| 其他摘要 | Lead, as a poisonous heavy metal element, is confined or forbidden to be used in electronic device in advanced industrial countries such as EU and Japan. Therefore, lead-free electronic packaging has entered the large-scale application stage. In other words, the traditional extensively used Sn-Pb electrodeposit finish and solders will be totally replaced by lead-free finish and solders. Unfortunately, with the lead-free trends of electronic packaging, some new technical problems arise, among which reliability problem resulted from tin whiskers is the most prominent one. Compared with tin-lead finish, the lead-free finish is more prone to grow tin whiskers spontaneously. Tin whisker, as an excellent metal conductor, may bridge adjacent legs, lead to electrical short-circuits, and result in failures of electronic components due to its growth and falling off. The spontaneous growth of tin whisker not only critically influences reliability of civilian electronic devices but also makes the security of satellite, missile and nuclear weapon system suffer potential risk. So, it has become a hot topic for both the scientists and industrial engineers to clarify the growth mechanism of tin whisker and develop an effective mitigation strategy.
The present study investigated whisker growth behaviors of several industrial used solders doped with trace amount of RE (rare-earth element); Especially, we investigated whisker growth on surface of Sn-RE intermetallic compounds, conducted in-situ observation of the whisker growth, investigated the effect of relative humidity of the storage conditions on whisker growth, and did TEM microstructure analysis of the whisker; Furthermore, whiskers growth on surface of In-RE and Pb-RE intermetallic compounds were also studied.
Experiment results showed that tin whiskers, some of which reached 100um, grew on all the industrial solder alloys (including Sn-Pb eutectic solder alloy) doped with trace amount of RE (even as few as 0.1 wt.%) and the whiskers were only observed to grow from Sn-RE intermetallic phase. Tin whiskers grown on the Sn-RE intermetallic compounds were found to be flourishing and XRD analysis showed that a new phase-RE(OH)3, appeared with the growth of tin whiskers. The continuous growth process of tin whiskers could be in-situ observed using OM. But, tin whisker stopped to grow immediately after SEM observation, in which an electron irradiation effect was suspected to stop whisker’s continuous growth. The relative humidity of the storage condition was found to heavily influence whisker growth behaviors, i.e., whiskers grown on sample stored in 30-54%RH were thin and with a high density, whiskers grown on sample stored in 20%RH were thick and with a low density. What’s more, no whisker was observed on the sample stored in dry oxygen condition. TEM results showed that there was a layer of Sn-oxide on surface of the whisker. Whiskers were also observed to grow on other intermetallic compounds. Based on the above results, a new growth mechanism of chemical reaction between Sn-RE intermetallic compounds and water vapor in air, i.e., a growth model of fresh Sn atoms nucleating and growing, was proposed to account for the experimental results. |
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