Mixed process essence
The mixing process is essentially a variable component solder joint formation process. Due to the different metal composition of the BGA solder balls and solder pastes, the components continuously diffuse and migrate to form a new "mixed alloy" during the melting of the solder/balls, that is, the composition and melting point are different in different layers of the solder joints.
According to the technical study of FASTPCBA, the mixing height is related to the welding peak temperature and the welding time. Temperature is a prerequisite and time is the acceleration factor. If the temperature is lower than 220 ° C, partial mixing may occur. According to this feature, the mixing process can be divided into two categories according to the welding peak temperature :
(1) Low-temperature welding process, that is, the welding peak temperature of is lower than 220 °C. Under these conditions, solder paste is generally difficult to spread evenly across the entire BGA solder ball height and will form a semi-fused solder joint. This solder joint has been evaluated for reliability. According to Intel research, reliability can be achieved as long as the height of the hybrid part is not less than 70% of the solder joint height, as shown in Figure 1.
(2) High-temperature welding process, that is, welding with a welding peak temperature greater than 220 °C. Under this condition, the solder paste and BGA solder ball components can be completely fused to form a uniform structure, as shown in Figure 2. If the temperature is higher than 245 ° C, the lead-rich phase segregation structure at the grain boundary will be discontinuous. The reliability of this organization is certainly no problem, but the process is relatively poor, and there is a risk of malignant massive IMC.
This classification is very meaningful for soldering lead-free BGA with lead solder paste. Because the BGA solder balls have to undergo at least two times reflows solderings, in many cases they will undergo three times reflow solderings. The BGA solder balls completely melted and semi-melted has greatly different impact on the thickness and morphology of the interface IMC on the BGA side of the solder ball , especially on the BGA carrier plate treated with OSP.