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The choice of solder is very important. At present, tin-silver-copper alloy series is mostly used. The liquid temperature is 217-22l (?), which requires high peak temperature of reflow soldering. As mentioned above, solder and conductor materials (such as copper foil) are easy to oxidize at high temperature, and intermetallic compounds grow rapidly.
Because in the welding process, when the melted solder contacts the welding substrate, a layer of intermetallic compound (IMC) will be formed at the interface due to the high temperature. Its formation is controlled not only by reflow temperature and time, but also by its thickness increasing with time in later use. The results show that intermetallic compounds at the interface are a key factor affecting the reliability of solder joints. The existence of overly thick intermetallic compound layer will seriously lead to the fracture of solder joints, the decrease of toughness and low cycle fatigue resistance, which will lead to the decrease of reliability of solder joints. In addition, because the composition of lead-free solders and traditional Sn-Pb solders are different, the reaction rates and products of lead-free solders and traditional Sn-Pb solders, such as Cu, Ni and AgPd, may be different, so that the solder joints will show different reliability.
At the same time, the compatibility of solder and flux will also have a great impact on the reliability of solder joints. Some studies have shown that the incompatibility between solder and flux components will lead to the decrease of adhesion. In addition, due to the mismatch of thermal expansion coefficient, the periodic fatigue failure of solder will be accelerated. Therefore, special attention should be paid to the selection of solders and flux with good compatibility in order to withstand the high temperature impact of lead-free reflow soldering. In addition, each welding interconnection component comes from different manufacturers, so the quality of components is inevitably uneven, such as poor solderability of pins, which has a great impact on the reliability of lead-free solder joints.
