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Factors Affecting Reliability of Lead-free Solder Joints
In the microelectronic packaging industry, SMT assembly technology and new chip size packaging (CSP), ball deletion array (BGA) and MCM packaging technology are widely used to improve the integration and reduce the size of devices. All these technologies directly realize electrical and mechanical connections between heterogeneous materials (mainly withstanding shear strain) through solder joint interconnection. Therefore, how to ensure the quality of lead-free solder joints is an important issue. Its quality and reliability largely determine the quality of electronic products.
Compared with the traditional lead-containing process, lead-free soldering will inevitably affect the reliability of solder joints due to the difference of solders and the adjustment of process parameters.
First of all, the melting point of lead-free solder is relatively high, generally around 2l7 C. The melting point of traditional Sn-Pb eutectic solder is l83 C. With the increase of temperature curve, the problems of easy oxidation of solder and rapid growth of intermetallic compounds will arise.
Secondly, because the solder does not contain lead, the wettability of solder is poor, which easily leads to the self-calibration ability, tensile strength, shear strength of solder joint can not meet the requirements. Taking a manufacturer as an example, the unqualified rate of solder joints in the original lead-containing process is generally around 0,05, while in the lead-free process, due to poor wettability of solder, the unqualified rate rises to 0,2-0,5%. There are still many problems in the reliability of lead-free solder joints, so it is necessary to analyze them. Similar to the traditional lead-containing process, the factors affecting the reliability of lead-free solder joints can be roughly divided into three aspects: design, solder selection and process.
