Study of wetting reaction between eutectic AuSn and Au foil

Wetting reactions between eutectic AuSn solder and Au foil have been studied. During the reflow process, Au foil dissolution occurred at the interface of AuSn/Au, which increases with temperature and time. The activation energy for Au dissolution in molten AuSn solder is determined to be 41.7 kJ/mol. Au₅Sn is the dominant interfacial compound phase formed at the interface. The activation energy for the growth of interfacial Au₅Sn phase layer is obtained to be 54.3 kJ/mol over the temperature range 360–440°C. The best wettability of molten AuSn solder balls on Au foils occurred at 390°C (wetting angle is about 25°). Above 390°C, the higher solder oxidation rate retarded the wetting of the molten AuSn solder.     

Microstructure evolution of gold-tin eutectic solder on Cu and Ni substrates

The microstructures of the eutectic Au20Sn (wt.%) solder that developed on the Cu and Ni substrates were studied. The Sn/Au/Ni sandwich structure (2.5/3.75/2 μm) and the Sn/Au/Ni sandwich structure (1.83/2.74/5.8 μm) were deposited on Si wafers first. The overall composition of the Au and the Sn layers in these sandwich structures corresponded to the Au20Sn binary eutectic. The microstructures of the Au20Sn solder on the Cu and Ni substrates could be controlled by using different bonding conditions. When the bonding condition was 290°C for 2 min, the microstructure of Au20Sn/Cu and Au20Sn/Ni was a two-phase (Au₅Sn and AuSn) eutectic microstructure. When the bonding condition was 240°C for 2 min, the AuSn/Au₅Sn/Cu and AuSn/Au₅Sn/Ni diffusion couples were subjected to aging at 240°C. The thermal stability of Au20Sn/Ni was better than that of Au20Sn/Cu. Moreover, less Ni was consumed compared to that of Cu. This indicates that Ni is a more effective diffusion barrier material for the Au20Sn solder.     

Controlling the microstructure from the gold-tin reaction

The microstructures from the reaction between Au and Sn under different conditions were studied. A Sn/Au/Ni sandwich structure (2.5/3.752 μm) was deposited over the Si wafer. The overall composition of the Au and Sn layers corresponded to the Au20Sn binary eutectic (wt.%). When the reaction condition was 290°C for 2 min, the microstructure produced was a typical two-phase (Au₅Sn and AuSn) eutectic microstructure over Ni. In contrast, when the reaction condition was 240°C for 2 min, a AuSn/Au₅Sn/Ni layered microstructure was produced. In both microstructures, a small amount of Ni was dissolved in Au₅Sn and AuSn. When the AuSn/Au₅Sn/Ni layered structure was subjected to aging at 240°C, the AuSn layer gradually exchanged its position with the Au₅Sn layer and eventually formed an Au₅Sn/AuSn/Ni three-layer structure in less than 9 h. The driving force for Au₅Sn and AuSn to exchange their positions is for the AuSn phase to seek more Ni. The dominant diffusing species for the AuSn and Au₅Sn has also been identified to be Au and Sn, respectively.     

Fabrication and Microstructures of Sequentially Electroplated Sn-Rich Au-Sn Alloy Solders

Three Sn-rich, Au-Sn alloy solders with eutectic, hypoeutectic, and hypereutectic Sn compositions were fabricated by sequential electroplating of Au and Sn and then the dual-layer films were reflowed at 250°C. The microstructures and phase compositions of the deposited Au/Sn dual-layer film and the reflowed Sn-rich Au-Sn alloys were studied. Microhardness values of the different phases or phase zones for the reflowed alloys were also tested. Finally, two Si wafers were bonded together with the eutectic Sn-rich Au-Sn alloy solder. For as deposited Au/Sn dual-layer films, reaction between Au and Sn occurs at room temperature leading to the formation of Au₅Sn, AuSn, and AuSn₂ at the Au/Sn interface. After reflowing at 250°C, two phases remain, Sn and AuSn₄, with the morphology and phase distribution depending on the original solder composition. In the Sn-rich, eutectic Au-Sn alloy, AuSn₄ particles are distributed uniformly in the Sn matrix. In the Sn-rich hypoeutectic/hypereutectic Au-Sn alloys, the proeutectic phase, AuSn₄ (Vickers hardness, Hv 125) or Sn (Hv 14.2), is larger in size and is surrounded by the eutectic zone (Sn + AuSn₄) (Hv 16.1). In all cases, the TiW adhesion and barrier layer remains intact during annealing. After reflowing at 250°C under a pressure of 13 kPa, two Si wafers are joined by the Sn-rich eutectic Au-Sn alloy solder, without crack or void formation at the Si wafer/solder interface or within the solder.     

Achievement of homogeneous AuSn solder by pulsed laser-assisted deposition

The deposition of AuSn solder at the eutectic composition (80 wt.% Au, 20 wt.% Sn) on a wetted, chemically inert metallic barrier has been studied in relation to its use in optoelectronic packaging. The bonding structure, consisting of a W barrier, the top part of which is doped with Ni (or Ti) to provide wetting by molten AuSn, and the homogeneous 80–20 AuSn solder several micrometers thick, has been grown by the Pulsed Laser-assisted Deposition (PLD) technique on 2″ silicon wafers. The composition of the AuSn layer was controlled within better than 1 wt.% as probed by EDX across the wafer diameter. The molten solder exhibited good wetting properties on the W modified layer and the whole structure was found to be chemically stable against thermal cycling at 320°C for over 3 min. The use of molten AuSn targets makes the PLD technique a most competitive one for the achievement of high quality and reliable AuSn solder.     

Intermetallic compounds formed during the interfacial reactions between liquid In-49Sn solder and Ni substrates

The interfacial reactions between liquid In-49Sn solder and Ni substrates at temperatures ranging from 150°C to 450°C for 15 min to 240 min have been investigated. The intermetallic compounds formed at the In-49Sn/Ni interfaces are identified to be a ternary Ni₃₃In₂₀Sn₄₇ phase using electron-probe microanalysis (EPMA) and x-ray diffraction (XRD) analyses. These interfacial intermetallics grow with increasing reaction time by a diffusion-controlled mechanism. The activation energy calculated from the Arrhenius plot of reaction constants is 56.57 kJ/mol.     

Growth of intermetallic compounds in the Sn-9Zn/Cu joint

We have studied the microstructure of the Sn-9Zn/Cu joint in soldering at temperatures ranging from 230°C to 270°C to understand the growth of the mechanism of intermetallic compound (IMC) formation. At the interface between the Sn-9Zn solder and Cu, the results show a scallop-type ε-CuZn₄ and a layer-type γ-Cu₅Zn₈, which grow at the interface between the Sn-9Zn solder and Cu. The activation energy of scallop-type ε-CuZn₄ is 31 kJ/mol, and the growth is controlled by ripening. The activation energy of layer-type γ-Cu₅Zn₈ is 26 kJ/mol, and the growth is controlled by the diffusion of Cu and Zn. Furthermore, in the molten Sn-9Zn solder, the results show η-CuZn grains formed in the molten Sn-9Zn solder at 230°C. When the soldering temperature increases to 250°C and 270°C, the phase of IMCs is ε-CuZn₄.     

Interfacial microstructure evolution in Pb-free solder systems

The Sn-3.5Ag and Sn-3.0Ag-0.5Cu ball-grid-array solder balls bonded onto Ni/Au metallization exhibited different interfacial morphology after both wetting and solid-state reactions. In contrast to the eutectic-SnPb solder system, both Pb-free systems showed higher solder-ball shear strength after annealing. Reprecipitation of Au as (Au,Ni)Sn₄ at the interface, as shown in the eutectic-SnPb solder system, was not observed in both Pb-free solder systems. Instead, Ni₃Sn₄ and Cu-Sn-Ni-Au intermetallic compounds (IMCs) were found in the SnAg and SnAgCu systems, respectively. In the SnAgCu system, a thick, acicular-Cu-Sn-Ni IMC formed after wetting, but a faceted-Cu-Sn-Ni-Au phase was found with longer annealing. The growth of this interfacial phase in the Sn-3.0Ag-0.5Cu solder system was also slightly inhibited by the addition of Cu, with a formation energy of about 200 kJ/mol.     

Power electronic assemblies: Thermo-mechanical degradations of gold-tin solder for attaching devices

The eutectic Au₈₀Sn₂₀ solder alloy has been applied in semiconductor assemblies and other industries for years. Due to some superior physical properties, Au/Sn alloy gradually becomes one of the best materials for soldering in electronic devices and components packaging but the voids growth in AuSn solder joints is one of the many critical factors governing the solder joint reliability. Voids may degrade the mechanical robustness of the die attach and consequently affect the reliability and thermal conducting performance of the assembly. Severe thermal cycles [− 55 °C/+175 °C] have highlighted degradations in AuSn die attach solder. The inspection of as-prepared die-attachments by X-ray and SEM (observation of cross-section) shows that the initial voids sizes were increased and a propagation of transverse cracks inside the joint between voids has appeared after ageing, it was featured also the existence of the IMC typical scallop-shape morphology with the phase structure of (Ni, Au)₃Sn₂ on as-reflowed joints. In this paper, we evaluate the origin of these degradations and ways to address them.     

Thin-film reactions during diffusion soldering of Cu/Ti/Si and Au/Cu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with Sn interlayers

The multilayer thin-film systems of Cu/Ti/Si and Au/Cu/Al₂O₃ were diffusion-soldered at temperatures between 250°C and 400°C by inserting a Sn thin-film interlayer. Experimental results showed that a double layer of intermetallic compounds (IMCs) η-(Cu_0.99Au_0.01)₆Sn₅/δ-(Au_0.87Cu_0.13)Sn was formed at the interface. Kinetics analyses revealed that the growth of intermetallics was diffusion-controlled. The activation energies as calculated from Arrhenius plots of the growth rate constants for (Cu_0.99Au_0.01)₆Sn₅ and (Au_0.87Cu_0.13)Sn are 16.9 kJ/mol and 53.7 kJ/mol, respectively. Finally, a satisfactory tensile strength of 132 kg/cm² could be attained under the bonding condition of 300°C for 20 min.     

Cross-Interaction Between Au/Sn and Cu/Sn Interfacial Reactions

The cross-interaction between Sn/Cu and Sn/Au interfacial reactions in an Au/Sn/Cu sandwich structure was studied. Field-emission electron probe microanalysis (FE-EPMA) revealed that the Cu content in the three Au-Sn phases (AuSn, AuSn₂, and AuSn₄) was very low, less than 1 at.%. This means␣that Cu from the opposite Cu foil did not participate in the interfacial reaction at the Sn/Au interface. On the opposite Sn/Cu side, Au-substituted (Cu,Au)₆Sn₅ formed within the initial 1 min of reflow. With prolonged reflow, the Au content in the Au-substituted (Cu,Au)₆Sn₅ increased and it transformed into a Cu-substituted (Au,Cu)Sn phase with 25 at.% Cu after 1 min of reflow at 250°C. The x-ray diffraction (XRD) pattern confirmed the phase transformation of Au-substituted (Cu,Au)₆Sn₅ to Cu-substituted (Au,Cu)Sn phase. In addition, there was greater Au consumption in the Au/Sn/Cu sandwich joint structure than in the single Au/Sn reaction case, due to some of the Au participating in the opposite Sn/Cu interfacial reaction.     

Au−Ni−Sn intermetallic phase relationships in eutectic Pb−Sn solder formed on Ni/Au metallization

Recent work has shown that a Au−Ni−Sn ternary compound with a nominal composition of Au_0.5Ni_0.5Sn₄ redeposits and grows at the interface between eutectic Pb−Sn solder and Ni/Au metallization during aging at 150°C. The present work verifies the existence of the Au_0.5Ni_0.5Sn₄ phase by examining the Sn-rich corner of the Au−Ni−Sn ternary phase diagram. The reconfiguration mechanism of the AuSn₄ from the bulk solder is also discussed, with detailed observations of the Au_0.5Ni_0.5Sn₄ microstructure. The results show that the Ni solubility limit in the AuSn₄ phase is approximately 12 at.% at 150°C and thus, the Au_0.5Ni_0.5Sn₄ phase is a ternary AuSn₄-based compound with high Ni solubility. Due to the slight solubility and the fast diffusion of Au in the eutectic Pb−Sn at 150°C, the AuSn₄ intermetallics in the bulk solder can reconfigure to form a Au_xNi_1−xSn₄ compound at the interface where Ni is available. The Au_xNi_1−xSn₄ compound layer consists of nanocrystals arranged in a larger grainlike morphology. It appears that the inherent lattice strain of the Au_xNi_1−xSn₄ compound and the volume change due to its formation results in a nanocrystalline microstructure.     

Solid state intermetallic compound growth between copper and high temperature,tin-rich solders—part I: Experimental analysis

An experimental study was performed which examined the solid state growth kinetics of the interfacial intermetallic compound layers formed between copper and the high temperature, tin-rich solders 96.5Sn-3.5Ag (wt.%) and 95Sn-5Sb. These results were compared with baseline data from the 100Sn/copper system. Both the 96.5Sn-3.5Ag and 95Sn-5Sb solders exhibited the individual Cu₃Sn and Cu₆Sn₅ layers at the interface; the thickness of the Cu₃Sn layer being a function of the aging time and temperature. The total thickness of the intermetallic compound layer formed in the 96.5Sn-3.5Ag solder/copper couple showed a mixture of linear and √t dependencies at the lower temperatures of 70,100, and 135°C, and a t^0.42 dependence at 170 and 205°C. The combined apparent activation energy was 59 kJ/mol, the Arrhenius plot showed a knee between the low and high temperature data. The total layer thickness of the 95Sn-5Sb/copper system exhibited √t dependence at the three lower temperatures and t^0.42 growth kinetics at 170 and 205°C. The combined apparent activation energy was 61 kJ/mol.     

Reactive wetting between molten Sn-Bi and Ni substrate

Wetting and interfacial reactions occurred when molten Sn-Bi alloys were in contact with a Ni substrate. Wetting properties of Sn-Bi alloys on the Ni substrate and their interfacial reactions were examined, and the effects of interfacial reactions on wetting properties were discussed. Couples made of various molten Sn-Bi alloys and Ni substrates were reacted at 300°C. It was found that, when the Bi content was greater than 98 wt.%, the intermetallic phase formed at the interface was NiBi₃ phase. The Ni₃Sn₄ phase was found in the other Sn-Bi/Ni couples that had Bi contents varied from 92 wt.% to 97.5 wt.%. An isothermal section at 300°C for the ternary Sn-Bi-Ni system was proposed to illustrate the reaction paths of Sn-Bi/Ni couples. Wetting properties, including wetting time and wetting force of molten Sn-Bi alloys, were determined by using a wetting balance. Surface tensions at 300°C were calculated based on the experimental measurements. For molten Sn-Bi alloys with Bi contents varying from 92 wt.% to 99 wt.%, their surface tensions were all about 0.34 N/m. On the basis of theoretical analysis and experimental observations, the surface tensions of molten metals, determined by using the wetting balance, are not significantly affected if the interfacial reaction is not excessive. However, the wetting time determined by using the wetting balance was altered by the reaction that occurred and the compound formed at the interface.     

进口AuSn20焊料环特性及焊缝化合物分析

AuSn20焊料环是高可靠密封工艺中一种常用的密封材料,采用差示扫描量热法对进口AuSn20焊料环进行熔化和凝固温度的检测,探明其熔化温度为280℃,凝固温度为277℃,AuSn20焊料环纯度很高几乎无杂质.通过对进口和国产AuSn20焊料环的表面状态形貌进行对比,发现均为AuSn和Au5Sn的均匀分布状态,未见明显区...     

The Melting Characteristics and Interfacial Reactions of Sn-ball/Sn-3.0Ag-0.5Cu-paste/Cu Joints During Reflow Soldering

In this work, the melting characteristics and interfacial reactions of Sn-ball/Sn-3.0Ag-0.5Cu-paste/Cu (Sn/SAC305-paste/Cu) structure joints were studied using differential scanning calorimetry, in order to gain a deeper and broader understanding of the interfacial behavior and metallurgical combination among the substrate (under-bump metallization), solder ball and solder paste in a board-level ball grid array (BGA) assembly process, which is often seen as a mixed assembly using solder balls and solder pastes. Results show that at the SAC305 melting temperature of 217°C, neither the SAC305-paste nor the Sn-ball coalesce, while an interfacial reaction occurs between the SAC305-paste and Cu. A slight increase in reflow temperature (from 217°C to 218°C) results in the coalescence of the SAC305-paste with the Sn-ball. The Sn-ball exhibits premelting behavior at reflow temperatures below its melting temperature, and the premelting direction is from the bottom to the top of the Sn-ball. Remarkably, at 227°C, which is nearly 5°C lower than the melting point of pure Sn, the Sn-ball melts completely, resulting from two eutectic reactions, i.e., the reaction between Sn and Cu and that between Sn and Ag. Furthermore, a large amount of bulk Cu₆Sn₅ phase forms in the solder due to the quick dissolution of Cu substrate when the reflow temperature is increased to 245°C. In addition, the growth of the interfacial Cu₆Sn₅ layer at the SAC305-paste/Cu interface is controlled mainly by grain boundary diffusion, while the growth of the interfacial Cu₃Sn layer is controlled mainly by bulk diffusion.     

Growth Mechanism of a Ternary (Cu,Ni)<Subscript>6</Subscript>Sn<Subscript>5</Subscript> Compound at the Sn(Cu)/Ni(P) Interface

The growth mechanism of an interfacial (Cu,Ni)₆Sn₅ compound at the Sn(Cu) solder/Ni(P) interface under thermal aging has been studied in this work. The activation energy for the formation of the (Cu,Ni)₆Sn₅ compound for cases of Sn-3Cu/Ni(P), Sn-1.8Cu/Ni(P), and Sn-0.7Cu/Ni(P) was calculated to be 28.02 kJ/mol, 28.64 kJ/mol, and 29.97 kJ/mol, respectively. The obtained activation energy for the growth of the (Cu,Ni)₆Sn₅ compound layer was found to be close to the activation energy for Cu diffusion in Sn (33.02 kJ/mol). Therefore, the controlling step for formation of the ternary (Cu,Ni)₆Sn₅ layer could be Cu diffusion in the Sn(Cu) solder matrix.     

Alloyed tin-gold ohmic contacts to n-type indium phosphide

The formation of alloyd ohmic contacts on n-InP using sequentially deposited Sn plus Au films was investigated. The specific contact resistance for metallizations with a Sn content of 5 at. % was determined for annealing temperatures between 250 and 500°C. The minimum specific contact resistance, r_c = (1.8±0.9) × 10^?6 ohm-cm² occurred for a narrow range of annealing temperatures between 380 and 410°C on substrates with n = 3 × 10¹⁸/cm³. For annealing temperatures 350°C the contacts were non-Ohmic and above 420°C the resistance increased dramatically. Contact morphology and metallurgy were studied by optical and scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy and Rutherford backscattering. Films annealed above 320°C contained several phases, mainly Au₄In, AuSn and polycrystalline InP. The contacts annealed at temperatures above 410°C were composed predominantly of the single phase Au₃In₂.     

Investigation of interfacial reaction between Au-Sn solder and Kovar for hermetic sealing application

The microstructural evolution and interfacial reactions of Au/Sn/Au/Au/Ni/Kovar joint were investigated during aging at 180 and 250 °C for up to 1000 h. The Au/Sn combination formed a rapid diffusion system. Even in non-annealed joint, three phases such as AuSn, AuSn₂ and AuSn₄ were formed. After initial aging at 180 °C, the AuSn, AuSn₂, AuSn₄, Au and Sn phases, which were formed after plating, were fully transformed into ζ-phase and δ-phase, and (Ni, Au)₃Sn₂ intermetallic compound (IMC) layer was observed between the ζ-phase and Kovar. As a whole, the microstructure of the joint was stable during aging at 180 °C. On the other hand, the solid-state interfacial reaction was much faster at 250 °C than at 180 °C. During aging at 250 °C, the Ni layer on the Kovar reacted primarily with the δ-phase in the solder, resulting in the formation and growth of the (Au, Ni)Sn IMC layer at the interface. After aging for 48 h, the Fe-Co-Ni-Au-Sn phase was formed underneath the (Au, Ni)Sn IMC layer. Furthermore, cracks were observed inside the interfacial layers after complete consumption of the Ni layer. The study results clearly demonstrate the need for either a thicker Ni layer or an alternative surface finish on Ni, in order to ensure the high temperature reliability of the Au/Sn/Au/Au/Ni/Kovar joint above 250 °C.     

Inhibiting the formation of (Au1−xNix)Sn4 and reducing the consumption of Ni metallization in solder joints

The effects of adding a small amount of Cu into eutectic PbSn solder on the interfacial reaction between the solder and the Au/Ni/Cu metallization were studied. Solder balls of two different compositions, 37Pb-63Sn (wt.%) and 36.8Pb-62.7Sn-0.5Cu, were used. The Au layer (1 ± 0.2 μm) and Ni layer (7 ± 1 μm) in the Au/Ni/Cu metallization were deposited by electroplating. After reflow, the solder joints were aged at 160°C for times ranging from 0 h to 2,000 h. For solder joints without Cu added (37Pb-63Sn), a thick layer of (Au_1−xNi_x)Sn₄ was deposited over the Ni₃Sn₄ layer after the aging. This thick layer of (Au_1−xNi_x)Sn₄ can severely weaken the solder joints. However, the addition of 0.5wt.%Cu (36.8Pb-62.7Sn-0.5Cu) completely inhibited the deposition of the (Au_1−xNi_x)Sn₄ layer. Only a layer of (Cu_1-p-qAu_pNi_q)₆Sn₅ formed at the interface of the Cu-doped solder joints. Moreover, it was discovered that the formation of (Cu_1-p-qAu_pNi_q)₆Sn₅ significantly reduced the consumption rate of the Ni layer. This reduction in Ni consumption suggests that a thinner Ni layer can be used in Cu-doped solder joints. Rationalizations for these effects are presented in this paper.