The beneficial effect of Zn additions on the microstructure of SnCu and SnCuNi solder joints to Cu substrates
The commercial success as Pb-free solders of alloys based on a trace addition of Ni to Sn-0.7Cu and the academic recognition of the metallurgical effects that deliver the excellent performance in soldering processes and in service, means that Sn-Cu-Ni is now well established as one of the main alloy systems on which the electronics industry depends for the assembly of reliable circuitry. One important effect of the Ni addition is the stabilization of the hexagonal form of the Cu6Sn5 phase that forms at the interface between the solder and the Cu substrate. However, with the limited amount of Ni available in a solder joint, growth of the intermetallic layer by reaction with a copper substrate can result in the Ni level in the Cu6Sn5 falling below that required for stabilization. Increasing the Ni in the solder alloy beyond the level required for optimization of properties such as fluidity that are important in soldering processes can result in the appearance of undesirable high melting point Sn-Ni phases, It would therefore be useful if a second stabilizing element could be introduced to the system to support the effect of the Ni. Ni works as a stabilizer by partially substituting for Cu in the Cu6Sn5 so potential additives were considered on the basis of reports of their partially replacing either Cu or Ni in that crystal structure. Zn was selected for this study because it can partially substitute for Sn in the Cu6Sn5 crystal, potentially complementing the role of Ni. The effect of a Zn addition was studied in the basic Sn-0.7Cu and when there was also an addition of Ni. By itself a Zn addition did not promote a eutectic microstructure in the way that a Ni addition does so powerfully but alone or with Ni it substantially reduced undercooling and in the alloy with both Ni and Zn there was an overall refinement in the near completely eutectic microstructure. In combination with the Ni the Zn significantly refined the grain size of the Cu6Sn5 that formed at the solder/Cu interface. By itself the Zn did not suppress the formation of Cu3Sn at the interface between the Cu and the Cu6Sn5 in the way that Ni does but it does not interfere with that beneficial effect of the Ni. Diffraction studies with high energy synchrotron-generated X-rays confirm that as well as stabilizing the hexagonal form of the Cu6Sn5, Zn reduces the anisotropy of this phase and increases the randomness of the crystal orientation in the intermetallic layer. These effects should result in a stronger intermetallic layer that is less likely to degrade during thermal cycling. The combination of all of these effects mean that an addition of Zn to the widely used Sn-Cu-Ni lead free solders should result in an overall increase in reliability of the solder joint, particularly in harsh environments.
The effect of Bi on the microstructure, electrical, wettability and mechanical properties of Sn-0.7Cu-0.05Ni alloys for high strength soldering
Abstract This paper elucidated the effects of the Bi element (0 wt%, 0.5 wt% and 1.5 wt%) on the microstructure, electrical, wettability and mechanical properties of the Sn-0.7Cu-0.05Ni as a high strength solder. Besides using the conventional cross-sectioned microstructure image, the real-time synchrotron radiation imaging and synchrotron micro-X-ray fluorescence (XRF) technique was also used to investigate the microstructure, focusing on the in-situ growth behaviour of the primary (Cu,Ni)6Sn5 intermetallic and elemental distribution that had occurred in the Sn-0.7Cu-0.05Ni-1.5Bi. Other essential properties of solder material, such as wettability, electrical resistance, and shear strength, were also determined. The results showed that the addition of 1.5 wt% Bi refined the primary (Cu,Ni)6Sn5 intermetallics formation in the solder joint, where it grew earlier and faster relative to that in the Sn-0.7Cu-0.05Ni/Cu joint. Additionally, the addition of 1.5 wt% Bi resulted with a 3% reduction of its electrical resistance while increasing the wettability of the solder alloy. 1.5 wt% addition of the Bi element also found to have contributed to a significant increment of shear strength relative to that of the Sn-0.7Cu-0.05Ni. The results confirmed that the developed material is applicable as a potential high strength solder material in the context of advanced interconnecting applications.
The crystal structure of stilpnomelane. Part II. The full cell
Summary The structural formula of stilpnomelane averaged from 37 representative literature analyses and based on the determined structure is (Ca,Na,K)4(Ti0·1Al2·3Fe35·5Mn0·8Mg9·3)[Si63Al9](O,OH)216·nH2O. Ferrostilpnomelanes have up to 12 R 3+ cations in the octahedral sheet, ferristilpnomelanes commonly have between 24 and 30 R 3+ cations, rarely up to 37. The two groups can be distinguished by accurate X-ray powder diffraction, ferrostilpnomelanes having d 001 less and a greater than have ferristilpnomelanes. The unit cell is triclinic, a = b∼21·8 Å, γ = 120°, d 001∼12·2 Å, P . Hexagonal groups or ‘islands’ of 24 silicon-oxygen tetrahedra co-ordinate to the octahedral sheet, the centres of the ‘islands’ are displaced by 5a/12, 8b/12 on either side of the octahedral sheet as a result of warping of this sheet to permit articulation with the smaller tetrahedral sheet. The 24-tetrahedra ‘islands’ are linked laterally by 6-member rings of inverted tetrahedra, which in turn link at their apices to like rings connecting the next sheet of ‘islands’. Thus a single tetrahedral layer is 4 tetrahedra thick and has symmetry 6/m. The 6-member rings of tetrahedra act as hinges permitting up to 0·5 Å extension or contraction of the tetrahedral sheet to accommodate variations in the octahedral sheet dimensions with varying R 2+/R 3+ ratio.