| Because of concerns about environmental pollution by lead and worldwide Pb-banning legislation,the electronics industry has been moving to adopt lead-free solders to replace the traditional Sn-Pbsolder. A useful approach to improve the performance of lead-free solders is to alloy suitable elementsinto these solders. Rare earth elements (REs), also known as the “vitamins” of metals, have beenwidely recommended by researchers to add into lead-free solders and have shown many beneficialeffects. But the reliability of RE-containing solders has seldom been studied. In this research, effectsof rare earth Pr addition on the microstructure, properties, joint strength and reliability issues of SZGsolder were studied. The phenomenon of spontaneous Sn whisker growth induced by RE addition inthe solders was discovered, the whisker growth behavior and mechanism were discussed.The wetting balance results showed that adding a proper amount of Pr into SZG solder greatlyimproved solder wettability on Cu substrate by increasing the wetting force and decreasing thewetting time, especially when the Pr content is0.08wt.%. This effect is due to the surface-activefeature of RE, which decreases the interfacial surface energy between molten solder and flux andprotects Zn from oxidation. Microstructure analysis results showed that an appropriate amount of Praddition also improved the solder microstructure due to the adsorption effect of RE. However, anexcessive content of Pr can lead to the formation of new RE-rich PrSn3IMCs in the solder.Determined by nanoindentation, it was found that the Young’s Modulus and Hardness of PrSn3were58.92GPa and1.33Gpa respectively, which were much higher than that of Sn matrix, thus anexcessive addition of Pr can worsen the solder microstructure and mechanical properties.Micro-joint strength test results showed that the Pr addition enhanced the shear bond strength ofsolder joints after both soldering and thermal aging at100℃especially when the Pr content is0.08wt.%. The addition of a proper amount of Pr inhibited the growth of Cu5Zn8interfacial IMLs andrefined the as-soldered microstructures during soldering, this is the main reason that joint strengthenhanced. While thermal aging led to the degradation of interfacial microstructures including theformation of micro voids and spalling of IMLs, it was found that the presence of Pr inhibited theformation of Cu6Sn5at the interfaces during aging. By means of FEA method and Weibull analysis,the joint reliabilities of SZG-Pr, Sn-Ag, Sn-Zn and Sn-Pb solders under thermal cycling wereevaluated and it was demonstrated that the SZG-0.08Pr solder showed the highest reliability. Based on 2-parameter Weibull model, the PDF function of solder joint was built asIt was found that an abnormal and spontaneous growth of Sn whiskers took place on the surface of solder and joint interface in high Pr-containing SZG-0.7Pr solder. It was further found that whiskers grew in different morphologies including needle-like, bent, twist, plate-like, hillock-like and many other complex shapes. The morphology mechanism analysis discussed that whisker morphology was governed by many factors such as the orientation and value of driving force, Sn source, initiation site and moment of gravity, any changes of the factors would result in the change in whisker morphologies. Considering the3D feature of whisker, a calculation method for whisker length based on2D measurements taken by SEM from two views and their3D geometry relationship was developed, which can avoid the measuring error effectively.The whisker growth behavior and effect factors were investigated by means of aging experiment, oxidation experiment and thermal cycling experiment. Aging results showed that the50℃aging condition accelerated whisker growth while whisker grew slower under room ambiance (25℃) and slowest under100℃and0℃aging condition. Oxidation results showed that the whiskers grew in hillock-type morphology under temperature humidity and in nodule-type under N2atmosphere while the typical needle-like whisker grew in room ambience. Unlike the needle-like whiskers grown mainly in ambience, it was found that coarse Sn hillocks were the dominant growth morphology under thermal cycling condition.The whisker growth mechanism was discussed based on the whisker growth behavior analysis. It was proposed that when the RE-containing solder is undergoing aging and oxidation processing, RE-induced growth of tin whiskers is governed by the oxidation of RE-rich IMCs, which generates compressive stress within the particles that creates fresh tin atoms and micro cracks on the IMC surfaces. The compressive stress then drives the newly released tin out from the weak points, forming the selective tin whisker growth from the RE-rich IMC surfaces on the bulk Sn-rich alloys. In the case of thermal cycling condition, it was discussed that the CTE mismatch between RE-rich IMC inclusions and surrounding solder matrix induced high thermal mismatch stress within the RE-rich IMCs, leading to the growth and propagation of cracks. The high local thermal stress and cracks ensured enough driving force and initiation sites for Sn hillock growth. |
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