We know that in the daily work of SMT chip processing, there are often cavities, cracks, etc., so what causes these conditions? Today, Sanjing takes everyone to discuss the causes of voids and cracks. There are mainly the following factors:

First, oxidation of the solder;

Second, there is poor wetting on the soldering surface (PCB pad and component soldering surface);

Third, the expansion coefficients of various materials on the welding surface are not matched, and the solder joints are not stable when solidified;

Fourth, the setting of the reflow soldering temperature curve failed to volatilize the organic volatiles and moisture in the soldering sound before entering the reflow zone.

At the same time, with the development of lead-free SMT, lead-free solders are becoming more and more widely used. The lead-free solder is characterized by high temperature, high surface tension, and high viscosity. The increase in surface tension will make it more difficult for the gas to escape during the cooling stage, and the gas will not be easily discharged, which increases the proportion of voids. Therefore, there are more pores and voids in lead-free solder joints in SMT patches.

In addition, because the lead-free soldering temperature is higher than the lead soldering temperature, especially for large-size, multi-layer boards, and components with large heat capacity, the peak temperature often reaches about 260 ℃, and the temperature difference between cooling and solidification to room temperature is large. Therefore, The stress of lead-free solder joints is also relatively large. Coupled with more IMC, the thermal expansion coefficient of IMC is relatively large, and it is prone to cracking under high temperature work or strong mechanical shock.

Due to the high melting point of lead-free solder and high-Sn solder, the dissolution rate of Cu during lead-free soldering is much higher than during Sn-Pb soldering. Studies have shown that the voids (micropores) in the welding interface are mainly caused by the high solubility of Cu. Such voids are so small that they can only be found by scanning electron microscopy (SEM). The location and distribution of the voids may be a potential cause of electrical connection failure. In particular, the voids of power components will increase the thermal resistance of the components and cause failures. Over time, these voids may weaken the reliability of solder joints. Therefore, the high solubility of lead-free solder determines to a certain extent that the generation of voids and micro-holes in SMT patches cannot be completely avoided. If you want to solve these problems, you can observe and test more after the solder joints are soldered to facilitate timely discovery Poor, to ensure product reliability.