Morphology of the Tin Whiskers on the Surface of a Sn-3Ag-0.5Cu-0.5Nd Alloy
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FOR the development of Pb-free solders, eutectic Sn-Ag-Cu alloys have been considered as the most promising system for application in electronic packaging. Many efforts have focused on the addition of fourth elements into ternary Sn-Ag-Cu alloys in order to improve their performances. Anderson et al. reported that the Co-doping Sn-Ag-Cu solder joints exhibited the least sensitivity of shear strength to cooling rate.[1] Zhao et al. showed that the addition of Bi into Sn-Ag-Cu alloy can render the beneficial effects of microstructure refinement and inhibition of interfacial intermetallics.[2] The excessive growth of intermetallic compounds at the interfaces of Sn-Ag-Cu joints with Cu pads during the reflowing[3] and aging processes[4,5] has also been reduced by Zeng et al. through the alloying of Sb into the solder alloys.[6] An innovative Sn-Ag-Cu solder doping with a trace amount of Ni element has also been proposed by Yamashita et al., showing improved strength and wettability.[7] In addition, trace doping with Ge can also lead to better mechanical properties and less dross formation.[7,8] Chuang et al. further indicated that the e-Cu3Sn intermetallic layer and Kirkendall voids that formed during the aging of an immersion-Ag surface finished Sn3Ag0.5Cu solder ball grid array (BGA) package were obviously inhibited in Sn-3Ag-0.5Cu-0.06Ni-0.01Ge specimens.[9] In a study of the aging of Sn-3Ag-0.5Cu-0.06Ni-0.01Ge solder joints with ENIG (Ni/Au)-surface finishing, Chuang et al. also showed that both the growth thickness of the interfacial TUNG-HAN CHUANG, Professor, and CHAO-CHI JAIN, PostDoctoral Fellow, are with the Institute of Materials Science and Engineering, National Taiwan University, 106 Taipei, Taiwan, R.O.C. Contact e-mail: [email protected] Manuscript submitted January 6, 2009. Article published online October 30, 2010 684—VOLUME 42A, MARCH 2011
Ni3Sn4 intermetallic phase and the consumption of the Ni film on Cu pads decreased slightly than those in undoped Sn-3Ag-0.5Cu.[10] In addition, the coarsening of brittle AuSn4 intermetallic compounds in the Sn3Ag0.5Cu solder matrix has been prevented in Sn-3Ag-0.5Cu-0.06Ni-0.01Ge.[10] Furthermore, rare earth (RE) elements have also been reported to show advantageous effects on Sn-Ag-Cu solders. Chen et al. found that the addition of 0.2 wt pct mixed metal (Ce, La) into a Sn-3.8Ag-0.7Cu alloy resulted in refinement of the microstructure and precipitation of Ag3Sn particles in the solder matrix, through which a sevenfold increase of creep rupture life was attained.[11] In another study, Chen et al. showed that the creep stress exponents for a Sn-3.8Ag-0.7Cu solder doped with 0.1 wt pct mixed metal are 8.2 and 14.6 at low and high stresses, respectively, both of which are higher than those of undoped Sn-3.8Ag-0.7Cu.[12] The most exciting result was illustrated by Dudek et al., who found that the addition of 0.5 wt pct La into a Sn-3.8Ag-0.7Cu alloy leads to an increase of rupture strain up to 150 pct over that of undoped solder, which is desirable for application in portable el
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