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Tin Pest: a Forgotten Issue in the Field of Applied Superconductivity?

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Tin Pest: a Forgotten Issue in the Field of Applied Superconductivity? correspondence Tin Pest: A Forgotten Issue in the Field of Applied Superconductivity? To the Editor — In the framework of Averaged results obtained from solders operating below 286 K. But the hybrid magnet project aiming to shear tests performed on the twelve even with a high concentration of lead produce 43 T in a 34 mm warm aperture dedicated samples are summarized such as in Sn63Pb37, tin pest can also at LNCMI-Grenoble, a novel type of in Fig. 1. A clear difference of the appear on long term8 restricting the superconducting conductor is being thermo-mechanical behavior between choice to tin-free solder alloys for high developed in collaboration with CEA- Sn96Ag4 and Sn60Pb40 samples tested reliability interconnects working at Saclay. It is based on the soft soldering at 300 K and 77 K can be observed. The cryogenic temperature. of a Nb-Ti flat Rutherford cable on a rupture shear strength of Sn96Ag4 Even if it is still unproven but not Cu-Ag hollow stabilizer that will be soldered samples decreases in average impossible that tin pest has destroyed cooled by superfluid helium i.e. a by 12.9 MPa at low temperature (-37 %) the buttons of uniforms of Napoleon’s Rutherford Cable on Conduit Conductor whereas for Sn60Pb40 ones an average army during its disastrous retreat from (RCOCC)1. During the study and the increase of 8.5 MPa is observed (+28 Russia in 1812, this problem is real and qualification of the soft soldering %). It can be added that the rupture known for a few hundreds of years. It process, it has been systematically shear force measured at 77 K for may have become a forgotten issue but observed that the rupture shear strength Sn96Ag4 soldered samples is in experiences currently a strong revival of of the joint of RCOCC samples agreement with results recently reported interest due to the increased prepared with the Sn96Ag4 solder alloy in a CERN internal note3. environmental and health concerns is (i) in the range 20 to 35 MPa at room regarding the toxicity of lead and the temperature depending on the bond possible alternative offered by tin based quality achieved in agreement with solders. published values 2 and, (ii) much lower In conclusion, more investigations at 77 K, i.e. with a reduction between 30 are required to definitely prove the role to 50 % with respect to room of tin pest in the weakening of the temperature values, a somehow opposite mechanical toughness of Sn-Ag behavior to the one usually known and soldered joints. As an immediate observed for “normal” materials. consequence it is strongly recommended To investigate the thermo- to avoid using Sn-Ag binary soldering mechanical behavior of Sn96Ag4 alloys for joints that will be used at soldered joints, shear tests have been cryogenic temperatures, especially if performed in dedicated samples they are not mechanically re-enforced or resulting from the assembly of two if they are submitted to mechanical copper (Cu) slabs of 99.9 % purity and loading. ❐ 3 dimensions of 155x15x2 mm . Twelve Figure 1│ Averaged rupture shear test samples have been prepared by strength measured on soldered samples. References joining Cu slabs with a lap joint of 1 P. Pugnat, et al. to appear in IEEE Trans. Appl. The origin of the difference between Supercond. (2012). 10 ± 2 mm length and 15 ± 0.1 mm 2 width. Six of them have been made with the thermo-mechanical behavior of both H. Wu, et al. IEEE Trans. Appl. Supercond. 21 1738– soldering alloys lies most probably in 1741 (2011). 0.1 mm thick ribbons of Sn96Ag4 3 S. Heck, et al. CERN-ATS-Note-2011-074, unpublished solder alloy covering the contact area the tin pest, namely the result of the (2011); phase transformation of tin appearing in http://cdsweb.cern.ch/record/1378836/files/ATS_Note_20 and the six other ones with the same 4,5,6,7 quantity of Sn60Pb40. In both cases, Sn96Ag4 below 286 K . This 11_074_ChrScheurlein.pdf 4 L.D. Brownlee, Nature 166(4220), 482 (1950). copper surfaces have been carefully allotropic change of the “normal” white 5 β J.H. Becker, J. Appl. Phys. 29(7), 1110 – 1121 (1956). cleaned with the flux F-SW 21/3.1.1 -tin (tetragonal structure, metallic 6 Y. Kariya, C. Gagg, and W.J. Plumbridge, Soldering (Norm DIN 8511/EN 29454). Samples compound) into gray α-tin (diamond and Surface Mount Technology, 13(1) 39–40 (2000); prepared with the Sn96Ag4 soldering like cubic structure, semiconductor http://www.smartgroup.org/pdf/tinpest.pdf compound) is accompanied by a 26 % 7 Y. Kariya, N. Williams, C. Gagg, W. Plumbridge, alloy (melting point Solidus/Liquidus 5 Journal of the Minerals, Metals and Materials Society increase in volume implying serious = 221/221°C) have been heated up to 53(6) 39–41 (2001); http://iweb.tms.org/PbF/JOM-0106- 226°C whereas for Sn60Pb40 ones problems when rupture shear strength 39.pdf (melting point Solidus/Liquidus = or lifetime of a soldered joint is 8 W.J. Plumbridge, Soldering & Surface Mount considered. Indeed this transformation Technology, 22(1) 56–57 (2010). 183/191°C), the temperature for the soldering has been fixed to 200°C. favors the apparition of cracks that R. Pfister, and P. Pugnat* Mechanical measurements have been weaken the mechanical toughness of Lab. National des Champs Magnétiques Intenses performed using a conventional tensile Sn96Ag4. In Sn60Pb40, lead is known (LNCMI), CNRS-UJF-UPS-INSA, BP166, 38042 to stabilize the β-tin phase like Grenoble Cedex 9, FRANCE testing machine equipped with a liquid 6,7 nitrogen reservoir for the tests at 77 K. antinomy (Sb) and bismuth (Bi) *e-mail: [email protected] making Sn-Sb and Sn-Bi based alloys possible alternatives for lead-free .
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