The microstructure of the solder joint is affected by the process used. Under all other conditions alloy, PCB weld surface treatment, components are same, the microstructure of the solder joint will change as the process parameters change. For a known system, in the process of forming a solder joint, process parameters affecting the formation of the solder joint microstructure include heating parameters and cooling parameters.
During the heating phase of the welding process, the key parameters are the peak temperature and the time at which the temperature is above the liquidus. Higher peak temperatures or longer liquidus times will create excessive intermetallic compounds at the interface of the solder joints and inside the solder joints. The thickness of the intermetallic compound at the interface increases under conditions that promote the formation of an excessive amount of intermetallic compound. When the peak temperature is sufficiently high and the temperature is higher than the liquidus, the intermetallic compound increases and migrates inside the pcb solder joint.
In extreme cases, intermetallic compounds can appear on the free surface of the solder, causing a change in the appearance of the solder joint. Changes in appearance directly reflect changes in microstructure. It is expected that the mechanisms and phenomena of all three forms of intermetallic compounds will adversely affect the solder joints, either in terms of the appearance of the solder joints or mechanical properties of the solder joints.
It must be noted that the plating properties of the PCB pads and the metallurgical affinity of the solder composition may affect the formation of the solder joint microstructure. Such as immersion tin, immersion silver, OSP, HASL surface treatment, they participate in the interfacial reaction is Cu, its diffusion rate in molten Sn is 8.6 times that of Ni, easy to form a relatively thick IMC layer, the thickness of the NiSn intermetallic compound formed by ENIG surface treatment is relatively thinner.
2. Cooling parameter
The faster the cooling rate, the finer the microstructure formed. For tin-lead eutectic alloys, the slow cooling rate brings the microstructure closer to equilibrium.
The microstructure of the eutectic solder is often composed of a characteristic thin layer aggregate represented by 63Sn/37Pb. As the cooling rate increases, the degradation of the thin layer aggregate structure increases and eventually disappears. For lead-free solders, such as SAC, faster cooling rates result in finer tin grains. It is widely believed that an increase in cooling rate will result in a finer grain (metallurgical) structure in the tin block, but this general rule tends to be complicated by the interface boundary and the metallurgical reaction at the solder joint interface.