Interfacial Intermetallic Compound Growth Research Articles

Formation and growth of interfacial intermetallics in Sn-0.3Ag-0.7Cu-xCeO2/Cu solder joints during the reflow process

The effects of adding CeO2 nanoparticles on the formation and growth of interfacial intermetallic compounds (IMCs) between Sn-0.3Ag-0.7Cu-xCeO2 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3 wt%) (SAC0307-xCeO2) solders and Cu substrates during reflow soldering were investigated. Scanning electron microscopy (SEM) was used to observe the microstructural evolution of the solder joints and measure the thickness and grain size of the intermetallic layers. The results show that the thicknesses and grain sizes of the IMCs decrease with CeO2 nanoparticle addition to the SAC0307 solder and that these parameters decrease significantly at a particle concentration of approximately 0.15 wt%. At higher concentrations, the thicknesses and grain sizes of the IMCs increase slightly. The growth exponents for the IMC layers and grains are determined by curve fitting to study the growth kinetics of the IMCs during reflow soldering. The results reveal that the growth exponents of the IMC layers range from 0.335 to 0.374 and those of the IMC grains range from 0.302 to 0.318, thus suggesting that the growth of the IMCs is controlled by the combined kinetic processes of atomic interdiffusion, interfacial reactions, and grain ripening. These data also show that among the tested solders, the SAC0307–0.15CeO2 solder joint has the smallest growth rate and exhibits the most significant effects in IMC growth suppression and grain size refinement. Based on the theories of heterogeneous nucleation and adsorption, a mechanism for reducing the IMC growth rate by the addition of CeO2 nanoparticles to the SAC0307 solder is proposed.

Influence of Interfacial Intermetallic Growth on the Mechanical Properties of Sn-37Pb Solder Joints under Extreme Temperature Thermal Shock

Solder joints in thermally uncontrolled microelectronic assemblies have to be exposed to extreme temperature environments during deep space exploration. In this study, extreme temperature thermal shock test from −196 °C to 150 °C was performed on quad flat package (QFP) assembled with Sn-37Pb solder joints to investigate the evolution and growth behavior of interfacial intermetallic compounds (IMCs) and their effect on the pull strength and fracture behavior of Sn-37Pb solder joints under extreme temperature environment. Both the scallop-type (Cu, Ni)6Sn5 IMCs at the Cu lead side and the needle-type (Ni, Cu)3Sn4 IMCs at the Ni-P layer side changed to plane-type IMCs during extreme temperature thermal shock. A thin layer of Cu3Sn IMCs was formed between the Cu lead and (Cu, Ni)6Sn5 IMC layer after 150 cycles. The growth of the interfacial IMCs at the lead side and the Ni-P layer side was dominated by bulk diffusion and grain-boundary diffusion, respectively. The pull strength was reduced about 31.54% after 300 cycles. With increasing thermal shock cycles, the fracture mechanism changed from ductile fracture to mixed ductile–brittle fracture, which can be attributed to the thickening of the interfacial IMCs, and the stress concentration near the interface caused by interfacial IMC growth.

Interfacial reaction and IMC growth between Sn-37 Pb and heterogeneous dual-phase substrate

This paper described microstructure evolution of interfacial intermetallic compound (IMC) layers in a series of Sn-37Pb/Cu-Co systems (the different amount of Co additions was a variable in this experiment, and we represented it with Cu-xCo in the rest of this article). With the same aging temperature (150 °C), the addition of Co significantly influenced the interfacial reaction between Sn-37 Pb solder and Cu-Co substrate. Through analyses of these changes, we found that Co addition was an important strategy to control IMC thickness and improve the interfacial wettability. Additionally, correlations between the thickness of IMC layers and aging time were established, and successfully fitted into linear functions, and this relationship corresponded to the characteristics of growth mechanism controlled by volume diffusion. Unfortunately, Kirkendall voids and microcracks were still inevitable in the current study, which could severely degrade the performance of solder joint, so further methods to improve these defects need to be explored.

Influence of diffusion brazing parameters on microstructure and properties of Cu/Al joints

The purpose of this article was to produce a sound Cu/Al joint by controlling interfacial inter-metallic compounds (IMCs) using suitable diffusion brazing parameter. An Al-8Si-4Cu-2Mg-1Ga-0.05Ce interlayer was used to diffusion braze copper to aluminum at a temperature above eutectic temperature and below melting point of aluminum under medium brazing pressure. The brazing pressure excludes eutectic structure and leaves a thin Cu9Al4/CuAl2 interfacial reaction layer in the joint. A short brazing time is enough to form a thin IMCs layer. However, increasing brazing time leads the rapid growth of CuAl2 and then results in some voids in the joint, which decreases the joint properties. It is not enough for a low brazing temperature to achieve a metallurgical bonding due to lack diffusion of atom. But, high brazing temperature results in excessive growth of IMCs, which is detriment to the bonding strength. The tensile strength and the bend ductility of Cu/Al joint diffusion brazed at 600 °C for 2 s under 9 MPa are equal to that of aluminum base metal and its conductivity is superior to that of aluminum base metal. Diffusion brazing with liquefaction in air can produce a sound Cu/Al joint both in mechanical and electrical properties.

Interfacial IMC growth of SAC305/Cu joint with a novel dual-layer of Ni(P)/Cu plating during solid-state aging

A dual-layer barrier of electroless-plated Ni(P)/electroplated Cu was prepared as under-bump metallization (UBM) joined with Sn-3.0Ag-0.5Cu (SAC305) lead-free solder. The interfacial reaction and growth mechanisms of the intermetallic compounds (IMCs) formed between the SAC305 solder and the dual-layer UBM during reflow and isothermal aging at different temperatures were systematically investigated. It was found that only a single (Cu,Ni)6Sn5 IMC layer was observed between the SAC305 solder and electroless Ni(P) layer. At low aging temperature (150 °C), there was a very slow IMCs growth due to the protection of thin (Cu,Ni)6Sn5 layer, and the surface morphologies of interfacial (Cu,Ni)6Sn5 grains exhibited hexagonal-type rod-like. While during aging process of 180 °C temperature, most IMC morphologies of (Cu,Ni)6Sn5 grains exhibited scallop-like and only a small part of IMCs exhibited hexagonal-like. And these prism hexagonal-like IMCs gradually transformed into scallop-like morphologies with increasing aging time. When the aging temperature reached up to 210 °C, the IMCs had the fastest grow rate and almost all IMC morphologies of (Cu,Ni)6Sn5 grains exhibited scallop-like in every stage of reactions. Compared with the interfacial IMCs layer evolutions of the Sn-based solders with bare Cu or Ni(P)/Cu UBMs, the interfacial IMCs layer between the dual-layer of Ni(P)/Cu plating and solder possessed better thermal stability, indicating that the dual-layer of electroless-plated Ni(P)/electroplated Cu was a excellent diffusion barrier in the microelectronics packaging.

Kinetic study of solid-state interfacial reactions of p-type (Bi,Sb)2Te3 thermoelectric materials with Sn and Sn–Ag–Cu solders

P-type (Bi,Sb)2Te3 material system is extensively used in commercial thermoelectric (TE) applications. Soldering is commonly applied to fabricate bulk TE elements into TE modules. This study carefully examined the solid-state interfacial reactions of the commercial (Bi,Sb)2Te3 substrate with Sn and Sn–3.0Ag–0.5Cu solders. The joining plane was along the c-axis of the highly oriented [1 1 0] TE substrate. For the reactions with pure Sn solder, both the intermetallic compounds (IMCs) of SnTe and SnSb were rapidly formed at the interface. Most importantly, it was clearly demonstrated that both of them exhibited a linear growth with the aging time. The growth rates of SnTe and SnSb were 15.1 μm/h and 4.3 μm/h, respectively, at 180 °C. The IMC growth kinetics was further investigated systematically. The activation energies for the SnTe and SnSb growths were 151.6 kJ/mol and 165.3 kJ/mol, respectively. For the Sn–3.0Ag–0.5Cu/(Bi,Sb)2Te3 reaction, the SnTe and SnSb were also formed simultaneously. Similarly, they also had a linear growth, but their growth rates were significantly suppressed. A thin stripe-like Ag-rich phase was formed at the SnTe/(Bi,Sb)2Te3 interface, which could be the cause of the suppression of IMC growth. Additionally, minor addition of 0.1 wt.% Ga in Sn was also found to effectively suppress the interfacial IMC growth.

Interfacial reaction and microstructure between the Sn3Ag0.5Cu solder and Cu–Co dual-phase substrate

In this study, interfacial reactions and microstructures of the Sn3Ag0.5Cu (SAC305)/Cu–xCo (x = 0, 30 and 50 wt%) systems were investigated during reflowing at 290 °C and solid-state aging at 150 °C for various time. The effects of different contents of Co in substrate on interfacial reaction in SAC305/Cu–xCo system were discussed. It was found that the addition of Co into pure copper substrate to achieve alloying would effectively inhibit the growth of IMC layers. Comparison among the thickness of the intermetallic compound (IMC) in three kinds of SAC305/Cu–Co systems indicated that the IMC layer of SAC305/Cu joint was thicker than that of the other two types of solder joints. The composition of the SAC305/Cu IMC layers was Cu6Sn5 and Cu3Sn. Three kinds of reaction phases (Cu,Co)6Sn5, (Cu,Co)Sn2 and (Cu,Co)3Sn were found at the interfaces of the SAC305/Cu-30Co and SAC305/Cu-50Co joints. Remarkably the (Cu,Co)Sn2 phase was found adjacent to the Co-rich phase after soldering and eliminated after the aging treatment. While the (Cu,Co)3Sn phase accumulated increasingly adjacent to the substrate with the increased aging time. The results suggested that the Co content increased from 30 to 50 wt% in substrate lead to significant restraint of the growth of interfacial IMC. In addition, the thickness of the interfacial IMC layer was linear with the square root of the aging time during the aging process. The reaction rate between Sn atoms in solder and Cu, Co atoms in substrate was quite different, which lead to the fact that the interface of SAC305/Cu–Co is uneven on the side of substrate after reflowing and aging.

Combined effects of surface oxidation and interfacial intermetallic compound growth on solderability degradation of electrodeposited tin thin films on copper substrate due to isothermal ageing

We report new insights into the solder wettability degradation of Sn thin films on Cu under 155 °C isothermal ageing. A multiscale wettability degradation model was established, reflecting quantitatively the surface oxidation and interfacial intermetallic compound (IMC) growth, on the basis of solder wetting behaviour. The thermal oxidation of Sn exhibited heterogeneous inward thickening, lateral expanding and outward platelet-like growth, forming nanocrystalline, oxygen-deficient SnO2 with pronounced voiding/cracking propensity. Unlike a commonly held belief that the initial wettability loss is due to surface-exposing and oxidation of IMCs, it was found from dual combined effects of inward surface oxidation and outward IMC growth.

Effect of the size of carbon nanotubes (CNTs) on the microstructure and mechanical strength of CNTs-doped composite Sn0.3Ag0.7Cu-CNTs solder

Carbon nanotubes (CNTs) with three different diameter ranges (10–20, 40–60, and 60–100 nm) were doped into tin-silver-copper (SAC) solder, to study the performance of the composite SAC-CNTs solder materials – as well as the effect of the size of the CNTs. It was found that all the CNTs-doped composite solder samples displayed refined microstructure, inhibited interfacial intermetallic compound (IMC) growth, and reinforced mechanical strength – while the melting point of the composite solder was close to that of the pristine solder. The reinforcement in mechanical strength was due to the doped CNTs pinned at the solder grain boundaries, which acted as second-phase particles that refined the microstructure and increased the dislocation density. The adsorbed CNTs destroyed the integrity of the interfacial IMCs, leading to reduced growth rate. Among these composite solders, CNTs with a diameter of 40–60 nm provided superior performance in refining the microstructure, lowering the IMC growth rate by 30.9% – and reinforcing the ball shear strength by 15.3% and the hardness by 16.1%. This size effect on the performance of composite solders was due to the various surface energy values for CNTs – that led to the agglomeration and adsorption of CNTs in the solder matrix and interfacial IMCs.

Interfacial reaction between liquid-state Sn-xBi solder and Co substrate

The interfacial reaction between liquid-state Sn-xBi (x = 0, 1, 5 and 10 wt%) solders and Co substrate at 260 °C for different soldering duration time was explored to investigate the effect of Bi addition and soldering time on the growth of intermetallic compound (IMC) and grains evolution by scanning electronic microscopy in this study. The results of research indicated that the Bi addition into Sn solder had not changed the formation of interfacial IMC and the single CoSn3 IMC layer formed at Sn-xBi/Co (x = 0, 1, 5 and 10 wt%) joints. The addition of 1 wt% Bi into Sn solder suppressed the growth of interfacial CoSn3 IMC, compared to Sn/Co system. Bi segregation occurred in Sn-10Bi/Co couple due to the supersaturated Bi element and would weaken the inhibiting effect on growth of IMC resulted from more Sn and Co atoms activated. The thickness of IMC at the interface of Sn-5Bi/Co at the minimum number was resulted from Bi solid solution strengthening, Bi segregation attributed to Bi supersaturation and gravity subsequently appeared and gradually influenced the IMC growth when the soldering time increased from 20 to 60 min. Accordingly, the law of IMC growth did not meet the linear relationship between the thickness of IMC and soldering time. Additionally, the thickness of interfacial IMC increased with extending soldering time, regardless of Bi content in Sn solder. Prism-like CoSn3 grain formed at Sn-xBi/Co (x = 0, 1, 5 and 10 wt%) joints and the law of grain mean size was in well agreement with the change of thickness of IMC in cross-sectional images.

Effect of Ni addition to the Cu substrate on the interfacial reaction and IMC growth with Sn3.0Ag0.5Cu solder

The formation and growth of intermetallic compound (IMC) layer at the interface between Sn3.0Ag0.5Cu (SAC305) solder and Cu–xNi (x = 0, 0.5, 1.5, 5, 10 wt%) substrate during reflowing and aging were investigated. The soldering was conducted at 270 °C using reflowing method, following by aging treatment at 150 °C for up to 360 h. The experimental results indicated that the total thickness of IMC increased with increasing aging time. The scallop-like Cu6Sn5 and planar-like Cu3Sn IMC layer were observed between SAC305 solder and purely Cu substrate. As the content of Ni element in Cu substrate was 0.5% or 1.5%, the scallop-like Cu6Sn5 and planar-like Cu3Sn IMC layer were still found between solder and Cu–Ni substrate and the total thickness of IMC layer decreased with the increasing Ni content. Besides, when the Ni content was up to 5%, the long prismatic (Cu,Ni)6Sn5 phase was the only product between solder and substrate and the total thickness of IMC layer increased significantly. Interestingly, the total thickness of IMC decreased slightly as the Ni addition was up to 10%. In the end, the grains of interfacial IMC layer became coarser with aging time increasing while the addition of Ni in Cu substrate could refine IMC grains.

Interfacial Reaction and IMC Growth of an Ultrasonically Soldered Cu/SAC305/Cu Structure during Isothermal Aging.

In order to accelerate the growth of interfacial intermetallic compound (IMC) layers in a soldering structure, Cu/SAC305/Cu was first ultrasonically spot soldered and then subjected to isothermal aging. Relatively short vibration times, i.e., 400 ms and 800 ms, were used for the ultrasonic soldering. The isothermal aging was conducted at 150 °C for 0, 120, 240, and 360 h. The evolution of microstructure, the IMC layer growth mechanism during aging, and the shear strength of the joints after aging were systemically investigated. Results showed the following. (i) Formation of intermetallic compounds was accelerated by ultrasonic cavitation and streaming effects, the thickness of the interfacial Cu6Sn5 layer increased with aging time, and a thin Cu3Sn layer was identified after aging for 360 h. (ii) The growth of the interfacial IMC layer of the ultrasonically soldered Cu/SAC305/Cu joints followed a linear function of the square root of the aging time, revealing a diffusion-controlled mechanism. (iii) The tensile shear strength of the joint decreased to a small extent with increasing aging time, owing to the combined effects of IMC grain coarsening and the increase of the interfacial IMC. (iv) Finally, although the fracture surfaces and failure locations of the joint soldered with 400 ms and 800 ms vibration times show similar characteristics, they are influenced by the aging time.

Synchrotron radiation imaging study on the rapid IMC growth of Sn–xAg solders with Cu and Ni substrates during the heat preservation stage

Synchrotron radiation X-ray real-time imaging technology was used to study the interfacial reactions of Sn and Sn3.5Ag solders with Cu and Ni substrates during the heat preservation stage. This experiment allows us to in-situ observe the interfacial intermetallic compounds (IMCs) growth, provides a stage to distinctly separate their growth at isothermal reflow and cooling durations, and thus avoids the measurement errors for size and morphology of interfacial IMCs. The results show that the Cu6Sn5 morphology is scalloped at Sn–xAg/Cu interfaces, whereas (Cu,Ni)6Sn5 is wide-stripe shaped at Sn–xAg/Ni interface during the heat preservation stage. It is interesting to note that the grain boundary molten liquid gaps is observed at both of the Sn3.5Ag/Cu and Sn3.5Ag/Ni interfaces. Compared with pure Sn solder, Sn3.5Ag solders are observed with interfacial IMC of greater thickness, smaller base width, lower aspect ratio and more substrate consumption. In addition, it is confirmed that Ag3Sn grains appear in Sn3.5Ag solder, but not on the surface of existing interfacial Cu6Sn5 grains at Sn3.5Ag/Cu interface during the heat preservation stage. All of the growth kinetic index (n) for Sn–xAg/Cu is close to 1/3 indicating that grain boundary diffusion determines the interfacial reaction at the heat preservation stage. The mechanism by which Ag addition affects interfacial reaction and changes the mirostructure of solder matrix through creation of grain boundary gaps and decrease of the growth activation energy of IMC is also clarified. The knowledge for morphology and thickness control of interfacial IMCs by alloy elements addition as outlined by this study can be utilized to improve the reliability of solder joints in electronic packaging and solar photovoltaic (PV) cells array manufacturing sectors.

Effect of Mn Nanoparticles on Interfacial Intermetallic Compound Growth in Low-Ag Sn-0.3Ag-0.7Cu-xMn Solder Joints

Interfacial intermetallic compound (IMC) growth between Cu substrates and low-Ag Sn-0.3Ag-0.7Cu-xMn (x = 0 wt.%, 0.02 wt.%, 0.05 wt.%, 0.1 wt.%, and 0.15 wt.%) (SAC0307-xMn) solders was investigated under different isothermal aging temperatures of 100°C, 150°C, and 190°C. Scanning electron microscopy (SEM) was employed to observe the microstructural evolution of the solder joints and measure the IMC layer thickness. The IMC phases were identified by energy-dispersive x-ray spectroscopy and x-ray diffraction. The results showed that a Cu6Sn5 IMC layer formed in the as-soldered solder joints, while a duplex structure consisting of a Cu6Sn5 IMC layer near the solder matrix and a Cu3Sn IMC layer was observed after isothermal aging. A considerable drop in the IMC layer thickness was observed when 0.1 wt.% Mn nanoparticles were added. Beyond this amount, the thickness of the IMC layer only slightly increases. Adding Mn nanoparticles can increase the activation energy and thus reduce the interdiffusion rates of the Sn and Cu atoms, which suppresses excessive IMC growth. The solder joint containing 0.1 wt.% Mn nanoparticles has the highest activation energy. SEM images revealed that the number of small particles precipitated in the channels between the Cu6Sn5 IMC layers increases with an increasing proportion of Mn nanoparticles. Based on the microstructural evolution of the solder joints, this study revealed that grain boundary pinning is one of the most important mechanisms for IMC growth inhibition when Mn nanoparticles are added.

Growth behavior of intermetallic compounds and early formation of cracks in Sn-3Ag-0.5Cu solder joints under extreme temperature thermal shock

The microstructure evolution and growth mechanism of interfacial intermetallic compounds (IMCs) as well as the mechanism for early formation of cracks in Sn-3Ag-0.5Cu solder joints of quad flat packages (QFPs) during extreme temperature thermal shock between 77K and 423K were investigated. The Cu-Sn IMC layer at the Cu lead/solder interface and the Ni-Cu-Sn IMC layer at the solder/ENIG pad interface gradually thickened as well as the IMCs morphologies changed during extreme temperature thermal shock. Scallop-like Cu-Sn IMC layer and needle-like Ni-Cu-Sn IMC layer both transformed to plane-like IMCs. New Cu3Sn phase was formed at the interface between Cu lead and Cu6Sn5 IMC layer after 250 cycles. The (Ni, Cu)3Sn4 IMC layer was completely converted into (Cu, Ni)6Sn5 IMC layer after 150 cycles resulting from the diffusion of Cu atoms from Cu lead and Sn-3Ag-0.5Cu solder to the solder/pad interface. The time exponent (n) values of Cu-Sn and Ni-Cu-Sn IMC layers were 0.66 and 0.34, respectively, indicating that the controlling mechanisms for Cu-Sn and Ni-Cu-Sn IMC growth were bulk diffusion and grain-boundary diffusion, respectively. Cracks were formed both at the solder/Cu-Sn layer interface and at the solder/Ni-Cu-Sn layer interface after 250 cycles, due to the CTE difference between the solder and IMC, and to the thickened and flattened IMC layer. The great stress concentration resulting from the large temperature variation (∆T = 346K) led to the early formation of cracks. With the increase of thermal shock cycles, the pull strength of Sn-3Ag-0.5Cu solder joints decreased and the fracture location changed from within the solder to partly in Cu-Sn IMC layer and partly along the solder/Ni-Cu-Sn layer interface, which indicated that the fracture mechanism transformed from ductile fracture mode to brittle fracture mode.

Effect of Ni-P Plating Temperature on Growth of Interfacial Intermetallic Compound in Electroless Nickel Immersion Gold/Sn-Ag-Cu Solder Joints

The growth of interfacial intermetallic compound and the brittle fracture behavior of Sn-3.0Ag-0.5-Cu solder (SAC305) joints on electroless nickel immersion gold (ENIG) surface finish have been investigated using Ni-P plating solution at temperatures from 75°C to 85°C and fixed pH of 4.5. SAC305 solder balls with diameter of 450 μm were mounted on the prepared ENIG-finished Cu pads and reflowed with peak temperature of 250°C. The interfacial intermetallic compound (IMC) thickness after reflow decreased with increasing Ni-P plating temperature. After 800 h of thermal aging, the IMC thickness of the sample prepared at 85°C was higher than for that prepared at 75°C. Scanning electron microscopy of the Ni-P surface after removal of the Au layer revealed a nodular structure on the Ni-P surface. The nodule size of the Ni-P decreased with increasing Ni-P plating temperature. The Cu content near the IMC layer increased to 0.6 wt.%, higher than the original Cu content of 0.5 wt.%, indicating that Cu diffused from the Cu pad to the solder ball through the Ni-P layer at a rate depending on the nodule size. The sample prepared at 75°C with thicker interfacial IMC showed greater high-speed shear strength than the sample prepared at 85°C. Brittle fracture increased with decreasing Ni-P plating temperature.

Improvement on interfacial structure and properties of Sn–58Bi/Cu joint using Sn–3.0Ag–0.5Cu solder as barrier

Sn–58Bi solder recently attracts much attention for low temperature soldering, but is also restricted by the degradation on Sn–Bi solder joint from the interfacial Bi embrittlement. This study introduced a thin solder layer with the composition of Sn–3.0Ag–0.5Cu (SAC) on Cu pad to improve the interfacial structure and shear properties of Sn–58Bi solder joints under soldering and the subsequent aging conditions. SAC solder layer with the thickness about 5 μm was coated on Cu pad using Hot Air Soldering Leveling (HASL). During soldering condition, the thickness of interfacial intermetallic compound (IMC) layer in Sn–58Bi joints increased with the soldering times, and there was no obvious difference on the bare and SAC-coated Cu. However, during aging condition, the growth on interfacial IMC layer was effectively retarded by the SAC coating compared to their bare Cu counterparts. SAC-coated samples that were reflowed with varying soldering times and then aged also showed a thinner IMC layer. The retarding effect on IMC growth from SAC-coated Cu pad was attributed to the reason that SAC solder on Cu produced an initial Cu6Sn5 layer with fewer scallops, fewer grain boundaries between scallops, and therefore fewer diffusion pathways for Cu atoms to diffuse into the solder and react to grow the intermetallic layer. Ball shear tests also showed that aged SAC-coated joints displayed higher shear strength and through-solder ductile failure while Sn–58Bi on bare Cu pad displayed lower shear strength and brittle failure through the interfacial IMC layer. Therefore, SAC Pb-free solder can be recommended as the surface finishing on Cu pad to improve the performance of Sn–58Bi joints because it can be easily realized by HASL method in electronic industry.

TEM investigation of interfacial microstructure and fracture mode of the Sn-Ag-Cu/Ni joint system

The effects of Cu concentration (x) on the interfacial microstructure between a molten Sn-3Ag-xCu alloy and an electrolytic Au/Ni/Cu multilayer and its mechanical reliability were investigated via transmission electron microscopy (TEM) and high-speed ball shear (HSBS) testing. The x values were 0, 0.3, 0.4, 0.5, 0.7, and 1.0 (wt%). An increase in x caused an interfacial intermetallic compound (IMC) transition from a dense (Ni,Cu)3Sn4 layer (x = 0 and 0.3) to the coexistence of (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5 (x = 0.4 and 0.5), and to a dense (Cu,Ni)6Sn5 layer (x = 0.7 and 1.0) after soldering reaction at 250°C for 2min. An increase in the reaction time enhanced the IMC growth and induced the nucleation of a (Ni,Cu)3Sn4 nanolayer beneath (Cu,Ni)6Sn5 in the high x case. The TEM and HSBS characterizations showed that a single, thin (Ni,Cu)3Sn4 layer grown at the interface possessed better shear resistance than a single (Cu,Ni)6Sn5 layer, and the presence of a dual layer structure of (Cu,Ni)6Sn5/(Ni,Cu)3Sn4 substantially degraded the mechanical properties of the joint interface. These observations indicated that the Cu concentration in Sn-3Ag-xCu alloy plays a crucial role in the interfacial reaction, which, in turn, dominates the mechanical reliability of microelectronic joints.

Effect of type of thermo-mechanical excursion on growth of interfacial intermetallic compounds in Cu/Sn-Ag-Cu solder joints

This study reports the effect of different types of thermo-mechanical excursion (TME) on growth of intermetallic compound (IMC) layer formed at the interface of Sn-3.0%Ag-0.5%Cu solder and Cu substrate. 1mm thick solder joints were prepared by reflowing at 270°C for either 60 or 90s. Solder joints were then exposed to one of the following TME: (i) isothermal aging at 60°C for 48, 96 and 144h, (ii) thermal cycling between −25 and 125°C for 100, 200 and 400cycles, and (iii) thermo-mechanical cycling between −25 and 125°C for 100, 200 and 400cycles, wherein a shear strain of 10% per cycle was imposed on the joint. Finite element analysis (FEA) was performed to ascertain the effects of imposed shear strain and volumetric expansion due to the formation of IMC on the stress field in the solder joint. Irrespective of the type of TME, the thickness of the IMC layer increased with time. However, IMC thickness increased relatively more rapidly under thermo-mechanical cycling condition, indicating strain enhanced coarsening of the interfacial IMC layer. FEA showed that high stresses were generated in the IMC layer and near solder-IMC interface due to the formation of IMC layer as well as imposed external strain, which might then not only enhance the IMC growth kinetics, but also affect the morphology of the IMC layer.

Effect of nano-Cu addition on microstructure evolution of Sn0.7Ag0.5Cu-BiNi/Cu solder joint

PurposeThe purpose of this paper is to focus on the fabrication of SnAgCu (SAC) nanocomposites solder and study the effect of Cu nanopowders (nano-Cu) addition on the microstructure evolution of resultant nanocomposite solder after reflow and thermal aging.Design/methodology/approachMechanical mixing is used in this work to incorporate nanoparticles into the solder and produce more homogeneous mixture. Standard metallographic procedures are applied for microstructural analysis of solder joints.FindingsIt is found that nano-Cu doped into Sn0.7Ag0.5Cu-BiNi solder has no appreciable influence on melting temperature of the composite solder. The addition of Cu nanoparticles refines the microstructure of bulk solder and suppresses the growth of interfacial intermetallic compound (IMC) layers. However, interfacial IMC grain size increases slightly after 1.0 per cent nano-Cu added.Originality/valueThe paper demonstrates a method of nano-composite solder paste preparation by means of mechanical mixing and a comparison study of the microstructure evolution of composite solder with the basic SAC solder.