intermetallic compounds are a key factor in the strength of solder joints, so many researchers specialize in studying the impact of changes in intermetallic compounds on the long-term reliability of solder joints. In order to protect the solderability of solder pads or component pins, their surfaces are generally coated with a tin lead alloy layer or an organic protective layer. For pins of non copper metal materials, a nickel plating layer is generally added between the pin coating and the metal as a blocking layer to prevent metal diffusion. This nickel coating is also used to block contact between metals that are not solderable or incompatible with the solder layer. Another issue related to the plating layer is the issue of the gold plating layer. If the gold content in the solder joint reaches 3-4% or more, there is a potential risk of increased brittleness in the solder joint.

To achieve good reflow soldering results, a good reflow temperature profile is necessary. So what is a good reflux curve? A good reflow curve should be a temperature curve that can achieve good soldering of various surface mount components on the PCB board to be soldered, and the solder joints not only have good appearance quality but also good internal quality.