Process of solder paste changes in circuit board reflow soldering

Reflow soldering first prints solder paste on the surface mount component (SMD) solder pads of the circuit board, and then places the SMD onto the pre printed solder pads using an automatic mounting machine. 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.

To gain a deeper understanding of circuit board solder paste reflow soldering, it is necessary to 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 and other metal alloy powders and fluxes. 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, called E-phase, will continue to be generated between the n-phase and copper layers, which will weaken welding force and reduce long-term reliability. In the metallographic diagram of the solder joint profile, this structure can be clearly seen. Cu Sn IMC displayed by scanning electron microscopy (SEM)

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. Some articles point out that 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.