High Temperature Solder Research Articles

Size effect on the fracture of sintered porous nano-silver joints: Experiments and Weibull analysis

Nano-silver paste is a promising alternative of high temperature solder material for the next generation electronic devices, which is composed of organic solvent and silver nanoparticles/sheets. The mechanical properties of nano-silver interconnections are essential for the bonding between chip and substrate by sintering. During sintering process, volatilization and combustion of organic solvent in nano-silver paste lead to development of abundant voids in the joint, forming a three-dimensional network structure, which has strong influence on the mechanical properties. Sintering temperature and cross section area can affect the distribution of voids in solder joint. In the current study, effects of sintering temperature (200 °C, 250 °C, 300 °C) on the tensile and shear failure strength of sintered nano-silver joints were investigated, the size effect caused by different cross section area (i.e. (0.5, 1, 2, 3) × 5 mm2) was studied. The volatilization of organic solvent in nano-silver paste is the main reason for the porosity diversity, and the difference of porosity is one of the key factors causing size effect. It also leads to dispersion of experimental data of failure strength, thus, statistical analysis was performed to the experimental data. Four universal estimators and Weibull distributions were adopted to analyze the tensile and shear failure strength of sintered nano-silver joints. Based on the Gurson-Tvergaard-Needleman model, the critical porosity equation of fracture was developed. Furthermore, the relationship between critical porosity and size effect was established considering porosity increases with the increasing of cross section area, and a modified Weibull model containing size effect was proposed, which showed good agreement compared with the experimental data.

Layup-only modulization for low-stress fabrication of a silicon solar module with 100 μm thin silicon solar cells

Despite the predominance of solar modules with bulk crystalline silicon solar cells as a result of their high photoconversion efficiency, long-lasting durability, and low cost per electrical watt, the competitive market has elevated the necessity of thinner silicon solar cells to further reduce direct material costs. However, the tabbing and stringing process using high melting temperature soldering in the conventional fabrication method of a silicon solar module is required to alternatingly interconnect adjacent silicon solar cells, which entails significant thermomechanical stress. The lifting up-and-down motions in the layup process of serially stringed silicon solar cells also lead to an unfavourable interfacial stress between metal ribbons and silicon solar cells. Consequently, a considerably poor yield of silicon solar modules with the unbearable breakage loss of thin silicon solar cells is ascribed to such processes. Herein, an innovative solution to interconnect thin silicon solar cells through virtually no thermomechanical stress is proposed by using encapsulants pre-attached with low melting temperature metal ribbons at 139 °C, which allow the interconnection of pre-laid silicon solar cells in the course of the vacuum lamination at 160 °C. The resulting silicon solar modules exhibit a photoconversion efficiency of 19.1 ± 0.1%, which represents 99.9 ± 0.6% of the benchmark photoconversion efficiency of conventional silicon solar modules. The presented layup-only modulization succeeds in fabricating silicon solar modules with 100 μm thin silicon solar cells and it is anticipated to curtail the breakage loss of thin silicon solar cells as well as elevate fabrication productivity by simplifying the fabrication steps of silicon solar modules.

Recent Low Temperature Solder of SnBi and Its Bonding Characteristics

A solder has been extensively used for long time in electronics packaging field, and there are various Sn-based lead-free solders such as Sn-Ag-Cu, Sn-Cu, Sn-Bi and etc. Recently, the low temperature solder, typically Sn- 58wt%Bi-X alloys, attract attention from industry because of flexible printed circuit board (FPCB) application and to avoid bending or warpage of big size electronic devices after high temperature soldering. In the Sn-58wt%Bi-X solder, basic important issues are melting temperature, wettability, microstructure, mechanical properties and etc. Specifically, the Sn-58wt%Bi-X solder receives much attention to improve the brittleness by grain refinement of Bi-phase. In this paper, recent studies about Sn-58wt%Bi-X solders were reviewed including melting temperature, microstructure and mechanical properties. Key words: Solder, Sn-58wt%Bi, Melting temperature, Microstructure, Mechanical property

Correction to: Characteristics of eutectic and near-eutectic Zn–Al alloys as high-temperature lead-free solders

Correction to: Characteristics of eutectic and near-eutectic Zn–Al alloys as high-temperature lead-free solders

Study on the reliability of novel Au–30Ga solder for high-temperature packaging

As a novel high-temperature solder with suitable eutectic temperature and excellent solderability, Au–30Ga eutectic alloy shows high application potential as a packaging material for high-power devices. In this work, the evolution of microstructure, shear strength and fracture mechanism of Au–30Ga/Ni solder joint during soldering and aging was studied. It was found that with the increase of soldering time, Ga was seriously consumed to form Ga–Ni interfacial intermetallic compounds (IMCs), resulting in the formation of Ga-poor region near the interface layer. Au7Ga3 phase would be first formed at the valley area of Ga3Ni2 IMC layer. The dominant IMCs growth mechanism eventually transformed from grain-boundary diffusion to volume diffusion after soldering for 0.49 min. The diffusion coefficients are 1.64 × 10−14 and 3.77 × 10−19 m2/s at 500 °C (soldering) and 300 °C (solid-state aging), respectively. With the coarsening of IMC layers, the fracture mechanism of joint changed from solder control mode to solder/IMC mixed control mode. When the thickness is 4.36 μm, the joint strength reaches the maximum value of 77.7 MPa. Furthermore, a reliability evaluation equation of Au–30Ga/Ni solder joint was established to optimize soldering process and solder composition.

The Microstructure, Thermal, and Mechanical Properties of Sn-3.0Ag-0.5Cu-xSb High-Temperature Lead-Free Solder

To obtain Sn-3.0Ag-0.5Cu-xSb (x = 0, 25, 28, and 31) high-temperature lead-free solder antimony was added to Sn-3.0Ag-0.5Cu solder. The microstructure, thermal properties, and mechanical behavior of the solder alloy prepared were studied by using JSM-5610LV scanning electron microscope, Germany STA409PC differential scanning calorimeter, AG-I250KN universal tensile testing machine, and other methods. The SEM-EDS results showed that after adding Sb, SnSb phase was formed in the β-Sn matrix phase. The newly formed SnSb phase and the existing Sb in the solder alloy can inhibit the generation of IMC and refine the IMC layer. The addition of Sb significantly increased the melting temperature of the solder alloy. Among them, the thermal performance of Sn-3.0Ag-0.5Cu-25Sb is the best. The melting temperature of Sn-3.0Ag-0.5Cu-25Sb is 332.91 °C and the solid–liquid line range of Sn-3.0Ag-0.5Cu-25Sb solder alloy is 313.28–342.02 °C. Its pasty range is 28.74 °C, lower than 30 °C, which is beneficial for soldering. The test results of the mechanical behavior of Sn-3.0Ag-0.5Cu-xSb solder alloy show that with the increase of Sb addition, the ultimate tensile strength of the solder alloy also increases. However, the change of the elongation of the solder alloy is the opposite. The ultimate tensile strength of the solder alloy increased from 29.45 MPa of Sn-3.0Ag-0.5Cu solder to 70.81 MPa of Sn-3.0Ag-0.5Cu-31Sb solder. The reason for the increase in the strength of the solder alloy is the reduction of the thickness of IMC and the solid solution hardening effect of Sb.

A Way to Reduce Leakage Current and Improve Reliability of Wire-Bonds for 300-A Multichip SiC Hybrid Modules

In this article, pressureless sintering of nanosilver paste is proposed to enhance the reliability of wire-bonds with SiC chips by both alleviating thermomechanical stresses and enhancing thermal coupling of bonding wires and chips. A multichip 1200-V/300-A SiC hybrid module with six Si IGBTs and 12 SiC Schottky barrier diodes (SBDs) is demonstrated. The lifetime of the wire-bonds with the SiC SBD bonded by the pressureless sintered nanosilver is ~27.9% longer than that by the high-temperature solder under power cycling with constant temperature swing. The lifetime of the wire-bonds with the IGBT also bonded by the pressureless sintered nanosilver is ~46.2% longer than that by the high-temperature solder under power cycling with constant conduction current. Furthermore, not only the cost of 1700-V SiC SBDs is quite similar to that of 1200-V ones but also the leakage current of the 1200-V/300-A SiC hybrid module can be reduced greatly using 1700-V SiC SBDs as free-wheeling diodes instead of the regular 1200-V ones. Considering the much larger leakage current of a SiC SBD compared with that of a Si p-i-n diode. It is feasible to use a SiC SBD with higher blocking voltage to reduce the significant leakage current of the free-wheeling SBD with almost no cost increase.

Investigations of high-temperature tensile properties of Zn–25Sn–x(0.1–0.2)Cu–y(0.01–0.02)Ti high-temperature Pb-free solders

The use of Pb-containing solders in electronic products has been restricted due to their harm to both human health and the environment. Although the Sn–37Pb solder has been well replaced by Sn–3.0Ag–0.5Cu or other Pb-free solders, there is no drop in replacement for high-temperature Pb-free solders. In this study, the microstructure and high-temperature tensile properties of the Zn–25Sn–x(0.1–0.2)Cu–y(0.01–0.02)Ti high-temperature Pb-free solders were investigated. The design of the moderate alloy composition prevented undesirable Cu- and Ti-containing intermetallic compound formation that may cause alloy embrittlement. The solders exhibit superior tensile strength and competitive elongation compared with the conventional high-Pb solders and the other potential candidates. The Zn–25Sn–xCu–yTi solders can be strengthened in terms of the tensile strength enhancement without a loss of ductility under both room-temperature and high-temperature testing conditions. The minor Cu and Ti elements served as heterogeneous nucleation sites for inducing the microstructure refinement of the primary (Zn) phase. The Cu-in-Zn solid solution phenomenon as well as the formation of deformation twins during the high-temperature tensile test also contributed to the superior tensile properties of the designed alloys.

Influence of Ag Layer on Microstructural Evolution and Joining Properties With Zn Foil for Power Electronics

Solid diffusion bonding of the copper–zinc system is preliminarily investigated as a possible bonding solution for power modules. This article proposed a method of bonding Zn foil on substrate-plated Ag to obtain the solder joints. The formation and transformation of intermetallic compounds (IMCs) in Cu–Zn and Ag–Zn joints were investigated. The results showed that the AgZn3 phase was compressed and moved into the solder side when competed with the Cu5Zn8 phase, relieved the Kirkendall voids, and enhanced the connection of joints. Besides, the electrical resistivity could maintain at a competitive level of $9~\mu \Omega \cdot {\mathrm {cm}} $ , and the shear strength reached 43–75 MPa. These properties were also appreciated even after the aging test. The deformation behaviors of different IMCs were tested by nanoindentation. This work provides a potential alternative to high-temperature solders used for high-power-density electronic packaging applications.

Effect of Bi addition on the corrosion resistance and mechanical properties of sintered NdFeB permanent magnet/steel soldered joints

In this study, novel Zn-Sn-xBi high-temperature solders were designed to join sintered NdFeB permanent magnets and steel. The effect of Bi addition on the corrosion resistance and mechanical properties of the soldered joints was systematically investigated. The results indicated that introducing Bi with specific content obviously decreased the undercooling of the solder. Reducing the undercooling by promoting the solidification process and the formation of Bi precipitates as nucleation sites could refine the microstructure. The improvement of corrosion resistance was attributed to the finer microstructure and the Bi precipitates serving as anodic barriers. Moreover, the sound soldered joints were obtained with finer β-Sn dendrites and a more uniform distribution of Bi precipitates when appropriate Bi was added. The shear strength of the soldered joint with 5 wt% Bi addition was 36.6% higher than the shear strength of the ZS soldered joint.

Microstructure and growth kinetic study in Sn–Cu transient liquid phase sintering solder paste

The feasibility of the highly reliable and replicable microstructure formation of the transient liquid phase sintering (TLPS) paste during the early soldering and isothermal aging on the Cu substrate had been successfully investigated in this study. By using the Sn–0.7 wt% Cu (SC) solder paste as the base material and the Cu particles in the production of the TLPS Sn–10 wt% Cu (SC10) solder paste, the ensuing Cu6Sn5 phase from the isothermal aging process was found to have reduced the β-Sn area of the bulk SC10 solder microstructure. The growth kinetic for TLPS SC10 resulted in a 26.76 kJ/mol of activation energy level. The real-time synchrotron radiation imaging technique that was employed in studying the growth and formation of the primary intermetallic phases at the solder joints had also discovered the primary intermetallic in TLPS SC10 was not only found to have experienced an early nucleation just after the solder had melted, but its growth was also restricted prior to the solidification of the liquid solder. Therefore, the relevance of the results that were obtained from this research may offer a possible solution for aiding the future development of highly reliable solder joints in high temperature solder applications.

Wetting behavior and vacuum soldering of novel Au-30Ga solder on Ni and Cu substrate

As a novel high-temperature solder with suitable eutectic temperature and high thermal conductivity, Au-30Ga solder is expected to be used in high-power device packaging. This research provides a timely and necessary study of its solderability. The wetting behavior and reactivity of Au-30Ga solder on Cu and Ni substrates was investigated through spreading tests in vacuum. The favorable wetting property of Au-30Ga solder on Ni and Cu substrate was observed. However, fundamental differences in the wetting behavior of liquid Au-30Ga alloy in contact with Ni and Cu substrates were immediately apparent. Their behavior changed from reactive wetting, characterized by formation of two thin Ni–Ga compound layers and a wide wetting ring, to dissolutive wetting, characterized by formation of a thick (Au, Cu)9Ga4 interface layer. Moreover, significant signs of solidification shrinkage of liquid solder are observed on the surface of wetting ring for the first time, which provides direct evidence for the formation mechanism of wetting ring. The wetting ring also reveals the microstructure of the Ni–Ga compounds during ripening. In addition, the soldering performance was evaluated by the shear strength of solder joints. The findings presented in this thesis add to understanding of solderability of the high-temperature Au-30Ga solder.

Rapid, high-temperature microwave soldering toward a high-performance cathode/electrolyte interface

Solid-state lithium batteries using inorganic electrolytes are expected to revolutionize energy storage systems due to their better safety and high energy density. However, their application is greatly hindered by the poor solid-solid interface between the solid-state electrolyte (SSE) and electrodes, particularly the cathode. Herein, we report a facile strategy to address the high cathode/SSE interfacial resistance through rapid, high-temperature microwave soldering. As a proof-of-concept demonstration, we soldered a garnet-type Li7La3Zr2O12 (LLZO) SSE with a V2O5 cathode, which feature high thermal stability and suitable melting temperatures. Our microwave soldering technique can selectively melt the surface of the granular V2O5 and rapidly form an intact and continuous cathode layer with tightly embedded carbon black nanoparticles, leading to a remarkable 690-time increase of the electronic conductivity of cathode. Additionally, the melted V2O5 cathode is conformally soldered to the garnet electrolyte, resulting in a 28-fold decrease of the cathode/garnet interfacial resistance (from 14.4 ​kΩ ​cm2to 0.5 ​kΩ ​cm2. As a result, this all-solid-state full cell displays a low overall resistance of 0.3 ​kΩ ​cm2 at 100 ​°C, which enables stable cyclability of the battery without the addition of liquid/polymer electrolyte. The fast microwave soldering strategy constitutes a significant step towards the development of the all-solid-state batteries.

Effect of Minor Addition of Ni on the Microstructure and Properties of Zn-Based High-Temperature Solder

Lead-based solders are widely used for making interconnections between electronic chips and a circuit board that is expected to perform at high temperatures. The substitution of traditional lead-bearing solders is one of the key challenges to electronic industries in the current drive for eco-friendly manufacturing. In the present study, a minor amount of Ni is alloyed with Zn to investigate the effect of Ni on the microstructure and the thermal, mechanical and electrical properties of the newly developed solder. The microstructures of the Ni-Zn solder alloys are composed of β-Zn and Ni-Zn intermetallics (γ-phase). The appearance of a γ-phase Ni-Zn intermetallic (NiZn5) instead of a δ-phase intermetallic in a eutectic structure, grain boundary particles and an irregular second phase is discussed in detail. Addition of Ni results in enhanced mechanical properties, keeping the electrical properties unchanged. The suitability of Ni-Zn alloy as high-temperature solder is evaluated in comparison with traditional Pb-Sn solders and other Zn-based alloys.

STUDY OF THE PROPERTIES A FRAGMENT OF THE HYDRAULIC CIRCUIT OF THE SPACECRAFT, OBTAINED BY MEANS OF ADDITIVE TECHNOLOGIES

The article discusses the course and results of experimental work on the initial study of the possibility of using one of the varieties of additive technologies – the method of layer-by-layer selective laser melting (SLM) in the manufacture of elements of heat exchangers and hydraulic circuits of spacecraft. Traditional manufacturing techniques for hydro-control elements and spacecraft heat exchangers are based on machining and high-temperature vacuum soldering, leading to a long cycle and high manufacturing costs. As an alternative, the method of layer-by-layer selective laser melting can be considered as a manufacturing method using a three-dimensional model of the product and not requiring additional equipment. This method is based on sequential layer-by-layer fusion of a metal powder with previous fused product layers under the action of a laser beam forming a local region of liquid melt. The article describes experimental work to assess the possibility of using the selective laser melting method. Assessed weld-ability of a sample made by selective laser fusion technology with tips made by traditional technology. Directions for testing the method of selective laser sintering on real structures of heat exchanging units of spacecraft have been determined. A technique is proposed and the results of a study of a sample synthesized by selective laser sintering are presented. Based on the results obtained, an analysis is made of the prospects for using this method in the production of elements of hydraulic circuits and heat exchange units of spacecraft.

A Method for Improving the Thermal Shock Fatigue Failure Resistance of IGBT Modules

High temperature application and long-term reliability are the future development trends of IGBT (insulated gate bipolar transistor) module. This article proposes a 1200-V/50-A IGBT module using the current-assisted sintered nanosilver as die attachment and high-temperature solder as substrate attachment. We measured and compared the electrical and the thermal characteristics, and the reliability of the proposed IGBT modules with those of the commercial IGBT module of the same level. The proposed IGBT modules show consistency with commercial IGBT module in electrical performance, which also proved the feasibility of the current-assisted sintering. The thermal impedance of the proposed IGBT module decreased by 17.2% compared with that of the commercial one. And, the thermal shock test shows that higher resistance to thermomechanical fatigue can be successfully achieved.

Беспотенциальный корпус силового модуля

In this paper we present an optimal design of a ceramic-metal sealed power module package. In our design the leads of the package are mounted on alumina ceramic substrates with through vias, which have been soldered by the high-temperature silver solder at the slots in the package base plate. Base plate is made of pseudoalloy, thermal expansion of which is in a good agreement with alumina ceramics. Substrate made of highly conductive aluminium-nitride ceramics, which is designed for mounting the functional elements of the module, is soldered to the flange with 450-500°С melting point solder. Suggested design provides resistance to cyclic temperature changes of -196+100°C, eliminates thermomechanical effects leading to damage to ceramic substrates during sealing by roll-seam welding, and ensures minimal bending of the bearing surface of the flange.

Characteristics of eutectic and near-eutectic Zn–Al alloys as high-temperature lead-free solders

Lead-based alloys are conventionally used for soldering interconnections that are expected to perform at high temperatures. Because of the concerns of lead toxicity, the development of lead-free solders became important. Till now, none of the proposed alloys have the characteristics that can completely substitute the Pb-bearing solders for high-temperature soldering application. Recently, researchers studied many ternary and quaternary systems near Zn–Al binary eutectic composition and reported prospects in soldering applications. However, the bulk solder properties of the Zn–Al binary system near the eutectic composition is rare in the literature. In the present study, the microstructure and thermal, mechanical and electrical characteristics of eutectic and near-eutectic Zn–Al alloys are investigated as replacement of traditional lead-bearing high-temperature solders. Mechanical and electrical properties of Zn was enhanced by alloying with Al, without altering much of the thermal expansion co-efficient. Finally, the material properties of the prepared Zn–Al alloys and other Zn-based alloys are compared with the conventional Pb–Sn solders. Research shows, 7 wt% Al–Zn alloy has multiple times higher magnitude of ultimate tensile strength, Vicker’s microhardness and electrical conductivity than the traditional Pb–Sn high-temperature solders. The melting temperature of the alloys was a bit higher than the Pb–Sn solders. A suitable ternary element addition to the system can potentially replace the conventional Pb-based high-temperature solders.

Effect of Zn addition on Cu3Sn formation in Sn-10Cu alloys

Sn-Cu alloys have generated interest as potential candidates for high-temperature soldering applications, as they are economically feasible and environmentally safe. Sn-Cu alloys with copper contents beyond 7.6wt.% solidify with a primary Cu3Sn phase, followed by peritectic and eutectic reactions. Typically, phases which form as elongated needles are undesirable, as they tend to be more brittle and in turn reduce the strength and toughness of the alloy. The effect of Zn on the peritectic reaction and phase formation in a Sn-10wt.%Cu alloy is investigated through cooling curves and microstructural analysis using optical and electron microscopy. Zn was alloyed in the range of 0.1-1wt.% to the base Sn-10Cu alloy system. The results indicate that the amount of the primary Cu3Sn phase that forms is inversely proportional to the concentration of Zn. Although complete suppression of Cu3Sn was not observed, there was a clear effect of Zn reducing the amount of the primary Cu3Sn phase, with the greatest reduction of Cu3Sn occurring with a 1wt.% Zn addition. The reduction of the elongated needle-shaped primary Cu3Sn phase may find application in the development of more reliable bulk solder microstructures.

Characterization of Electroless Nickel Plated Materials During High Temperature Solder Processing

Electroless nickel immersion gold (ENIG) has emerged as a leading surface finish for under bump metallization (UBM). An ENIG surface typically exhibits excellent planarity, good electrical characteristics, wettability for solder, and wire bond-ability. Reliability and performance of ENIG systems have been extensively reported at temperatures below 250°C; however, information is not readily available about its reliability at higher temperatures. In this paper, thermal behavior of electroless Ni/Au, Ni/Pd and Ni/Pd/Au during solder reflow processing at temperatures of up to 350°C is characterized. Wafer level evaluations were completed using optical microscopy and FIB cross-sectional SEMs, while packaged devices were subjected to standard reliability tests. Electroless Ni began to exhibit signs of cracking when exposed to temperatures of 350°C under certain conditions. Based on these findings, we propose electroless Ni material structures and maximum processing temperatures to minimize stress cracking at high reflow temperatures. This paper builds on our previous work by expanding the tested material structures to a gold-free process of Ni/Pd as well as 5um Ni/Pd/Au. Power devices manufactured with electroless Ni metallization can be capable of passing JEDEC qualification when implemented with these design parameters.