Publications
162 results found
Shang H, Ma ZL, Belyakov SA, et al., 2017, Grain refinement of electronic solders: The potential of combining solute with nucleant particles, JOURNAL OF ALLOYS AND COMPOUNDS, Vol: 715, Pages: 471-485, ISSN: 0925-8388
Sn-Ag-Cu electronic solder joints typically solidify from a single nucleation event producing either a single βSn grain or twinned grains. With so few and variable βSn orientations, each joint is mechanically unique due to the anisotropy of tetragonal βSn. Here we explore the potential of increasing the number of βSn orientations in solder joints by promoting heterogeneous nucleation during solidification. It is shown that large (60 g) samples can be grain refined effectively by combining growth restricting solute with potent heterogeneous nucleant particles introduced by alloying additions of Zn, Ti, Co, Ir, Pd, or Pt. These mechanisms are discussed with reference to the grain refinement of structural casting alloys. In 500 μm solder balls, these grain refinement approaches were less effective and a relatively high cooling rate of 17 K/s combined with solute and nucleant particles were required to trigger multiple nucleation events. With this approach, up to 12 independent grains formed in 500 μm balls of Sn-3Ag-0.5Cu alloyed with Pt, Co, or Ti, compared with one independent grain in balls without nucleant particle additions.
Nogita K, Salleh MAAM, Zeng G, et al., 2017, Real time X-ray imaging of soldering processes at the SPring-8 synchrotron, International Conference on Electronics Packaging (ICEP), Publisher: IEEE, Pages: 144-147
Nogita K, Tran XQ, Yamamoto T, et al., 2017, Reply to ' Comments on "Evidence of the hydrogen release mechanism in bulk MgH2"', Scientific Reports, Vol: 7, ISSN: 2045-2322
In a comment on our Article “Evidence of the hydrogen release mechanism in bulk MgH2”, Surrey et al. assert that the MgH2 sample we studied was not MgH2 at any time but rather MgO; and that the transformation we observed was the formation of Kirkendall voids due to the outward diffusion of Mg. We address these issues in this reply.
Xian J, Belyakov S, Ollivier M, et al., 2017, Cu6Sn5 crystal growth mechanisms during solidification of electronic interconnections, Acta Materialia, Vol: 126, Pages: 540-551, ISSN: 1359-6454
The growth mechanisms of primary Cu6Sn5 are studied in Sn-Cu alloys and solder joints by combining EBSD, FIB-tomography and synchrotron radiography. With increasing cooling rate and Cu content, Cu6Sn5 crystals developed from faceted hexagonal rods to grooved rods, in-plane branched faceted crystals and, finally, to nonfaceted dendrites. This range of growth morphologies has been rationalised into a kinetic microstructure map. Cu6Sn5 hexagonal rods grew along [0001] bounded by View the MathML source facets and Cu6Sn5 dendrites branched along <405> in the View the MathML source planes. The faceted to nonfaceted transition indicates a kinetic interface roughening transition and a gradual change in mechanism from lateral growth governed by anisotropic attachment kinetics to continuous growth governed by diffusion and curvature. Finally, it is shown that the full range of Cu6Sn5 morphologies that grew for different composition and cooling rate combinations in bulk alloys can be engineered to grow in solder joints made with a single composition (Sn-0.7 wt%Cu/Cu) by altering the peak temperature and the cooling rate.
Ma Z, Gourlay C, 2017, Nucleation, grain orientations, and microstructure of Sn-3Ag-0.5Cu soldered on cobalt substrates, Journal of Alloys and Compounds, Vol: 706, Pages: 596-608, ISSN: 0925-8388
The potential of cobalt substrates to control the microstructure of 550 μm Sn-3Ag-0.5Cu ball grid array (BGA) joints is explored. It is shown that cobalt substrates give a small and reproducible nucleation undercooling for tin and prevent the formation of large primary Ag3Sn blades and Cu6Sn5 rods. βSn dendrites grew from the CoSn3 reaction layer and the βSn grains are shown to inherit their orientation from the CoSn3 layer due to heterogeneous nucleation with the following orientation relationship: (100)Sn||(100)CoSn3 with [001]Sn||[001]CoSn3. Changes in the dendrite and eutectic microstructure are shown to be caused by the reduced nucleation undercooling for βSn and the altered solidification path due to cobalt substrate dissolution.
Xian JW, Ma ZL, Belyakov SA, et al., 2017, Nucleation of tin on the Cu6Sn5 layer in electronic interconnections, Acta Materialia, Vol: 123, Pages: 404-415, ISSN: 1359-6454
A Cu6Sn5 layer is an integral part of many electronic interconnections. Here we show that, although primary Cu6Sn5 is not a potent nucleant for Sn, the Cu6Sn5 layer plays a key role in Sn nucleation and microstructure formation in solder joints. Combining thermal analysis, FIB-tomography and EBSD, we show that conical cavities are present between the scallops of the Cu6Sn5 layer that act as geometric nucleation sites for Sn, that Sn grows from the Cu6Sn5 layer, and that reproducible nucleation orientation relationships (ORs) exist between Cu6Sn5 and Sn. With these ORs, a near-random distribution of Sn orientations is predicted from joint to joint even for Cu6Sn5 layers with a strong [0001] fibre texture. It is shown that the nucleation undercooling is strongly affected by manipulation of the Cu6Sn5 layer shape, and that it is possible to prevent nucleation on the Cu6Sn5 layer by adding more potent nucleants.
Mohd Salleh MAA, Gourlay CM, Xian J, et al., 2017, In situ imaging of microstructure formation in electronic interconnections, Scientific Reports, Vol: 7, ISSN: 2045-2322
The development of microstructure during melting, reactive wetting and solidification of solder pastes on Cu-plated printed circuit boards has been studied by synchrotron radiography. Using Sn-3.0Ag-0.5Cu/Cu and Sn-0.7Cu/Cu as examples, we show that the interfacial Cu6Sn5 layer is present within 0.05 s of wetting, and explore the kinetics of flux void formation at the interface between the liquid and the Cu6Sn5 layer. Quantification of the nucleation locations and anisotropic growth kinetics of primary Cu6Sn5 crystals reveals a competition between the nucleation of Cu6Sn5 in the liquid versus growth of Cu6Sn5 from the existing Cu6Sn5 layer. Direct imaging confirms that the β-Sn nucleates at/near the Cu6Sn5 layer in Sn-3.0Ag-0.5Cu/Cu joints.
Zeng G, Xian JW, Gourlay CM, 2017, Growth of Al8Mn5 Intermetallic in AZ91, Symposium on Magnesium Technology / In-Situ Methods for Unraveling Structure-Property Relationships in Light Metals Symposium held during the 146th TMS Annual Meeting and Exhibition, Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 85-92, ISSN: 2367-1181
Zeng G, Nogita K, Belyakov S, et al., 2017, Real-Time Observation of AZ91 Solidification by Synchrotron Radiography, Symposium on Magnesium Technology / In-Situ Methods for Unraveling Structure-Property Relationships in Light Metals Symposium held during the 146th TMS Annual Meeting and Exhibition, Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 597-603, ISSN: 2367-1181
Belyakov SA, Xian JW, Sweatman K, et al., 2016, Influence of bismuth on the solidification of Sn-0.7Cu-0.05Ni-xBi/Cu joints, JOURNAL OF ALLOYS AND COMPOUNDS, Vol: 701, Pages: 321-334, ISSN: 0925-8388
The improvement of solder joint mechanical performance due to bismuth additions is of ongoing interest. This paper investigates the influence of 0–14 wt% Bi on microstructure formation in Sn-0.7 wt%Cu-0.05 wt%Ni-xBi solder balls on Cu substrates. It is shown that Bi additions reduce the Ni content of (Cu,Ni)6Sn5, increase the size of primary (Cu,Ni)6Sn5 rods and reduce the thickness of the Cu6Sn5 reaction layer, although these effects are subtle until the Bi content reaches ∼5 wt%Bi and Bi additions of ≤5 wt% did not significantly affect the positive effects of Ni. Higher Bi additions (≥8 wt% Bi) caused a transition of the tin growth texture from columnar grains with a <110> fibre texture towards a single grain with subgrains. Bi additions increased the non-equilibrium freezing range and caused a nonequilibrium ternary eutectic reaction in joints containing ≥2 wt% Bi. Bi contents less than 2 wt% are required to prevent (Bi) phase formation during solidification.
Kareh KM, O'Sullivan C, Nagira T, et al., 2016, Dilatancy in semi-solid steels at high solid fraction, Acta Materialia, Vol: 125, Pages: 187-195, ISSN: 1873-2453
The study of mushy-zonedeformation in steels is important forlimitingdefect formation in continuous casting. Here, we use in situsynchrotron radiography to quantify the shear deformation mechanisms of an equiaxedcarbon steel at asolid fraction >0.9 and to understand how these mechanisms lead to casting defects. We show that the grain assembly undergoes shear-induced dilation(Reynolds’dilatancy)which opens liquid-filled fissures andcracksat high solid fraction. We further show a complex interaction between grain rearrangement, grain deformation and local coarsening, where rearrangement reducesthe grain-grain contact area andcoordination number which alters the stress networkand also drives coarsening as grains move apart.
Ollivier M, Harker RM, Gourlay C, 2016, Reaction-induced surface reconstruction of silver in contact with zirconium, Journal of Alloys and Compounds, Vol: 691, Pages: 624-633, ISSN: 1873-4669
When two solid metals are in contact at high temperature, interdiusion occurs leading in some cases to the growth of intermetalliccompounds. The study of nucleation, growth and properties of these intermetallic compounds are of interest since it can be criticalfor many applications in industries. Yet, the eect of these reactions on the initial surfaces of both metals is not well understood andparticularly when surfaces are not perfectly flat and for short contact time. The purpose of the present study is to demonstrate thatthe growth of an intermetallic compound layer between to solid metals can lead to the surface reconstruction of one of them. Thesilver–zirconium system will be presented in order to illustrate this new phenomenon. The eect of contact point on the diusion-reaction process has been modelled by patterning the Zr surface. The nucleation and growth of the intermetallic compounds occuralong the contact points which leads to silver surface reconstruction with the growth of the preferential crystal planesf111gadf100g. A model explaining this new phenomenon is developed based on the minimisation of Gibbs energy and the diusion rates afboth Ag & Zr atoms in the binary system Ag/Zr.
Belyakov SA, Gourlay CM, 2016, Effect of Bi and In on microstructure formation in Sn-3Ag-3Bi-3In/Cu and /Ni solder joints, Key Engineering Materials, Vol: 700, Pages: 142-151, ISSN: 1013-9826
Sn-3Ag-3Bi-3In solder has been investigated to improve the understanding of microstructure formation in this solder during solidification and soldering to Cu and Ni substrates. The as-solidified microstructures of Sn-3Ag-3Bi-3In samples were found to consist of a significant fraction of βSn dendrites with a complex eutectic between the dendrites. In total five phases were observed to form during solidification: βSn, Ag3Sn, Bi, ζAg and a "Sn-In-Bi" ternary compound. Soldering of Sn-3Ag-3Bi-3In to substrates changed the phase equilibria in the system and caused the formation of additional phases: Cu6Sn5 during soldering to Cu and Ni3Sn4 and metastable NiSn4 during soldering to Ni. It is shown that metastable NiSn4 forms as a primary phase in a complex 5-component Sn-3Ag-3Bi-3In-Ni system. In and Bi were detected in solid solution in the βSn matrix in amounts of ∼1.5-2at% and ∼1.2at% respectively. Bi also existed as fine particles of two distinct types. (i): sub-micron (<500nm) coral-like particles and (ii) facetted particles measuring up to 7-8 μm.
Salleh MAAM, McDonald SD, Gourlay CM, et al., 2016, Suppression of Cu6Sn5 in TiO2 reinforced solder joints after multiple reflow cycles, Materials & Design, Vol: 108, Pages: 418-428, ISSN: 0261-3069
In the current generation of 3D electronic packaging, multiple reflows are often required during soldering. In addition, electronic packages may be subjected to additional solder rework or other heating processes. This paper investigates the effects of multiple reflow cycles on TiO2 reinforced Sn–0.7Cu solder fabricated by a powder metallurgy microwave sintering technique. Compared to TiO2-free equivalents, a relative suppression of the Cu6Sn5 phase, both as primary crystals and as an interfacial layer was observed. The likely mechanism relates to the TiO2 nanoparticles promoting nucleation and decreasing the amount of time that liquid is in contact with the interfacial layer. The TiO2 particles appear to stabilise the interfacial Cu6Sn5 layer and result in a more planar morphology. The suppression of Cu6Sn5 results in TiO2 reinforced solder joints having a higher shear strength after multiple reflow cycles compared to Sn–0.7Cu solder joints.
Belyakov SA, Nishimura T, Sweatman K, et al., 2016, Influence of Bi additions on the distinct βsn grain structure of Sn-0.7Cu-0.05Ni-xBi (x = 0-4wt%), ICEP 2016, Publisher: IEEE, Pages: 222-225
Effects of Bi additions to Ag-containing lead-free solders have been the focus of a considerable amount of past investigation. However, the influence of Bi on Sn-Cu-Ni solders has not been studied extensively. In the present study, we explore the influence of Bi on microstructure formation of Sn-0.7Cu-0.05Ni/Cu solder joints both in the bulk and at the interface. It is shown that (i) Sn-0.7Cu-0.05Ni solidifies to produce a markedly different grain structure to Ag-containing lead-free alloys, with 5-8 independent βSn grains in each joint; (ii) Bi additions to Sn-0.7Cu-0.05Ni maintain this distinct βSn grain structure and had no discernible effect on the (Cu,Ni)6Sn5 interfacial intermetallic layers or primary intermetallic crystals.
Gourlay CM, Ma ZL, Xian JW, et al., 2016, Solidification of Sn-3Ag-0.5Cu and sn-0.7Cu-0.05Ni solders, Materials Science Forum, Vol: 857, Pages: 44-48, ISSN: 0255-5476
The solidification of Sn-3Ag-0.5Cu and Sn-0.7Cu-0.05Ni are overviewed and compared. In joints on Cu substrates, both solders begin solidification with primary Cu6Sn5 growing in the bulk liquid prior to tin nucleation. In freestanding balls and joints, SAC305 generally solidifies with a single tin nucleation event and exhibits a mutually-twinned tin grain structure. In contrast, SN100C BGA balls and joints exhibit multiple independent tin grains that grow as a columnar array in joints.
Ollivier M, Harker RM, Chater RJ, et al., 2016, Thermal etching of silver: Influence of rolling defects, Materials Characterization, Vol: 118, Pages: 112-121, ISSN: 1044-5803
Silver is well known to be thermally etched in an oxygen-rich atmosphere and has been extensively studied in the laboratory tounderstand thermal etching and to limit its effect when this material is used as a catalyst. Yet, in many industrial applicationsthe surface of rolled silver sheets is used without particular surface preparation. Here, it is shown by combining FIB-tomography,FIB-SIMS and analytical SEM that the kinetics of thermal etch pitting are significantly faster on rolled Ag surfaces than on polishedsurfaces. This occurs due to range of interacting phenomena including (i) the reaction of subsurface carbon-contamination withdissolved oxygen to form pores that grow to intersect the surface, (ii) surface reconstruction around corrosion pits and surfacescratches, and (iii) sublimation at low pressure and high temperature. A method to identify subsurface pores is developed to showthat the pores have {111} and {100} internal facets and may be filled with a gas coming from the chemical reaction of oxygen andcarbon contamination.
Ma ZL, Belyakov SA, Gourlay CM, 2016, Effects of cobalt on the nucleation and grain refinement of Sn-3Ag-0.5Cu solders, Journal of Alloys and Compounds, Vol: 682, Pages: 326-337, ISSN: 1873-4669
We show that cobalt-microalloying causes significant grain refinement in large samples (60 g) of Sn-3Ag-0.5Cu solder. Nucleation occurs on the (100) facet of αCoSn3 crystals with a reproducible orientation relationship of (100)Sn ‖ (100)CoSn3 with [001]Sn ‖ [001]CoSn3, which has a planar lattice mismatch of ∼4%. In 550 μm solder joints on Cu substrates, Co microalloying alters the βSn nucleation location to an αCoSn3 particle in the bulk liquid and increases the number of βSn nucleation events when the cooling rate is higher than ∼5 K/s. However, Co-microalloying caused only weak grain refinement in sample volumes relevant to solder joints which is explained using nucleation and grain refinement theories.
Ng WCW, Sweatman K, Akaiwa T, et al., 2016, Dissolution in Service of the Copper Substrate of Solder Joints, 18th IEEE Electronics Packaging Technology Conference (EPTC), Publisher: IEEE, Pages: 388-393
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Ollivier M, Harker RM, Gourlay CM, 2015, Etch pitting and subsurface pore growth during the thermal etching of silver, Philosophical Magazine Letters, Vol: 95, Pages: 547-554, ISSN: 0950-0839
The thermal etching of silver was extensively studied in the middle of twentieth century, revealing the key role of oxygen on surface morphology changes including: grains striation, grain-boundary grooving and etch pitting. Here we probe the role of the subsurface and defects induced by rolling on the thermal etching of pure silver sheet. Nanometre- and micrometre-sized faceted pores have been observed in the subsurface region after heat treatments in air. The relationship between these subsurface pores and rolling defects is demonstrated. A mechanism for the formation of subsurface pores based on the precipitation/reaction of dissolved oxygen is suggested which can also explain the aligned etch pitting observed.
Salleh MAAM, McDonald SD, Gourlay CM, et al., 2015, Effect of Ni on the Formation and Growth of Primary Cu6Sn5 Intermetallics in Sn-0.7 wt.%Cu Solder Pastes on Cu Substrates During the Soldering Process, Journal of Electronic Materials, Vol: 45, Pages: 154-163, ISSN: 1543-186X
This paper investigates the effect of 0.05 wt.% Ni on the formation and growth of primary Cu6Sn5 in Sn-0.7 wt.%Cu solder paste soldered on a Cu substrate, using a real-time synchrotron imaging technique. It was found that small additions of Ni significantly alter the formation and growth of the primary Cu6Sn5 intermetallics, making them small. In contrast, without Ni, primary Cu6Sn5 intermetallics tend to continue growth throughout solidification and end up much larger and coarser. The primary effect of the Ni addition appears to be in promoting the nucleation of a larger amount of small Cu6Sn5. The results provide direct evidence of the sequence of events in the reaction of Ni-containing Sn-0.7 wt.%Cu solder paste with a Cu substrate, and in particular the formation and growth of the primary Cu6Sn5 intermetallic.
Xian JW, Belyakov SA, Gourlay CM, 2015, Controlling Bulk Cu6Sn5 Nucleation in Sn0.7Cu/Cu Joints with Al Micro-alloying, Journal of Electronic Materials, Vol: 45, Pages: 69-78, ISSN: 1543-186X
We show that dilute Al additions can control the size of primary Cu6Sn5 rodsin Sn-0.7Cu/Cu ball grid array joints. In Sn-0.7Cu-0.05Al/Cu joints, thenumber of primary Cu6Sn5 per mm2 is 7 times higher and the mean threedimensionallength of rods is 4 times smaller than in Al-free Sn-0.7Cu/Cujoints, while the area fraction of primary Cu6Sn5 is similar. It is shown thatepitaxial nucleation of primary Cu6Sn5 occurs on d-Cu33Al17 or c1-Cu9Al4particles, which are stable in the Sn-0.7Cu-0.05Al melt during holding at250C. The observed facet relationships agree well with previously determinedorientation relationships between d-Cu33Al17 and Cu6Sn5 in hypereutectic SnCu-Alalloys and result in a good lattice match with<2.5% lattice mismatchon two different interfacial planes.
Belyakov SA, Gourlay CM, 2015, The Influence of Cu on Metastable NiSn4 in Sn-3.5Ag-xCu/ENIG Joints, Journal of Electronic Materials, Vol: 45, Pages: 12-20, ISSN: 1543-186X
We have investigated the effect of small amounts of Cu on suppression ofmetastable bSn-NiSn4 eutectic growth in solder joints between Sn-3.5Ag-xCusolders and Ni-based substrates. For Sn-3.5Ag/electroless nickel immersiongold (ENIG) and Sn-3.5Ag/Ni solder joints we showed that the eutectic mixturecontains bSn, Ag3Sn, and metastable NiSn4. It was found that addition ofonly 0.005 wt.% Cu to Sn-3.5Ag-xCu/ENIG or Sn-3.5Ag-xCu/Ni joints promotedformation of a stable bSn-Ni3Sn4 eutectic and that both Ni3Sn4 andNiSn4 occur in the eutectic at this Cu level. We also showed that for completeprevention of formation of metastable NiSn4 during eutectic solidification ofthe solder joint, addition of at least 0.3 wt.% Cu was required.
Gourlay CM, Belyakov SA, Ma ZL, et al., 2015, Nucleation and growth of tin in Pb-free solder joints, JOM, Vol: 67, Pages: 2383-2393, ISSN: 1047-4838
The solidification of Pb-free solder joints is overviewed with a focus on the formation of the βSn grain structure and grain orientations. Three solders commonly used in electronics manufacturing, Sn-3Ag-0.5Cu, Sn-3.5Ag, and Sn-0.7Cu-0.05Ni, are used as case studies to demonstrate that (I) growth competition between primary dendrites and eutectic fronts during growth in undercooled melts is important in Pb-free solders and (II) a metastable eutectic containing NiSn4 forms in Sn-3.5Ag/Ni joints. Additionally, it is shown that the substrate (metallization) has a strong influence on the nucleation and growth of tin. We identify Co, Pd, and Pt substrates as having the potential to control solidification and microstructure formation. In the case of Pd and Pt substrates, βSn is shown to nucleate on the PtSn4 or PdSn4 intermetallic compound (IMC) reaction layer at relatively low undercooling of ~4 K, even for small solder ball diameters down to <200 μm.
Belyakov SA, Gourlay CM, 2015, NiSn<inf>4</inf> in solder joints between Sn-3.5Ag and Ni, ENIG or ENEPIG, Pages: 1273-1279, ISSN: 0569-5503
Sn-3.5Ag solder is commonly used with ENIG (electroless nickel, immersion gold) and ENEPIG (electroless nickel, electroless palladium, immersion gold) platings. Most research has focused on the interfacial reactions with the substrate and the microstructure development. In the present study we describe a new phenomenon that has been missed in past research: the formation of metastable eutectic βSn+Ag3Sn+NiSn4 instead of the equilibrium βSn+Ag3Sn+Ni3Sn4 eutectic during solder bulk solidification in Sn-3.5Ag/Ni, Sn-3.5Ag/ENIG and Sn-3.5Ag/ENEPIG joints. It is shown that metastable NiSn4 transforms during ageing at 150°C and that the transformation occurs in two stages (i) coarsening of NiSn4 eutectic sheets into tile-like crystals and (ii) decomposition of the coarsened NiSn4 crystals into Ni3Sn4 and βSn. It is further shown that Pd additions of ∼2at% can stabilize NiSn4, preventing decomposition into Ni3Sn4. However, Pd from the ENEPIG substrate also causes a relatively large volume fraction of NiSn4 to form which coarsens rapidly at 150°C.
Belyakov SA, Gourlay CM, 2015, Metastable eutectic in Pb-free joints between Sn-3.5Ag and Ni-based substrates, Materials Letters, Vol: 148, Pages: 91-95, ISSN: 1873-4979
Microstructure development in solder joints between Sn–3.5Ag and Ni-based substrates has been widelyreported. However, in the present study we illustrate a new phenomenon: that during soldering ofSn–3.5Ag to Ni or ENIG (electroless nickel, immersion gold), the bulk solder solidifies to contain ametastable eutectic consisting ofβSnþAg3SnþNiSn4instead of theβSnþAg3SnþNi3Sn4, expected ofequilibrium solidification. It is shown that metastable NiSn4coarsens and then decomposes into Ni3Sn4andβSn during aging at 150 and 2001C and that coarsened NiSn4particles deteriorate impact shearproperties.
Nogita K, Tran XQ, Yamamoto T, et al., 2015, Evidence of the hydrogen release mechanism in bulk MgH<sub>2</sub>, SCIENTIFIC REPORTS, Vol: 5, ISSN: 2045-2322
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Xian JW, Belyakov SA, Britton TB, et al., 2015, Heterogeneous nucleation of Cu<sub>6</sub>Sn<sub>5</sub> in Sn-Cu-Al solders, JOURNAL OF ALLOYS AND COMPOUNDS, Vol: 619, Pages: 345-355, ISSN: 0925-8388
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Belyakov SA, Gourlay CM, 2015, Metastable NiSn<inf>4</inf> in Sn-3.5Ag/ENIG joints
Sn-Ag solders are a popular Pb-free choice in power electronics. While significant research has focused on the formation and growth of an interfacial Ni3Sn4 intermetallic layer, little work has considered the Ni-based intermetallics that form in the bulk solder during soldering of Sn-Ag alloys to Ni-based substrates. In the present study we demonstrate an important phenomenon that have been missed in past research: The formation of metastable Sn-Ag3Sn- NiSn4 eutectic throughout the solder bulk when Sn-3.5Ag is soldered to ENIG (electroless nickel, immersion gold) or Ni surface finishes. It is shown that the metastable NiSn4 eutectic phase rapidly coarsens and transforms into Ni3Sn4 and βSn during ageing at 150°C. Primary NiSn4 crystals or coarsened NiSn4 eutectic particles can significantly compromise solder joint reliability as demonstrated by impact shear tests. In addition to its metastable nature, NiSn4 is a brittle intermetallic compound with a weak interface with the βSn matrix that serves as the path for crack propagation during shear tests. It is further shown that Cu additions to Sn- 3.5Ag solder promote equilibrium Sn-Ag3Sn-Ni3Sn4 eutectic growth.
Belyakov SA, Gourlay CM, 2015, The Influence of Alloying Elements on Metastable NiSn<sub>4</sub> in Sn-Ag Solders on Ni-containing Metallizations, 17th IEEE Electronics Packaging and Technology Conference (EPTC), Publisher: IEEE
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