Primary Cu6Sn5 Research Articles

Suppression of Cu<sub>3</sub>Sn with Ni in High Cu Containing Sn-Cu Solder Alloys

This research examines the relationship between Cu and Ni concentration and the formation of primary Cu3Sn in high Cu containing Sn-Cu solder alloys. Through thermal analysis and optical microscopy, it was determined that Ni additions still have a significant effect in minimising or eliminating Cu3Sn for Cu concentrations as high as 30wt%. In addition, it is clear that a relationship exists between Cu concentration and the effect of Ni addition and the volume fraction of Cu3Sn increases as the Cu content increases. It is likely that the Ni addition has a significant effect on the interdiffusional coefficients of the diffusing species of Cu3Sn and Cu6Sn5, slowing the growth of Cu3Sn and encouraging primary Cu6Sn5 nucleation.

Thermodynamic properties and primary phase structures of ternary Cu90−xSnxSb10 alloys investigated by differential scanning calorimetry and laser flash method

Ternary Cu–Sn–Sb alloys are a good candidate for solder materials without Pb element. Therefore, the knowledge of thermodynamic properties and phase transition characteristics of this alloy system is essential not only for the fundamental research but also for the industry applications. In this letter, we have measured the liquidus and solidus temperatures, enthalpy of fusion and thermal diffusivity for ternary Cu90−xSnxSb10 alloys by differential scanning calorimetry (DSC) and laser flash methods, respectively. Moreover, the typical growth morphologies of various intermetallic compounds are characterized. It is found that the primary Cu3Sn, Cu6Sn5, Cu4Sn, Cu3Sb and SnSb phases display faceted growth mode, and develop into long strips or large blocks. By contrast, the primary (Cu) phase forms by non-faceted way into very coarse dendrites.

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

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.

Controlling Bulk Cu6Sn5 Nucleation in Sn0.7Cu/Cu Joints with Al Micro-alloying

We show that dilute Al additions can control the size of primary Cu6Sn5 rods in Sn-0.7Cu/Cu ball grid array joints. In Sn-0.7Cu-0.05Al/Cu joints, the number of primary Cu6Sn5 per mm2 is ∼7 times higher and the mean three-dimensional length of rods is ∼4 times smaller than in Al-free Sn-0.7Cu/Cu joints, while the area fraction of primary Cu6Sn5 is similar. It is shown that epitaxial nucleation of primary Cu6Sn5 occurs on δ-Cu33Al17 or γ 1-Cu9Al4 particles, which are stable␣in the Sn-0.7Cu-0.05Al melt during holding at 250°C. The observed facet relationships agree well with previously determined orientation relationships between δ-Cu33Al17 and Cu6Sn5 in hypereutectic Sn-Cu-Al alloys and result in a good lattice match with <∼2.5% lattice mismatch on two different interfacial planes.

Directional Solidification of Sn-Ag-Cu Alloys

The alloys of Sn-Ag-Cu family (SAC) are candidates for lead-free replacement of Sn-Pb eutectic used in welding processes of electronic devices. These alloys, with eventual adding of Zn, In and Bi highly improves the morphological stability and the corrosion resistance. Close to the eutectic ternary composition, the solidification occurs with three distinct phases: Sn-rich dendritic primary phase, Ag3Sn and Cu6Sn5 intermetallic phases, which have limited solubility in the Sn phase, in such a way that some authors consider these alloys as compounds of Sn with the mentioned intermetallic phases. In this aspect, the directional solidification is a suitable tool to study the solidification under strictly controlled conditions. In this work, samples of Sn-Ag-Cu composition were grown to analyze the mechanisms involved in the solidification of these alloys. The typical microstructure agrees the above description, with a spacing that depends mainly on the advance interface velocity of the growth.

Growth behavior of Cu6Sn5 in Sn–6.5 Cu solders under DC considering trace Al: In situ observation

High resolution time-resolved X-ray imaging with synchrotron radiation has been used to in situ observe the growth behavior of Cu6Sn5 intermetallic compounds (IMCs) during solidification in Sn–6.5 Cu and Sn–6.5 Cu–0.2 Al (wt. %) solders under applied direct current (DC) field. The morphological evolution of Cu6Sn5 with I-like, X-like, Y-like and bird-like shapes is directly observed. It is shown that trace levels of Al have a marked effect on the solder microstructures and refining the size of the primary Cu6Sn5. The solidification pathway leading to the refinement is observed in real time using synchrotron microradiography. After adding the trace Al, I-like shapes bifurcate into X-like shapes. Furthermore, when DC field with 10 A/cm2 is applied, both the growth rate and the mean size of Cu6Sn5 are increased but decreased when 100 A/cm2 is applied. Meanwhile, the effect of thermodynamic potential barrier caused by DC field on the growth behavior of Cu6Sn5 is discussed.

Heterogeneous nucleation of Cu6Sn5 in Sn–Cu–Al solders

Cu6Sn5 is a common intermetallic in Pb-free soldering. We explore the influence of dilute aluminium additions on the heterogeneous nucleation and grain refinement of primary Cu6Sn5 in Sn–4Cu–xAl solders. For cooling rates relevant to soldering, it is found that 0.02 and 0.2wt%Al cause significant grain refinement of Cu6Sn5 with 0.2wt%Al increasing the number of Cu6Sn5 grains per unit area by a factor of ∼8. Grain refinement is shown to be due to heterogeneous nucleation of Cu6Sn5 on either delta-Cu33Al17 or gamma1-Cu9Al4, coupled with significant constitutional supercooling ahead of growing Cu6Sn5 crystals. Reproducible orientation relationships are measured between the Cu–Al intermetallics and Cu6Sn5, with a planar lattice mismatch of less than 2.5%. The role of potent nuclei and solutal growth restriction are discussed with reference to the grain refinement of structural casting alloys.

Microstructural development and mechanical properties of hypereutectic Sn–Cu solderalloys

In this study a transient directional solidification system was used, which permitted the assessment of microstructures along castings lengths of hypereutectic Sn–2.0wt% and 2.8wt% Cu alloys, for a wide spectrum of experimental tip growth rates (VL) and tip cooling rates (T•). The primary Cu6Sn5 IMC developed typical M-shaped or H-shaped morphologies. However, rod-like particles prevailed along the Sn-rich β matrix of both alloys evaluated. Smaller inter-branch spacings (λ) are shown to be associated with the alloy having higher Cu content, and the presence of Cu3Sn (ε) intermetallic compound (IMC) was detected by X-ray diffraction (XRD). Such IMC developed probably due to the combination of high cooling rates during solidification and incomplete peritectic reaction along the Cu-enriched regions of the casting, which provided less η than predicted under equilibrium conditions. Hall–Petch type relationships are proposed interrelating Vickers hardness (HV), ultimate tensile strength (σu) and elongation to fracture against λ. The two alloys examined showed roughly similar trends considering HV and σu, despite having a very different ductility behavior. However, the mechanical strength of each alloy is shown to be controlled by different microstructure features. The higher fraction of eutectic rod-like Cu6Sn5 intermetallics observed for the Sn–2.0wt% Cu alloy was shown to be associated with the corresponding higher experimental mechanical strength.

Real Time Synchrotron X-Ray Imaging for Nucleation and Growth of Cu&lt;sub&gt;6&lt;/sub&gt;Sn&lt;sub&gt;5&lt;/sub&gt; in Sn-7Cu-0.05Ni High Temperature Lead-Free Solder Alloys

This paper demonstrates how recent progress for real-time solidification observation at SPring-8 synchrotron has contributed to the development of Sn-7wt%Cu-0.05wt%Ni high temperature lead-free solder alloys. Lead-free solder alloys in the composition range Sn-0.7 to 7.6wt%Cu that consist of primary Cu6Sn5in a eutectic Sn-Cu6Sn5matrix have been proposed as solders for application at temperatures up to 400°C for the assembly high current semiconductors. It is shown that trace levels of Al have a marked effect on the solder microstructure and refine the size of the primary Cu6Sn5. The solidification pathway that leads to the refinement was observed in real-time using X-ray synchrotron observations.

Influence of Composition on the Morphology of Primary Cu6Sn5 in Sn-4Cu Alloys

Alloys from the composition range Sn-(0.7 wt.% to 7.6 wt.%)Cu consist of primary Cu6Sn5 surrounded by a eutectic Sn-Cu6Sn5 mixture. The primary Cu6Sn5 intermetallics commonly adopt a coarse elongated morphology, which is not optimal for the mechanical properties of the soldered joint. This paper investigates the effect of trace elemental additions on the size and morphology of the primary Cu6Sn5 in Sn-4 wt.%Cu alloy with and without Ni additions. Elements investigated include ppm additions of Al, Ag, Ge, and Pb. It is shown that Al has a marked effect on the solder microstructure and refines the size of the primary Cu6Sn5, even at very low addition levels, in both binary Sn-Cu alloys and those containing additional Ni. The effect of Al is confirmed using real-time x-radiographic synchrotron observations of solidification.

Formation of Primary Intermetallic Compounds in Sn-Ag-Cu Alloys

Sn-Ag-Cu alloys are considered one of the most favorable lead-free solder systems. In slowly-cooled eutectic Sn-Ag-Cu alloys, sometimes large primary Ag3Sn or Cu6Sn5 intermetallic compounds (IMCs) form. These IMCs may affect the mechanical properties of solders. However, explanations for the formation of these IMCs are still not clear. This study deals with interrupted tests in order to clarify the nucleation of IMCs in the liquid phase. In this study, Sn-4.41Ag-0.63Cu and Sn-3.30Ag-1.47Cu alloys were prepared. According to the thermodynamic calculation, Pandat, the equilibrium solidification paths are described as follows: Sn-4.41Ag-0.63Cu :L → primary Ag3Sn → binary eutectic (Ag3Sn +Sn) → ternary eutectic; Sn-3.30Ag-1.47Cu :L → primary Cu6Sn5 → binary eutectic (Cu6Sn5 + Sn)→ ternary eutectic. The actual solidification process was different from the estimation from the equilibrium phase diagram. In the case of Sn-4.41Ag-0.63Cu, only Ag3Sn grew as a primary phase in the liquid, while in the case of Sn-3.30Ag-1.47Cu, not only primary Cu6Sn5 but also pseudo-primary Ag3Sn grew in the liquid. Ag3Sn may nucleate easily in the liquid phase, but Cu6Sn5 would not nucleate in the liquid.

凝固中断実験による三元共晶点付近の Sn-Ag-Cu 系合金の凝固過程の解析

Near-ternary eutectic Sn-Ag-Cu alloys are leading candidates for Pb-free solders. These alloys have three solid phases: β-Sn, Ag3Sn and Cu6Sn5. In order to reveal the solidification process of the alloy, the technique for quenching samples during freezing was used to evaluate the freezing characteristics of near eutectic ternary Ag-Cu-Sn alloys. In the present study, Sn-3.5Ag-0.5Cu and Sn-3.5Ag-1.5Cu (mass%) samples were investigated. These samples were subjected to the thermal program to give a cooling rate of about 0.05°C/s. In the case of Sn-3.5Ag-0.5Cu alloy, the solidification structure almost corresponded with the estimation from equilibrium phase diagram, though the solidification process was different. In this study the Sn-Ag3Sn eutectic nucleated immediately after primary β-Sn solidified. On the other hand, eutectic Cu6Sn5 did not nucleate easily, though the temperature of the specimen indicated at 217°C (ternary eutectic temperature). In the case of Sn-3.5Ag-1.5Cu alloy, the solidification structure as well as solidification process were quite distinct from the estimation of equilibrium phase diagram. Primary Cu6Sn5 was the first phase formed in the liquid. This was followed by the nucleation and growth of psud-primary Ag3Sn. The recalescence was observed when β-Sn nucleated and grew as a halo. The ternary eutectic structure emerged after Sn-Ag3Sn binary eutectic solidification.

凝固中断実験による Sn-Cu 系合金の凝固過程の解析

Recently, lead-free solders have been used in electronic equipments. There are many reports on mechanical properties of lead free solders. It is important to understand the evolution of solidified structure, though, such reports are few. In order to understand the solidification process, the Sn-Cu alloys, which are one of the candidate alloys for lead-free solders, have been used in this study. Three alloys, hypo-eutectic, eutectic, and hyper-eutectic, have been prepared. The specimen was quenched during solidification. Then the solidified structures of interrupted specimens were compared to thermal histories. In the case of hypo-eutectic and hyper-eutectic alloys, it was found that the undercooling of approximately 10 degrees was necessary for nucleation of β-Sn as a primary dendrite or as a halo. On the other hand, the undercooling was not necessary for crystallization of primary Cu6Sn5. The latter indicates that the primary Cu6Sn5 is easy to nucleate and grow even it is a facet. Primary Cu6Sn5 may be a starting site of a halo, but it doesn't facilitate nucleation of β-Sn. In the case of the hypo-eutectic alloy, a small undercooling for nucleation of eutectic structure was observed. Such undercooling was not observed in eutectic and hyper-eutectic alloys. This may be interpreted by the difference in the liquid composition when the eutectic solidification starts.

レーザー顕微鏡によるSn-Cu系合金の凝固現象の解析

The Sn-Cu Alloy is one of the candidate alloys for lead-free solder. It is important to understand the solidified structure of this alloy, but such reports are few. In order to make in-situ observation during solidification of this alloy, a laser confocal microscope has been used. Three alloys, hypo-eutectic, eutectic and hyper-eutectic alloys have been prepared. In the case of hypo-eutectic alloy, primary β-Sn dendrite and interdendritic eutectic were observed. A large recalescence was observed when the primary β-Sn nucleated in thermal histories. In the case of eutectic alloy, only eutectic structure was observed by the laser microscope. However, primary β-Sn dendrites were observed on the cross section. There was a large recalescence during solidification. This corresponded to the nucleation of β-Sn. In the case of hyper-eutectic alloy, fibrous primary Cu6Sn5 grew and then β-Sn dendrite grew as halo. This followed by the formation of eutectic. It was observed that a large recalescence after growth of primary Cu6Sn5 precedes the nucleation of β-Sn as halo.

Formation of bulk Cu6Sn5 intermetallic compounds in Sn–Cu lead-free solders during solidification

The influence of cooling rate and Cu content on the microstructures of the solidified Sn–Cu alloys was investigated in terms of microstructural observation, thermal analysis and theoretical calculation. It was found that bulk Cu6Sn5 intermetalic compounds (IMCs) formed only in the Sn–1.0Cu alloy with lower cooling rate. At higher cooling rate, the actual eutectic point of Sn–Cu solder alloy shifts to the direction of higher Cu concentration. Thus the eutectic and hypereutectic alloys experience a metastable pseudoeutectic solidification route instead. The volume fraction of the bulk Cu6Sn5 IMCs formed in the Sn–1.0Cu alloy, measured by thermal analysis, is larger than that predicted by the equilibrium phase diagram. This could be attributed to eutectic Cu6Sn5 phases, which cling to the primary Cu6Sn5 crystal during the eutectic reaction due to their matching crystalline orientation relationship.

一方向凝固によるSn-Cu二元共晶系合金のカップルドゾーンの解明

The Sn-Cu binary eutectic alloy is one of candidate alloys for lead-free solder. In order to investigate the evolution of solidified structure of Sn-Cu binary eutectic alloy, the unidirectional solidification technique has been applied using a Bridgman type furnace. Primary β-Sn exhibits dendrite morphology, on the other hand, a primary Cu6Sn5 indicates a faceted crystal, the cross section of which is typically H-shaped and eutectic structure is rod-like. Coupled growth zone of this alloy system is found to be extended to Cu-rich side and to be rather symmetric. This may be due to the difference in undercooling for growth of Cu6Sn5, β-Sn and eutectic phases. Jackson parameter of Cu6Sn5 is deduced to be high comparing with β-Sn.

Impact of printed wiring board coatings on the reliability of lead-free chip-scale package interconnections

When lead-free solder alloys mix with lead-free component and board metallizations during reflow soldering, the solder interconnections become multicomponent alloy systems whose microstructures cannot be predicted on the basis of the SnPb metallurgy. To better understand the influences of these microstructures on the reliability of lead-free electronics assemblies, SnAgCu-bumped components were reflow-soldered with near-eutectic SnAgCu solder pastes on Ni(P)|m.Au- and organic solderability preservative (OSP)-coated printed wiring boards and tested under cyclic thermal shock loading conditions. The reliability performance under thermomechanical loading was found to be controlled by the kinetics of recrystallization. Because ductile fracturing of the as-soldered tin-rich colonies would require a great amount of plastic work, the formation of continuous network of grain boundaries by recrystallization is needed for cracks to nucleate and propagate intergranularly through the solder interconnections. Detailed microstructural observations revealed that cracks nucleate and grow along the grain boundaries especially between the recrystallized part and the non-recrystallized part of the interconnections. The thermal cycling test data were analyzed statistically by combining the Weibull statistics and the analysis of variance. The interconnections on Ni(P)|m.Au were found out to be more reliable than those on Cu|m.OSP. This is due to the extensive dissolution of Cu conductor, in the case of the Cu|m.OSP assemblies, into molten solder that makes the microstructure to differ noticeably from that of the Ni(P)|m.Au interconnections. Because of large primary Cu6Sn5 particles, the Cu-enriched interconnections enhance the onset of recrystallization, and cracking of the interconnections is therefore faster. The solder paste composition had no statistically significant effect on the reliability performance.

Investigation of vibration fracture behavior of Sn–Ag–Cu solders under resonance

This study investigated the vibration fracture properties of Sn–Ag–Cu alloys with various Cu contents. Results show that the microstructure becomes finer with a higher Cu content. This leads to a lower damping capacity, higher deflection amplitude, and thus inferior vibration fracture resistance under a constant vibration force. It is of interest that when the Cu content reaches 1.5 wt%, the specimen possesses the highest damping capacity and greatest vibration life. The presence of massive primary Cu6Sn5 intermetallics probably accounts for this phenomenon.

Microstructure of a lead-free composite solder produced by an in-situ process

A composite solder, which consisted of solder matrix and intermetallic reinforcements, was manufactured by an innovative method. The cast ingot of the Sn-6.9Cu-2.9Ag alloy has primary Cu6Sn5 intermetallics in dendritic form. After rolling, the intermetallic dendrites are crushed into fine particles and distributed uniformly in the solder matrix. The crushed intermetallic particles do not melt, and they retain their shape during spheroidizing and reflow because of their high melting temperature. Coarsening and gravitational segregation of the particles was observed during reflow and analyzed theoretically.