Size Effect of Rare-Earth Intermetallics in Sn-9Zn-0.5Ce and Sn-3Ag-0.5Cu-0.5Ce Solders on the Growth of Tin Whiskers
- PDF / 606,505 Bytes
- 5 Pages / 593.972 x 792 pts Page_size
- 17 Downloads / 144 Views
TRODUCTION
ALLOYING with rare earth (RE) elements in Pb-free solders has been reported to have beneficial effects on their physical and mechanical properties. For the eutectic Sn-9Zn solder, Wu et al. found that the addition of 0.05 to 0.1 wt pct mixed metal (Ce, La) causes refinement of b-Sn grains and an increase in tensile strength.[1] In addition, the wettability and bonding strength of Sn-9Zn-RE solder have been improved due to the decrease of the interfacial energy between solder and flux, thereby facilitating the interfacial reaction.[2] Chen et al. reported that RE elements induced the refinement of the microstructure and the formation of needlelike Ag3Sn precipitates in a Sn-3.8Ag-0.7Cu with 0.2 wt pct mixed metal addition, through which a sevenfold increase of creep rupture life was attained.[3] In another study on the constructive relations of creep behaviors, Chen et al. indicated that the creep stress exponents for Sn-3.8Ag-0.7Cu solder joints were increased through the addition of 0.1 wt pct mixed metal.[4] Summarizing these experimental results, Wu et al. suggested in a review article that RE elements would play an important role in the improvement of electronic interconnections.[5] Recently, Dudek et al. have also presented evidence that the strain of failure of a Sn-3.8Ag-0.7Cu-0.5La solder is increased by nearly 150 pct over that of undoped Sn-3.8Ag-0.7Cu alloy.[6] From their results, they concluded that such novel RE-containing Pb-free solders could provide better reliability for portable electronic devices. T.H. CHUANG, Professor, and H.J. LIN, Doctoral Candidate, are with the Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted January 3, 2007. Article published online September 27, 2008 2862—VOLUME 39A, DECEMBER 2008
Unfortunately, as we observed by accident, long fiber-shape whiskers grow very rapidly in a Sn-3Ag0.5Cu-0.5Ce solder ball grid array package after air storage at room temperature for a short time.[7,8] Further study on a Sn-3Ag-0.5Cu-1.0Ce package revealed that coarse hillock-shaped whiskers appeared on the surface of solder joints stored at 150 C within a few minutes.[9] According to our observations, the tin whiskers formed on the surface of CeSn3 intermetallics in these solders. It has been proposed that the whisker growth in this case is attributed to the oxidation of CeSn3 intermetallics, which releases the tin atoms as the source of these whiskers. The compressive stress induced by the diffusion of oxygen atoms into the CeSn3 lattice squeezes the tin whiskers out of the oxidized CeSn3 intermetallics. In the present study, we have found that the size of CeSn3 and (Ce,Zn)Sn3 phases formed in Sn-3Ag-0.5Cu0.5Ce and Sn-9Zn-0.5Ce solders, respectively, can influence their whisker growth. The results provide us a clue for the inhibition of tin whiskers in RE elementcontaining solders.
II.
EXPERIMENTAL
For the experiments, Sn-9Zn-0.5Ce and Sn-3Ag0.5Cu-0.5Ce solders were prepared by m
Data Loading...
