Diffusion Soldering Research Articles

Composite soldering materials for high-temperature stable solder joints

The thermal load on electronic assemblies is constantly increasing. The reasons for this increase are, on the one hand, the integration of power electronic components in an ever smaller space and thus an increasing power density and, on the other hand, the increasingly harsh environmental conditions with high temperature load. In addition to electronic components and substrate materials, the soldered connections are also exposed to this stress and must withstand it. The thermal stability is primarily determined by the melting temperature of the solder material or by the remelting temperature of the final solder interconnection. The remelting temperature can be purposefully increased through diffusion soldering. The advantage of diffusion soldering is that the operating temperature of the final solder joint can exceed the joining process temperatures. By using the composite soldering materials and diffusion soldering process it is possible to produce the solder interconnections that can withstand the high thermal and thermomechanical stress [1]. In this work, the composite solder material, consisting of the base solder alloy BiSnAg in eutectic composition with a melting point of 139 °C and added copper particles, was examined. The added copper particles have a direct influence on the dynamics of the diffusion process. Diffusion can also be influenced by adjusting the soldering process parameters, such as maximum temperature and time above liquidus of the base solder alloy, with the aim of achieving isothermal solidification. The solidification can take place through the parallel reactions: the reaction between tin and copper with the formation of high-melting intermetallic phases Cu3Sn and Cu6Sn5 and the growth of bismuth crystals through coarsening of the structure and tin depletion in the original eutectic solder alloy [2].

Improving the reliability of power modules through layered diffusion solder interconnects – Comparative study based on experiments and FE-simulation

The demand for reliable, lead-free and high temperature stable solder interconnects for power electronic application has increased in recent years. The die attach solder materials are exposed to higher electrical currents and temperatures due to advancing designs and wide band-gap semiconductors. The diffusion soldering material Sn-Cu-Sn with its high post processing melting temperature is a promising option for high temperature die attach interconnects. The aim of this work is to predict potential failure mechanisms in the die attach of automotive power modules due to simulation based thermo-mechanical analysis of the diffusion solder Sn-Cu-Sn layer and SAC. The FE-model is validated with experimental results from active power cycling tests. The simulation indicates a potential for lifetime increase by usage of layered diffusion solder. The created FE-simulation model could be used for simulation-based design for reliability to reduce material costs and development time.

Composite Soldering Materials Based on BiSnAg for High-Temperature Stable Solder Joints

The thermal load on electronic assemblies is constantly increasing. The reasons for this increase are, on the one hand, the integration of power electronic components in an ever smaller space and, thus, an increasing power density and, on the other hand, the increasingly harsh environmental conditions with high temperature load. In addition to electronic components and substrate materials, the soldered connections are also exposed to this stress and must withstand it. The thermal stability is primarily determined by the melting temperature of the solder material or by the remelting temperature of the final solder interconnection. The remelting temperature can be purposefully increased through diffusion soldering. The advantage of diffusion soldering is that the operating temperature of the final solder joint can exceed the joining process temperatures. By using the composite soldering materials and diffusion soldering process, it is possible to produce the solder interconnections that can withstand the high thermal and thermomechanical stress. In this work, the composite solder material, consisting of the base solder alloy BiSnAg in eutectic composition with a melting point of 139 degrees C and added copper particles, was examined. The added copper particles have a direct influence on the dynamics of the diffusion process. Diffusion can also be influenced by adjusting the soldering process parameters, such as maximum temperature and time above liquidus of the base solder alloy, with the aim of achieving isothermal solidification. The solidification can take place through the parallel reactions: the reaction between tin and copper with the formation of high-melting intermetallic phases Cu3Sn and Cu6Sn5 and the growth of bismuth (Bi) crystals through coarsening of the structure and tin depletion in the original eutectic solder alloy.

The research and development of Soldering materials applied in IGBT modules packaging

Power modules have attracted much attention in the market, especially for high-voltage applications like switch mode power, automotive powertrain. As the requirement of high reliability, high-temperature operation improves, the packaging materials should develop to meet the applications with high properties. In this paper, the soldering materials such as Sn based lead free solder, SnPb solder, Ag-sinter are demonstrated. Soldering materials reliability is described on reliability testing especially for power cycling, temperature cycling testing, thermal shock testing. The concept is based on different applications and soldering process on Si power modules and SiC power modules. Important materials properties of thermal conductivity, CTE (coefficient of thermal expansion), elastic modulus, melting point are discussed. By comparison, Ag sintering process has a good application in high power modulus especially for SiC. SnAg solder also has the potential application in SiC modulus by IMC diffusion soldering technology.

Development of a Simplistic Method to Simulate the Formation of Intermetallic Compounds in Diffusion Soldering Process

A simplistic simulation technique has been developed for computing the individual intermetallic compound (IMC) thickness which is formed in substrate-solder (Cu-Sn) systems during the diffusion soldering process in high-temperature power electronic applications. The method requires the time-dependent temperature profile for the soldering process and the growth rate parameters (e.g. concentration gradient, diffusion coefficient, activation energy, etc.) for the development of IMC layers as input. The method is suitable for predicting the thickness of an intermetallic phase layer during the diffusion soldering process. As such, it can be used in high-temperature power electronic application’s solder processing to enhance the reliability and lifetime of solder interconnections by allowing the control of the thickness of IMC layers. The method is demonstrated for IMC growth between pure copper as substrate and pure Sn as solder material. The growth behavior of the IMC layer is increased with increasing temperature over time according to the Arrhenius theory in the temperature range between 24°C to 260°C. To simulate the formation of IMC thickness in diffusion soldering interconnections, a simplistic way has been attempted using the popular commercial finite element simulation tool Comsol Multiphysics and scientific computing application ‘Matlab’. By means of transient thermal input, the diffusion-controlled intermetallic phase formation is simulated here. Few assumptions are taken care of this simulation process, for example, no convection, no reaction, solid-solid diffusion, no the pressure effect on the computational domain.

Effects of Aspect Ratio on Microstructural Evolution of Ni/Sn/Ni Microjoints

Under the simultaneous influence of volume shrinkage and surface solder diffusion, the microstructural evolution in Ni/Sn/Ni microjoints exhibits a conformable tendency among different joint sizes. This conformable tendency is the necking of the Sn layer and the initiation of voids which occur only near the periphery of the Sn layer. The reasons for this joint-size dependency have been theoretically evaluated and proposed in this study. This dependency remains applicable until the Ni3Sn4 layers growing in opposite directions impinge on each other. Afterward, microvoids are able to form everywhere along the centerline of the joints. It is hypothesized that the electrical and mechanical performance of microjoints is significantly related to the microstructures in these joints. With the help of the joint-size dependency proposed in this study, it is expected that the dimensional design of microjoints is capable of being optimized to assure high reliability.

Highly variable Sn-Cu diffusion soldering process for high performance power electronics

A Sn-Cu-based diffusion soldering process is presented that is capable of producing high temperature resistant joints for small area dies, like MOSFETs as well as for larger IGBTs and even large area baseplate or cooler-contacts. It is shown how the lateral infiltration process, needed to produce these joints, is influenced by the choice of the respective Cu-paste system. Furthermore, materials analysis on bulk material samples, including tensile testing, metallography and fracture analysis, show how the choice of paste system also affects the microstructure and hence the mechanical behavior of the bulk material. First results from power-cycling investigations on one of the selected paste-systems show a more than tenfold increase of lifetime compared to a SnAgCu305 solder reference.

Advanced wire bonding for high reliability and high temperature applications

Abstract The next generation of switches for power electronic will be based on white band gap (WBG) semiconductor GaN or SiC. This materials supports higher switching current and high frequency. White band gap semiconductors enables higher application temperature. Certainly, high temperature capability is also to discuss in combination with high number of thermal cycles. For a frame module concept shows these paper a comparison of different joining techniques with the focus on the reliability issue on wire and ribbon bonding. Beside to the 1000 passive thermal cycles from −40°C to +125°C there are active thermals cycles for technology qualification required [3]. Depending on the application and mission profile a high thermal cycling capability is necessary. For this reason, new high temperature joining techniques for die attach, e.g. Silver sintering or diffusion soldering, were developed in the recent past [4]. All of this new joining techniques focusing on higher electrical, thermal and thermo-mechanical performance of power modules. By using an optimized metallization system for the WBG the numbers of thermal cycles can be increased and the maximum operating temperature advanced up to 300°C. In these new temperature regions silicon semiconductors will be substituted by WBG semiconductors. The present work shows an active power cycling capability of different wire and ribbon bonds and the failure mechanism will be discussed. A calculation model explained the reliability for the different wire diameter and the impact of bonding materials. This reliability calculation explain the thermo-mechanical effects and based on materials and geometry data and is not optimized for evidence. Through these physical background understanding more than 1.000.000 thermal cycles with a 150 K temperature swing from +30°C to +180°C are now possible. These is a the basic knowledge for a design for reliability based on current, mission profile and reliability optimization for future high end applications with wire or ribbon bonding technique.

Intermetallic Reactions during the Solid-Liquid Interdiffusion Bonding of Bi2Te2.55Se0.45 Thermoelectric Material with Cu Electrodes Using a Sn Interlayer

The intermetallic compounds formed during the diffusion soldering of a Bi2Te2.55Se0.45 thermoelectric material with a Cu electrode are investigated. For this bonding process, Bi2Te2.55Se0.45 was pre-coated with a 1 μm Sn thin film on the thermoelectric element and pre-heated at 250 °C for 3 min before being electroplated with a Ni barrier layer and a Ag reaction layer. The pre-treated thermoelectric element was bonded with a Ag-coated Cu electrode using a 4 μm Sn interlayer at temperatures between 250 and 325 °C. The results indicated that a multi-layer of Bi–Te–Se/Sn–Te–Se–Bi/Ni3Sn4 phases formed at the Bi2Te2.55Se0.45/Ni interface, ensuring sound cohesion between the Bi2Te2.55Se0.45 thermoelectric material and Ni barrier. The molten Sn interlayer reacted rapidly with both Ag reaction layers to form an Ag3Sn intermetallic layer until it was completely exhausted and the Ag/Sn/Ag sandwich transformed into a Ag/Ag3Sn/Ag joint. Satisfactory shear strengths ranging from 19.3 and 21.8 MPa were achieved in Bi2Te2.55Se0.45/Cu joints bonded at 250 to 300 °C for 5 to 30 min, dropping to values of about 11 MPa for 60 min, bonding at 275 and 300 °C. In addition, poor strengths of about 7 MPa resulted from bonding at a higher temperature of 325 °C for 5 to 60 min.

A Comprehensive Study on the Automation Potentials and Complexities of Advanced and Alternative Die-Attach Technologies for Power Electronic Applications

The field of power electronics packaging presents intricate and interdisciplinary challenges. System costs, reliability and performance are strongly determined by various aspects such as mechanical design, materials, thermal management and interconnect technologies. The overall costs of the product depend mainly on the complete process chain in the module development. Automation as well plays an important role and facilitates higher production rates, efficient use of materials, better product quality, and reduced factory lead times. This paper focuses on emerging interconnection technologies of bonding semiconductor components to power electronic substrates like diffusion soldering, conductive adhesive bonding and reactive multi-layer bonding. An overview on the automation potentials and complexities in individual technologies for the manufacturing of reliable and cost-effective power modules is given and discussed. Thus, a basis is created for choosing optimal die-attach technology depending on economic and technologic demand by comparing the state-of-the-art and advanced technologies.

Barrier Properties of Electroless Ni-B UBM for Solder Joining

As demands accelerate for high density, high speed transmission and low power integrated circuits, 3D-ICs with through-silicon via (TSV) is pursued. In the structure of 3D-ICs, the first die is attached to the second die with micro bump, and the second die is attached to the circuit substrate with a C4 solder bump. The electrode structure of the second die is Cu/Ni UBM. The stress of the Ni-B layer is less than that of the Ni-P layer, and the Ni-B layer can suppress stress and die warpage. The purposes of our study are to clarify the difference in the barrier properties of the Ni-B UBM and Ni-P under bump metal (UBM) and the relevance of the barrier properties of Ni UBM and intermetallic compound (IMC) growth. It was found that an electroless Ni-B plating layer is superior to a Ni-P plating layer for UBM in liquid phase diffusion and in solid phase diffusion, and that a segregated B layer is formed under the IMC layer of a Ni-B land due to reflow soldering. It was estimated that this B layer plays the role of being a barrier layer for solder diffusion.

Formation, processing and characterization of Co–Sn intermetallic compounds for potential integration in 3D interconnects

Cobalt exhibits good wetting by lead-free solders (Humpston, 2010) [1], generates Sn-rich intermetallic compounds (IMC’s) and has lower dissolution in solders than Cu and Ni, thus potentially being a more effective solder diffusion barrier with improved electro- and thermo-migration resistance in interconnect applications (Ishida et al., 1991; Okamoto et al., 2006; Wang et al., 2013; Kanatzidis et al., 2005; Liu et al., 2004; Labie, 2007) [5–7]. In this work, we extend prior data (Vakanas et al., 2013; M.O. et al., 2012; Vakanas et al., 2014) on formation, processing, characterization and quantification of Co–Sn IMC phases generated upon deposition, reflow and aging of electroplated and solder paste screen printed samples. Cobalt oxides growth (vs. Cu) was characterized by X-ray Photoelectron Spectroscopy (XPS) as a function of multiple reflows. Cobalt oxides cleaning by microwave Ar/H2 plasma was also demonstrated. Co–Sn IMC growth rate (vs. Cu–Sn and Ni–Sn) was quantified after one reflow cycle and aging in solid-state temperature range of 180–220°C. Relevant kinetic parameters were calculated assuming a diffusion-driven, parabolic growth model. IMC phases were identified by Energy-Dispersive X-ray Spectroscopy (EDS) verifying Sn-rich Co IMC’s. Young’s modulus (E) and hardness (H) were measured by nanoindentation indicating that Co–Sn IMC’s are more compliant and softer than Cu–Sn IMC’s. Implications of the findings for material selection in 3D micro-bumping and stacking integration are discussed with a future outlook and recommendations to improve interconnect reliability.

Electrodeposition and Characterisation of Novel Ni-NbOx Composite Coatings as a Diffusion Barrier for Liquid Solder Interconnects - Part II: Diffusion Barrier Performance

The control of interfacial microstructural stability is of utmost importance to the reliability of liquid solder interconnects in high temperature electronic assemblies. This is primarily due to excessive intermetallic compounds (IMCs) that can form and continuously grow during high temperature operation, which renders conventional barrier metallisations inadequate. With the intention of reducing such excessive IMC growth, electrically conducting, NbOx containing Ni coatings were developed using electrodeposition and were assessed as solder diffusion barrier layers in terms of their electrical conductivity and barrier property. The present work adopts a novel electrochemical route to produce Ni-NbOx composite coatings of good uniformity, compactness and purity, from non-aqueous glycol-based electrolytes consisting of NiCl2 and NbCl5 as metal precursors. The effects of cathodic current density and NaBH4 concentrations on the surface morphology, composition and thickness of the coatings were examined. Increased NaBH4 concentrations were found to elevate the maximum deposit thickness (above 10 µm), although these led to a reduction in the co-deposited Nb content. The composite coatings generally exhibited good electrical conductivity. The reaction between a liquid 52In-48Sn solder and Ni-NbOx, with Nb contents up to 6 at.%, was studied at 200ºC. The results indicate that, Ni-NbOx with sufficient layer thickness and higher Nb content, offered longer service lifetime. Nb enrichment was generally observed at or close to the reaction front after high temperature storage which suggests evident effectiveness of the enhanced diffusion barrier characteristics. A mechanism for the role of Nb as a barrier performance enhancer was postulated.

Diffusion Barrier Characteristics of Ni-NbOx Composite Electrodeposits for Liquid In-Sn Solder Interconnects

The control of interfacial microstructural stability is of utmost importance to the reliability of liquid solder interconnects in high temperature electronic assemblies. This is primarily due to excessive intermetallic compounds (IMCs) that can form and continuously grow during high temperature operation, which renders conventional barrier metallizations inadequate. With the intention of reducing such excessive IMC growth, electrically conducting, NbOx containing Ni coatings were developed using electrodeposition and were assessed as solder diffusion barrier layers in terms of their electrical conductivity and barrier property. The present work adopts a novel electrochemical route to produce Ni-NbOx composite coatings of good uniformity, compactness and purity, from non-aqueous glycol-based electrolytes consisting of NiCl2 and NbCl5 as metal precursors. The effects of cathodic current density and NaBH4 concentrations on the surface morphology, composition and thickness of the coatings were examined. Increased NaBH4 concentrations were found to elevate the maximum deposit thickness (above 10 μm), although these led to a reduction in the co-deposited Nb content. The composite coatings generally exhibited good electrical conductivity. The reaction between a liquid 52In-48Sn solder and Ni-NbOx, with Nb contents up to 6 at.%, was studied at 200°C. The results indicate that, Ni-NbOx with sufficient layer thickness and higher Nb content, offered longer service lifetime. Nb enrichment was generally observed at or close to the reaction front after high temperature storage which suggests evident effectiveness of the enhanced diffusion barrier characteristics. A mechanism for the role of Nb as a barrier performance enhancer was postulated.

Joining and package technology for 175 °C Tj increasing reliability in automotive applications

For the task of improving the reliability and robustness of power electronic semiconductor devices, the main focus is on packaging and joining technology. The improvement is made necessary by various applications which require higher active and passive temperature cycles capability. For example: Automotive applications require a minimum life time of 15years, wind power 25 and traction applications, 30–40years and longer.Furthermore, higher reliability is demanded of new semiconductor technology, because maximum junction temperatures will, sooner or later, exceed currently recognized limits.In the near future, silicon devices will reach a maximum junction temperature of up to 200°C. And the new wideband-gap devices have the potential to exceed even this limit. That is why a new package and joining technology is mandatory.There are new developments in the pipeline:For the die contact, planar wire technology and copper based bond wires can be expected in the long term.This new technology is based on new material science and printed circuit board process techniques.As a robust joining method, sinter or diffusion solder will be in use soon.But on the semiconductor module level, further action can be taken now, or in the near future, to increase reliability significantly.This paper presents improvements, based on material science, which are already available and can be used now, or in the near future.

High Stroke and High Deflection Bulk-PZT Diaphragm and Cantilever Micro Actuators and Effect of Pre-Stress on Device Performance

This paper presents the design, simulation, fabrication, and experimental characterization of high-performance piezoelectric diaphragm and cantilever beam out-of-plane micro actuators that are fabricated from bulk lead zirconium titanate (PZT) films integrated on silicon. The utilized fabrication technology involves low-temperature diffusion solder bonding of a bulk piezoelectric ceramic on silicon, and subsequent lapping to achieve a desired PZT thickness. Different piezoelectric actuation modes (conventional longitudinal and transverse modes, and a novel shear mode) are explored and compared in terms of displacement range, and the diaphragm structures and electrodes are optimized via finite-element analysis (FEA). The effect of bonding prestress on the device performance is analytically characterized and verified through measurements. The close match between test data and simulation results suggests that the piezoelectric properties of integrated bulk-PZT5A films are mostly preserved without any necessity of re-polarization. Fabricated devices are tested for dynamic displacement range, power consumption, and temperature response, and FEA is used to evaluate the actuation forces. A 25- μm thick 1- mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> sized diaphragm can provide 12 μmPP displacement at 111 kHz with power consumption, while 1-3.5 mm long cantilever beams of similar thickness provide 0.1-1 mmPP displacement at resonance frequencies of 0.7-7.4 kHz. The introduced devices can be leveraged in various ultrasonic and acoustic applications.

Investigation of Process Parameters and Characterization of Nanowire Anisotropic Conductive Film for Interconnection Applications

In this paper, the assessment of the electrical and mechanical performance of the Cu nanowire anisotropic conductive film (NW-ACF) flip-chip interconnects has been performed. The influence of bonding conditions (bonding temperature and bonding force) has been studied in order to understand their impact on bonding resistance and shear strength for two types of NW-ACF based on different templates with varying porosity and thickness. An overgrowth-stripping method has been employed to obtain uniform NW (200 and 220 nm) arrays in the track-etch polymer membranes. The bonding substrate was coated with indium so that a diffusion soldering between Cu NWs and bulk In film can occur at a relatively low eutectic point (156.6 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula> ). The bonding interface and the fractural surface of NW-ACFs have been characterized by scanning electron microscope and energy dispersive X-ray analysis. Under optimized fabrication conditions, process at the bonding profile of 200 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula> , 20 N shows that the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$z$</tex></formula> -axis resistance can be below 1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm m}\Omega$</tex></formula> and the shear strength can be above 10 MPa, which is very promising for the fine pitch, 3-D interconnection applications in the future.

Morphology and Chemical Composition of Ag/In/Ag Interconnections

One of the environment protection’s main aims is working out soldering materials able to replace the Sn-Pb solders commonly used so far. The joint obtained using diffusion soldering fulfills these conditions and takes up 6 times less space than in the case of conventional soldering. Moreover, it can work at the temperatures higher than 350 [°C] and it often shows mechanical and thermal stability at temperatures 2-3 times higher than the joining temperature. This paper presents the morphology and chemical composition Ag/In/Ag joint diffusion-soldered at 193 [°C] for 1.5 and 2 hours. Two intermetallic phases, namely AgIn2 and Ag2In were identified within the joint using an energy dispersive X-ray microanalysis and selected area electron diffraction techniques. Most probably the AgIn2 phase was formed during cooling of the joints from the soldering temperature because its melting point is 166 [°C]. On the other hand, the Ag2In phase is desired as its thermal stability is over 300 [°C]. This phase grows into the solder material (In) relatively fast. The exponential factor n determined from the appropriate kinetic equation was found to be 0.41, which suggests some contribution of the grain boundary diffusion process in addition to the volume diffusion (n=0.5).

Morphology and chemical composition of Cu/Sn/Cu and Cu(5 at-%Ni)/Sn/Cu(5at-%Ni) interconnections

In the present paper, scanning and transmission electron microscopies aswell as energy dispersive X-ray spectroscopy investigations were performedto describe the morphology and chemical composition of the intermetallic phasesgrowing in Cu/Sn/Cu and Cu(Ni)/Sn/Cu(Ni) interconnectionsduring the diffusion soldering process. The obtained results revealed thateven a small amount of Ni addition (5 at-%) to the Cu substratetotally changes the morphology and the rate of formation of the intermetallicphase layers in the solder/substrate reaction zone of the interconnectionsprepared at the same time and joining temperature conditions. The presentedstudies are promising in terms of the shortening of the soldering time inthe electronic industry.

Process challenges in 0-level packaging using 100μm-thin chip capping with TSV

This paper presents the process challenges for the fabrication of 0-level packages using chip capping with through silicon via's (TSV) in the capping chip/wafer. The key processing challenges addressed include the fabrication of TSVs on capping wafers that are thinned down to 100μm and further, the Cu/Sn/Cu diffusion soldering to make an intermetallic bond for the chip capping. Integration of the TSVs on the cap wafer enables the vertical interconnection, thus making the package with a small form factor and readiness for 3D integration. The Cu/Sn/Cu bond and seal process provides a hermetic package and at the same time provide electrical connection between the cap and the device wafer. Adequate processing conditions are proposed.