SnAg Solder Research Articles

Pure Chemical Reduction of Tin Oxides to Metallic Tin by Atmospheric Plasma to Improve Interconnection Reflow of Pb-free Solders

Tin alloys are widely used as solder for electronic interconnections. Tin solder surfaces tend to have tin oxides which need to be removed to improve the yield of interconnection reflow processes such as flip chip joining. Conventionally, a strong flux is employed to remove these oxides, however this process has the drawbacks of leaving flux residue which can cause underfill delamination or require a high-cost cleaning process. As solder bump volumes and bump-to-bump spacing decrease, these problems become more difficult to manage in manufacturing. We propose the use of Atmospheric Plasma to reduce these oxides from the bump surfaces to enable the use of very light fluxes, or no flux at all. This process has the advantages of plasma surface preparation without the cost and throughput penalty of vacuum plasma processes. Such a process can increase throughput and yield while reducing the cost. We describe an experiment in which tin foils were treated with a reducing-chemistry Atmospheric Plasma process and then analyzed with X-ray photoelectron spectroscopy (XPS) and Auger Electron Spectroscopy (AES). AES depth profile analyses indicate that the thickness of tin oxides was significantly reduced by the plasma. There was no evidence for any etching of underlying elemental tin. These results suggest that tin oxides are reduced to metallic tin without etching of the underlying tin metal. In another similar experiment using semiconductor chips with SnAg solders, XPS results suggest that the tin oxides were again reduced to metallic tin. In flip chip joining, the joining process with such Atmospheric Plasma-treated chips achieved high interconnect yield, even in the case of poor quality solder balls with excessive oxidation. It is our understanding that the pure chemical reduction of tin oxides with atmospheric plasma in ambient had not been previously reported.

Asymmetrical Precipitation of Ag3Sn Intermetallic Compounds Induced by Thermomigration of Ag in Pb-Free Microbumps During Solid-State Aging

Three-dimensional integrated circuit technology has become a major trend in electronics packaging in the microelectronics industry. To effectively remove heat from stacked integrated circuitry, a temperature gradient must be established across the chips. Furthermore, because of the trend toward higher device current density, Joule heating is more serious and temperature gradients across soldered joints are expected to increase. In this study we used heat-sink and heat-source devices to establish a temperature gradient across SnAg microbumps to investigate the thermomigration behavior of Ag in SnAg solder. Compared with isothermal conditions, small Ag3Sn particles near the hot end were dissolved and redistributed toward the cold end under a temperature gradient. The results indicated that temperature gradient-induced movement of Ag atoms occurred from the hot side toward the cold side, and asymmetrical precipitation of Ag3Sn resulted. The mechanism of growth of the intermetallic compound (IMC) Ag3Sn, caused by thermomigration of Ag, is discussed. The rate of growth Ag3Sn IMC at the cold side was found to increase linearly with solid-aging time under a temperature gradient. To understand the force driving Ag diffusion under the temperature gradient, the molar heat of transport (Q*) of Ag in Sn was calculated as +13.34 kJ/mole.

A time dependent damage indicator model for Sn3.5Ag solder layer in power electronic module

This paper reviewed the existing damage evolution models in the literature for solder layer in microelectronics and then proposed a two dimensional approximate time dependent damage indicator model for Sn3.5Ag type lead free solder layer in power electronic module application. The proposed time dependent damage indicator model is influenced by inelastic strain from microstructural evolution Anand viscoplastic model. The experimental evaluation of parameter values of the proposed damage indicator model was not feasible. Hence, we adopted a numerical approximation methodology to extract the parameter values of the damage model. A MatLab code was generated to simulate the stress versus strain curve of the solder layer during the thermal variance loading. A data from public domain for crack initiation and crack propagation of SnAg solder layer was also utilised to estimate the parameter values of damage indicator model. The developed approximate time dependent damage model was numerically compared with a damage model in the literature based on Coffin Manson and Paris law fatigue model for prediction accuracy.

The Effect of Bi/Sn Ratio of Bi–Sn–Ag Solder on the Mechanical and Electrical Properties of Interfaces Between Solder/Thermoelectric Material and Solder/Electrode

The Effect of Bi/Sn Ratio of Bi–Sn–Ag Solder on the Mechanical and Electrical Properties of Interfaces Between Solder/Thermoelectric Material and Solder/Electrode

Back Cover: Advanced Engineering Materials 8∕2015

Advanced Engineering MaterialsVolume 17, Issue 8 p. 1244-1244 Back CoverFree Access Back Cover: Advanced Engineering Materials 8∕2015 First published: 06 August 2015 https://doi.org/10.1002/adem.201570031AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract Micromechanical testing enables us to determine the mechanical properties of small volumes, microstructural phases, and constituents. Although traditionally limited to time independent deformation, we have now used the micropillar compression technique to characterize the stress and temperature dependent creep properties of microscopic phases (shown in the graph). Many materials, such as Sn-Ag solder alloys, have complex inhomogeneous microstructures (as shown in the background). For the first time ever, this new technique allowed us to probe and understand the creep properties of individual constituents, in this case pure tin and eutectic regions (shown in the inset SEM images). For details see the article by N. Chawla and co-workers on page 1168. Volume17, Issue8August 2015Pages 1244-1244 RelatedInformation

Thermal property measurement of solder joints fabricated by self-propagating exothermic reaction in Al/Ni multilayer film

To realize the application of reactive joint process to electronic packaging, we analyze thermal resistance for solder joints fabricated by using Al/Ni self-propagating exothermic reaction. Laser flash method with response function analysis is used for thermal resistance measurement. The specimens with 4–8-µm-thick Sn–3.5Ag solder joint have thermal resistances ranging from 5.0 × 10−6 to 8.1 × 10−6 m2 K/W. In the specimen with 12-µm-thick SnAg solder joint, periodical heating analysis suggests that high and low thermal resistance areas happen to coexist due to incomplete bonding. Micro thermo-reflectance (m-TR) analysis suggests that thermal effusivity of reacted NiAl layer is lower than expected. Thermal resistance of the joints will be influenced by voids, cracks, and grain boundaries in reacted NiAl layer.

Mechanical properties of interfacial phases between Sn-3.5 Ag solder and Ni-18 at. % W barrier film by nanoindentation

Purpose – The purpose of this paper is to investigate the hardness and elastic modulus on interfacial phases formed between Sn-3.5Ag solder and Ni-18 at. % W alloy film by nanoindentation. It has been found that a ternary amorphous Sn-Ni-W layer formed below Ni3Sn4 IMC at the interface. In this study, mechanical properties of the IMC formed between SA solder and Ni-18 at. % W film after six times reflows were performed by nanoindentation. Design/methodology/approach – The characterization was carried at 25°C, and 100 indents were generated. The elastic modulus and hardness were investigated. Findings – The results showed that hardness of Ni3Sn4 IMC was higher than amorphous Sn-Ni-W phase. A slight bigger indent was observed on the Sn-Ni-W layer compared with that on the Ni3Sn4 IMC. Lower topographical height in the Sn-Ni-W layer indicated that the Sn-Ni-W phase was softer compared with the Ni3Sn4 IMC. The lower hardness and soft Sn-Ni-W phase is significantly related to the amorphous structure that formed through solid-state amorphization. Originality/value – There are no publications about the indentation on the interfacial between the Ni-W layer and the Sn-Ag solder.

J2210203 薄膜はんだ接合に向けた自己伝播発熱反応接合における熱的考察

J2210203 薄膜はんだ接合に向けた自己伝播発熱反応接合における熱的考察

Interfacial reaction and intermetallic compound formation of Sn–1Ag/ENIG and Sn–1Ag/ENEPIG solder joints

Interfacial reactions and intermetallic compound (IMC) formation of Sn–1wt.%Ag solder with two different surface finishes, electroless nickel–immersion gold (ENIG) and electroless nickel–electroless palladium–immersion gold (ENEPIG), were evaluated during a reflow process. We first compared the interfacial reactions of the two solder joints and identified a relationship between the interfacial reaction behavior and solder/substrate compatibility. The Sn–1Ag/ENIG joint exhibited stable IMC growth and superior interfacial stability compared with the Sn–1Ag/ENEPIG joint. The growth rate of the interfacial IMC layer and consumption rate of the Ni(P) layer were closely related to the interfacial microstructure and morphology of the IMCs. The layer-type well-attached interfacial IMCs had advantages with respect to IMC growth rate and Ni(P) consumption rate. Compared to the previous Sn–3.0Ag–0.5Cu study, ENEPIG performed very well with Sn–Ag–Cu solder alloy. On the other hand, ENEPIG proved not to be a suitable surface finish for Sn–Ag solder alloy in this study.

Dissolution of Copper and Formation of IMC in Bulk Lead-Free Solders

Besides the popular reflow soldering practice, wave soldering is another established technique in the electronic packaging industry for obtaining the solder joints. In this study the dissolution behavior of copper and the growth of intermetallic compound (IMC) during wave soldering process have been investigated. The study involved two different solder compositions, viz. pure Sn and eutectic Sn–Ag solder. Dipping experiments were conducted for the temperature range of 221–300 °C and for dwell time starting form 5 s to 6 min. The average thickness of the intermetallic compound formed and the amount of copper dissolved was determined. Experimental results indicated that the amount and rate of copper dissolution were strongly affected by temperature of molten solder and dwell time. The initial rate of copper dissolution was very high but it decreased with time. Based on the experimental results, the kinetic parameters governing the dissolution of copper and growth of the IMC were determined for the two solders.

IMC and creep behavior in Lead-free solder joints of Sn-Ag and Sn-Ag-Cu alloy system by SP method

The creep properties of Lead-free solder joints were evaluated by Shear Punch (SP) method. Lead-free solder joints with Sn-4Ag, Sn-4Ag-0.5Cu and Sn-4Ag-1.0Cu solder tested and compared to the lead solder joint with Sn-37Pb. SP creep tests were performed at temperature of 30, 50 and 80°C. For the SP method, constant shear stresses from 3 to 22 MPa were applied into the SP specimen(10 × 10 × 0.5 mm). From the SEM, IMC layer of Cu6Sn5 were observed on the interface between solder and Cu. From the SEM and EDX, the Lead-free solder joints have much thicker interfacial IMC layer than the Sn-37Pb solder joint. From the power law relationships, the Lead-free solder joints show superior creep resistance than the Sn-37Pb solder joint. And among Lead-free solder joints, at 30 and 50°C, the ternary SnAgCu solder joints have high creep resistance than the binary SnAg solder joint. Sn-4Ag-1.0Cu solder joint shows the best creep resistance at 30°C, and Sn-4Ag-0.5Cu solder joint shows the best creep resistance at 50°C. However, at high temperature 80°C, all of the Lead-free solder joints have similar creep properties. The activation energy were expressed with creep strain rate and temperature. The ternary SnAgCu solder joints have the higher activation energy than the binary SnAg solder joint at 17.68 and 18.72 MPa. Average activation energy of Lead-free solder joints were lower than that of Sn-37Pb 75.11 kJ/mol. The average activation energy of Sn-4Ag-1.0Cu, Sn-4Ag-0.5Cu and Sn-4Ag solder joint at high stress is 102.68, 62.52 and 58.61 kJ/mol, respectively.

Study of the UBM to copper interface robustness of solder bumps in flip chip packages

Shear tests on SnAg solder bumps were performed with a reduced height to the surface for a high shear force on the under bump metallurgy (UBM) to redistribution layer (RDL) copper interface. By this the failure mechanism of UBM–RDL delamination after stress tests simulating several assembly reflows could be reproduced. A design of experiment was done with corner wafers at worst case conditions for topography and interface clean. TEM cross sections confirmed nano scale carbon residues in the interface when reducing the clean efficiency. This results in a mechanically weakened interface with a present electrical contact. The shear test with reduced height is a more severe test beyond the JEDEC test to verify the bump robustness. This is important when existing bump technologies are used for flip chip package solutions with increased solder reflow requirements.

Comparison of Cu-Sn Compound Formation Between Snag and Pure Sn Solder Bumps by EBSD

Anisotropic behavior of Sn based solder during electromigration has been studied by many researchers, and it is very important to control the microstructure for more reliable bump joints[1]. Here, we prepared various SnAg bumps with different Ag concentrations including pure Sn[2]. Unary electroplating has various advantages such as easy handling bath, negligible bump composition and maintaining equipment with securing about environmental health and safety issue.Figure 1 shows our preliminary experiments for the cross-sectional EBSD and EDS measurement of Cu pillar with pure Sn bump, and it is reflowed at 235°C for 6 min. In this study, we will report comparison of Sn microstructures according to various Ag inclusions by using cross-sectional EBSD(electron backscattered diffraction) analysis and EDS(energy dispersive spectroscopy) measurement. We carried out multiple reflows at various atmosphere how this impact IMC(intermetallic compound) growth along with microstructures. Through this crystallographic aspect approach, we will present optimum condition for resistance of electro migration and void formation.[1] Yiwei Wang, Kuan H. Lu, Vikas Gupta, Leon Stiborek, Dwayne Shirley, Seung-Hyun Chae, Jay Im, Paul S. Ho, J. Mater. Res., 27, 1131 (2008)[2] Ui-Hyoung Lee, Moongi Cho, Woojin Choi, Ha-Young You, Jinho Choi and Jaihyung Won, 224th ECS meeting, San Francisco, Oct. (2013)

High Speed Electrochemical Deposition Bath for Sn-Ag Alloy and Evaluation of Micro Solder Bump Plated on Ni-Coated Si Wafer

In flip chip packaging, especially high density interconnection, electrodeposition of Sn-Ag alloy has been attracted increasing attention from semiconductor industry as a replacement of toxic tin-lead solders. With aim of improving plating speed, life-time, and enhancing quality of micro solder bump, various electrochemical characteristics of strong acid electroplating bath composing of methane sulfonic acid (MSA), Ag-MSA, Sn-MSA, and several modification agents were investigated by using linear voltammetry techniques, cyclic voltammetric stripping (CVS) with controlled hydrodynamic condition with RDE. In addition, surface morphology, metallurgical structure and composition of Sn-Ag solder bump formed on Si wafer and deposition properties were characterized by using XRD, WDXRF, and SEM. The CVS measurements showed that suppressing the hydrogen evolution reaction at high cathodic current above 30mA/cm2 was critical, and small amount, less than 0.6 g/L, of benzylidene acetone was effective for this troublesome. The fundamental electrochemical responses revealed that the concentration of metallic complex ions, silver (I) and tin (II & IV), and its stability are crucial for better electroplating. In this study, the effects of complexing agent of thiourea, anti-oxidizer of hydroquinone, and several surfactants were also examined in terms of potential shift, current density, solution stability, and deposition rate. The results also showed that most electrochemical reactions in this bath composition are influenced by rotation speed, hence hydrodynamic conditions. With optimized bath solution, electroplating parameters, such as current efficiency, deposition rate, and elemental decay, and bath lifetime were also measured on micro-patterned Si wafer, and plating performance was compared with electrochemical properties. The optimized plating bath showed high deposition rate of 0.8μm/min/ASD, longer lifetime of 95 AH/L, proper eutectic alloy composition of 3.5 w/o and suitable for micro solder bump size of less than 20 μm. In this study, the relation of electrochemical and deposition characteristics with various additives and electrical properties of micro solder bumps will be discussed.

Effect of Isothermal Aging on the Long-Term Reliability of Fine-Pitch Sn–Ag–Cu and Sn–Ag Solder Interconnects With and Without Board-Side Ni Surface Finish

The combined effects on long-term reliability of isothermal aging and chemically balanced or unbalanced surface finish have been investigated for fine-pitch ball grid array packages with Sn–3.0Ag–0.5Cu (SAC305) (wt.%) and Sn–3.5Ag (SnAg) (wt.%) solder ball interconnects. Two different printed circuit board surface finishes were selected to compare the effects of chemically balanced and unbalanced structure interconnects with and without board-side Ni surface finish. NiAu/solder/Cu and NiAu/solder/NiAu interconnects were isothermally aged and thermally cycled to evaluate long-term thermal fatigue reliability. Weibull plots of the combined effects of each aging condition and each surface finish revealed lifetime for NiAu/SAC305/Cu was reduced by approximately 40% by aging at 150°C; less degradation was observed for NiAu/SAC305/NiAu. Further reduction of characteristic life-cycle number was observed for NiAu/SnAg/NiAu joints. Microstructure was studied, focusing on its evolution near the board and package-side interfaces. Different mechanisms of aging were apparent under the different joint configurations. Their effects on the fatigue life of solder joints are discussed.

Low melting point nanocrystalline Sn–Ag solder synthesized by a refined chemical reduction method

The commercial market of Sn–Pb solder is gradually decreasing due to its toxicity, calling for Pb-free substitute materials. Sn–Ag alloy is a potential candidate in terms of good mechanical property. The major problematic issue of using Sn–Ag is their high melting temperature, consequently this study is dedicated to lowering the melting temperature of Sn3.5Ag (wt%) alloy by developing nanomaterials using a chemical reduction approach. The resultant nanocrystalline Sn3.5Ag is characterized by field emission scanning electron microscope. The size dependence of the melting temperature is discussed based on differential scanning calorimetry results. We have reduced the melting temperature to 209.8 °C in the nanocrystalline Sn3.5Ag of (32.4 ± 8.0) nm, compared to ~221 °C of the bulk alloy. The results are consistent with the prediction made by a relevant theoretical model, and it is possible to further lower the melting temperature using the chemical reduction approach developed by this study.

Electromigration in Sn–Ag solder thin films under high current density

The electro-migration behavior of a Sn–Ag solder thin film stripe that is deposited on a glass substrate has been investigated under a high current density in the absence ofthermo-migration. The distribution of voids and hillocks at current densities of 4.4–6.0×104A/cm2 has been analyzed optically and using electron microscopy. The voids mainly formed at the cathode side of the stripe where maximum current density was predicted but voids also formed along a line that crosses the stripe. This was explained in terms of the initial voids forming at locations of maximum current density concentration, altering these locations, and then expanding into them. The movement of the maximum current density location is caused by redistribution of current as the voids form. An atomic migration model has been developed and used in this work. It was found that if thermal gradients were completely neglected, the model was unable to account for the divergence of atomic flux density which is necessary for void nucleation. However, the temperature dependence of the diffusivity of atoms is sufficient to account for void nucleation within the timescale of the experiments.

Effect of rare earth metal Ce addition to Sn-Ag solder on interfacial reactions with Cu substrate

The effect of adding a small amount of rare earth cerium (Ce) element to low Ag containing Sn-1wt%Ag Pb-free solder on its interfacial reactions with Cu substrate was investigated. The growth of intermetallic compounds (IMCs) between three Sn-1Ag-xCe solders with different Ce contents and a Cu substrate was studied and the results were compared to those obtained for the Ce-free Sn-1Ag/Cu systems. In the solid-state reactions of the Sn-1Ag(-xCe)/Cu solder joints, the two IMC layers, Cu6Sn5 and Cu3Sn, grew as aging time increased. Compared to the Sn-1Ag/Cu joint, the growth of the Cu6Sn5 and Cu3Sn layers was depressed for the Ce-containing Sn-1Ag-xCe/Cu joint. The addition of Ce to the Sn-Ag solder reduced the growth of the interfacial Cu-Sn IMCs and prevented the IMCs from spalling from the interface. The evenly-distributed Ce elements in the solder region blocked the diffusion of Sn atoms to the interface and retarded the growth of the interfacial IMC layer.

Effects of silver addition on Cu–Sn microjoints for chip-stacking applications

The solid-state reactions under confined space between Cu substrate and Sn and Sn–Ag solders are systematically investigated, with the objective of providing fundamental data that are critical to the design of robust microjoints for chip-stacking applications. Sandwich structures of Cu/Sn/Cu and Cu/Sn3.5Ag/Cu are prepared by thermal compression bonding. The thickness of Sn and Sn3.5Ag layer is controlled at 10μm. High temperature storage tests are conducted at 120°C, 150°C, 180°C, and 200°C for different time periods. Unlike the well-known void formation in the Ni/Sn/Ni system, there is no such void at all in the Cu/Sn/Cu system. The addition of Ag consistently slows down the growth kinetics of intermetallics. The time-to-impingement of intermetallics is established for the first time for both systems under solid-state reactions.

Effect of aging treatment on microstructure and creep behaviour of Sn–Ag and Sn–Ag–Bi solder alloys

The present study was undertaken to investigate the influence of aging temperature on the creep behaviour of Sn–Ag and Sn–Ag–Bi solder alloys at testing temperatures ranging from 333 to 363 K under constant stress of 7·80 MPa. The steady state creep rate was found to increase continuously with increasing aging temperature at all testing temperatures. Results show that addition of Bi to the binary Sn–Ag solder alloy led to a significant increase in the strength and improvement in the creep resistance. The activation energy for the creep process of Sn–Ag and Sn–Ag–Bi solder alloys was found to have an average value of 36 and 45 kJ mol−1 respectively. This might be characterised by diffusion of Ag in Sn. The microstructure of the aged samples for both alloys examined by X-ray diffraction measurements supported the improvement in the creep resistance for Sn–Ag alloy by adding a small trace of Bi.