Solder System Research Articles

Microstructural Study of Sn-3.0Ag0.5Cu Alloy on Cu/NiP-Coated Substrate by Addition of Fe, Sb and Ce

In this study, the effects of alloying elements Fe (0.2 wt.%), Sb (0.5 wt.%) and Ce (0.5 wt.%) on the Sn-3.0Ag-0.5Cu lead-free solder system were investigated. The solder alloy was reflowed on electroless Cu/NiP-coated substrates and their behavior such as microstructure, wetting angle and intermetallic compound (IMC) formation were analyzed. The results showed that the presence of Fe, Ce and Sb reduced the grain size of β-Sn as compared to solder without addition of alloying element. The addition of Fe and Ce was the most significantly refined the microstructure of Sn-3.0Ag-0.5Cu solder alloys. Sn3.0Ag0.5Cu0.2Fe0.5Ce solder was revealed better spreading behavior on Cu/NiP coated substrates which have the lowest IMC thickness of 0.558 μm.

Phase-field simulations of intermetallic compound evolution in Cu/Sn solder joints under electromigration

In this work, we present a preliminary model based on the multiphase-field formalism that is used to investigate the evolution of intermetallic compound (IMC) layers in the Cu/Sn solder system under electromigration conditions. The simulation considers the concurrent evolution of the Cu3Sn and Cu6Sn5 IMC layers under electrochemical driving forces. We use CALPHAD descriptions for the thermodynamics of the bulk phases (Cu, Sn and both IMCs) and incorporate electrochemical contributions to the total free energy of the microstructure in order to simulate the evolution of the interfacial microstructure due to mass–current couplings. The simulation considers grain-boundary diffusion as the dominant mechanism for chemical-potential-driven mass flux. The simulation considers the effect of current density and polarity on the growth of IMCs. The simulations are in turn compared to experimental measurements of growth rates, morphology of IMC grains and evolution of interface roughness. The analysis of experiments and simulations suggest that back-stress resulting from the non-equilibrium vacancy generation due to diffusive flux imbalance among the individual diffusants (Cu and Sn) plays a fundamental role in the evolution of the IMC layers. Overall, the simulations and experiments show good qualitative agreement.

Electrodeposition of lead‐free solder alloys

PurposeThe purpose of this paper is to enhance the understanding on the electrodeposition of various lead (Pb)‐free solder alloys, so that new studies can be carried out to solve processing issues.Design/methodology/approachThe paper reviews the available reports on the electrodeposition of tin (Sn)‐based solder systems and identifies the challenges in this area.FindingsCompositional control remains a major challenge in this area, where the achievement of desired composition for binary and ternary alloys is subjected to uncertainties. The use of chelating agents in the bath and optimization of parameters can assist the achievement of near‐desired alloy composition. Acidic plating baths are preferred due to their compatibility with photoresists but oxidation of stannous ions causes poor bath stability. Antioxidants, reducing agents and low oxygen overpotential anodes can suppress the oxidation rate and increase the lifespan of plating baths. Apart from chelating agents and antioxidants, various categories of additives can be added to improve quality of deposits. Surfactants, grain refiners and brighteners are routinely used to obtain smooth, fine‐grained and bright deposits with good thermo‐mechanical properties.Originality/valueThe paper provides information on the key issues in electrodeposition of Pb‐free solder alloys. Possible measures to alleviate the issues are suggested so that the electrodeposition technique can be established for mass production of a wider range of solder alloys.

Effect of Joint Size on Microstructure and Growth Kinetics of Intermetallic Compounds in Solid-Liquid Interdiffusion Sn3.5Ag/Cu-Substrate Solder Joints

The effect of joint size on the interfacial reaction in the Sn3.5Ag/Cu-substrate soldering system was examined. An experiment was conducted in which parameters such as bonding time, temperature, and pressure were varied at multiple levels. The morphology and thickness of all intermetallic compounds (IMC) were analyzed using the scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) techniques. An examination of the microstructures of solder joints of different sizes revealed that the size of the solder joint has no effect on the type of IMCs formed during the process. It was found that the joint size significantly affected the thickness of the intermetallic layers. The Cu3Sn intermetallic layers formed in the smaller sized solder joints were found to be thicker than those in the larger sized solder joints. In all specimen sizes, the increase in the thickness of Cu3Sn intermetallic layers with soldering time was found to obey a parabolic relationship. Additionally, for the cases when eutectic solder is available in the joints, a similar soldering time and temperature dependency were found for the Cu6Sn5 IMC phase. The intermetallic growth of the Cu3Sn phase was under a volume-diffusion controlled mechanism. The growth rate constants and activation energies of intermetallic layers were also reported for different joint thicknesses. Furthermore, the growth rate constants of the Cu3Sn intermetallic layer were found to depend upon the size of the joints.

The effect of micro via‐in pad design on surface‐mount assembly defects: part II – voiding and spattering

PurposeThe purpose of this paper is to propose a solution procedure to minimize/eliminate voiding and spattering defects in the assembly of 0201 chip components with micro via‐in pads and 95 wt.%Sn‐5 wt.%Sb solder alloy.Design/methodology/approachIn total, four different micro via‐in pad designs were compared (via‐hole opening size): ultra small via‐in pads (d: 10 μm), small via‐in pads (d: 20 μm), and large via‐in pads (d: 60 μm), as well as designs with no via‐in pads and capped via‐in pads. Two process variables were also evaluated for the goal of achieving a high‐yield assembly solution in micro via‐in pad and lead‐free solder systems. Potential factors, such as the preheat conditions of the reflow profile and stencil aperture size, which might affect voiding and spattering in solder joints with micro via‐in pad, were investigated. Solder voiding frequency and size were also determined from X‐ray inspection and sample cross‐section analysis.FindingsThe results indicated that larger via‐holes were seen to create bigger voiding than smaller via‐holes. For smaller via‐holes, spattering is a greater problem than voiding in solder joints. Ultra small via‐in pads generated higher spattering compared to no via‐in pads and capped via‐in pads. Capped via‐in pads exhibited the best results in preventing voiding and flux spattering, and provided a wide process window for the selection of process parameters. It is also indicated that spattering was found to rapidly reduced with both increasing stencil opening size and use of reflow profile with long‐preheat conditions.Originality/valueThe findings provide certain process guidelines for surface‐mount assembly with via‐in pad substrate design. The strategy is to prevent voiding and spattering by adopting capped via‐in pads, if possible, when applying micro via with the 95 wt.%Sn‐5 wt.%Sb solder alloy system.

Recent Development of Novel Lead-Free Composite Solders Using Microwave-Assisted Sintering Powder Metallurgy Route

This review paper discusses on the recent lead-free composite solders developed using powder metallurgy (PM) routes. However, the review will focus on the sintering part in the powder metallurgy techniques which the application of microwave technology can be utilized in the development of robust lead-free composite solders. Comprehensive reviews of the published literatures were carried out to understand the requirements and problems in fabricating the lead-free composite solder by using microwave-assisted sintering PM route. This paper describes the improvement of the recent development of lead-free composite solders using microwave-assisted sintering application. It can be summarized that by adding appropriate amounts of a third element as reinforcement in lead-free base matrix system has mostly improved mechanical properties of the monolithic solder. Nevertheless, the review discovered with microwave-assisted sintering, most lead-free bulk samples exhibited a superior microstructural feature which has brought to surprisingly a better mechanical properties in the solder compared to conventionally sintered samples. With microwave-assisted sintering application, the issues related to the implementation of microwave-assisted sintering in lead-free solder development based solder system reinforced with ceramic particulates are also introduced. Furthermore, with the implementation of new hybrid microwave sintering technique which arises from the coalition of conventional and microwave sintering, will yield a staggering advancement in lead-free nanocomposite solder development.

Influence of TiO2 nanoparticles on IMC growth in Sn–3.0Ag–0.5Cu–xTiO2 solder joints in reflow process

The influence of TiO2 nanoparticles on the growth kinetics of intermetallic compound (IMC) between Sn–3.0wt.% Ag–0.5wt.% Cu–xwt.%TiO2 (x=0, 0.02, 0.05, 0.1, 0.3, and 0.6) solder and copper substrate during reflow process has been investigated in this study. Scanning electron microscope (SEM) was applied to observe the microstructural evolution of the solder joints, to measure the thickness of the IMC layer, and to estimate the grain size of the IMC layer. The IMC phases were identified by energy-dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD). Results show that both the thickness and grain size of IMC decrease when TiO2 nanoparticles are added into the Sn–3.0Ag–0.5Cu solder system, and have a significant drop when the weight percentage of TiO2 nanoparticles is about 0.1wt.%. Beyond this amount, the thickness and grain size of IMC increase slightly. The growth exponents for both the IMC layers and grains are determined by curve-fitting to study the growth kinetics of IMC in the soldering reaction process. Results reveal that the growth exponents of the IMC layer range from 0.354 to 0.358, and those of the IMC grains range from 0.332 to 0.346, which suggests that the growth of the IMC layer is controlled by the combined kinetics process of atomic interdiffusion, interfacial reaction, and grain ripening. These data also show that Sn–3.0Ag–0.5Cu with about 0.1wt.% TiO2 nanoparticles solder system exhibits the smallest growth rate and gives the most prominent effect in suppressing IMC growth and refining IMC grain size. Based on the theory of adsorption, heterogeneous nucleation, and Ostwald ripening, a mechanism for retarding the IMC growth rate due to TiO2 nanoparticles addition is proposed.

Research Development of Solder Materials and its Intermetallic Compound (IMC) Study

Nowadays, the formation and the subsequent growth of the intermetallic compounds (IMCs) and the development of new lead-free solder systems is a major issue in soldering. The excessive growth formation of intermetallic compounds will increase the brittleness of solder joints and detrimentally affects its mechanical properties. This paper reviews the latest fabrication method for solder materials and the literatures of bulk IMCs study in most solder materials by other researchers. Explanation on solder fabrication by using powder metallurgy method to produce solder materials and IMCs study were explained in detail in this paper.

Characterization of a soldering system consisting of a glass from the system CaOAl2O3B2O3SiO2 and wollastonite

Abstract In this work, the characterization of a soldering system with wollastonite as filler added as powder to the powdered base glass is shown. A soldering system consisting of the base glass and a coexisting crystalline phase is created which is then pressed to pellets and sintered to maximum density. Wollastonite increases the thermal expansion coefficient of the base glass after sintering the samples. In the present paper, the sintering behavior of the soldering system and possible interactions between glass and filler are analyzed. Also, density and porosity after sintering and the resulting crystalline phases are characterized. The mixtures with wollastonite contents up to 30% showed suitable sintering behavior and good density (low porosity). From 40% wollastonite on, the sintering of the samples was not satisfying and porosity was still high after sintering. The main crystalline phases were anorthite, wollastonite and pseudowollastonite, which occurred in all samples. This shows that all three phases are coexisting phases of the glass composition.

The Interphases Formed During the Very Early Stage Liquid Solder/Metal Substrate Interaction of the Soldering Process

The initial step of the soldering process may consist of dissolution of metal substrate and the subsequent counterdiffusion between substrate atoms and solder elements. The counterdiffusion gives rise to the formation of the various interphases (phases at the interfacial region). A close investigation of the interphases formed at the very early stage of soldering will provide the root understanding to the soldering process. No in situ observation at the liquid/solid interaction was possible thus far. A number of studies attempted to adopt rapid quench of the soldered specimen at the very early stage of interaction, around 10 s or less after solder melts, so as to freeze the occurrences. The current article provides a review of the achievements of these studies, although not many, and the interphases formed during the very early stage of the soldering process for various solder systems.

Physical and electrical attributes of sintered Ag80–Al20 high temperature die attach material with different organic additives content

In this work, the primary focus was to establish a relationship between the post-sintered physical attributes of the high temperature Ag80–Al20 die attach material and its electrical performance. The post-sintered Ag80–Al20 die attach material depicted the formation of Ag2Al and Ag3Al compounds. The melting point and maximum operational temperature for the Ag80–Al20 die attach material was determined to be 518 ± 1 °C and approximately 400 °C respectively, whereby the maximum operational temperature was based on a homologue temperature ratio of 0.85. The die attach material also demonstrated good electrical properties, i.e., an electrical conductivity value of 1.005 × 105 (ohm–cm)−1, which is higher than or equal to that of most solder systems. By varying the nanoparticle versus organics content between 83.3 and 87.0 %, it was seen that the surface morphology of the die attach material changed and the root-mean-square roughness values reduced to 175.1 nm. A similar observation was seen as the sintering temperature increased between 100 and 380 °C. This reduction in surface roughness proved that there was grain growth and particle coalescence within the die attach material. This translated to a reduction in electrical resistivity. Die attach area and thickness simulations found that smaller and thinner die attach areas are preferred for the Ag80–Al20 die attach material, whereby the highest recorded electrical conductivity value was 1.006 × 105 (ohm–cm)−1 for an area of 0.2 × 0.2 cm2 and thickness of 25.4 μm.

Systematic mechanical design approach for a flexible printed circuit board assemblies (PCBA) rework cell: part II – conceptual design of soldering and desoldering system

PurposeThe purpose of this paper is to apply the developed systematic mechanical design methodologies, that are obtained in part I, to investigate their success in designing mechanics of a flexible printed circuit board assembly (PCBA) rework cell.Design/methodology/approachThe decision of soldering and desoldering tool, which is the most critical function of a PCBA rework or remanufacturing cell, significantly influences overall design concept. Therefore, the paper starts by applying the design methodology to the soldering and desoldering function. The same study is repeated for the rest of the sub‐functions but only the results are provided.FindingsAn application of rework machine design methodology for the design of a PCBA rework cell has been made available. In addition to this, the embedded knowledge, such as the requirements list, the function structure, the function/means tree, the weighted objective tree and evaluation chart for the soldering and desoldering function are provided.Practical implicationsThe paper is the first work providing both embedded knowledge and the application of the systematic design methodology for the design of a fully automated flexible PCBA rework cell. The methodology leads rework machine designers in a well‐controlled and structured design environment.Originality/valueThe design methodology can be applied to all functions or targeted on key weighted areas to ensure that the designed rework machine meets the key areas of concerns. Furthermore, the methodology is generic and may be used to develop other complex manufacturing sytems.

Development of Metallic Hermetic Sealing for MEMS Packaging for Harsh Environment Applications

Hermetic sealing of microelectromechanical system sensors is indispensable to ensure their reliable operation and also to provide protection during fabrication. This work proposes two prospective candidates for hermetic sealing for rugged environment applications, i.e., Al-Ge and Pt-In. Al-Ge was chosen due to its compatibility with complementary metal–oxide–semiconductor technology. Pt-In possesses the highest remelting temperature among all the solder systems, which is desired for high-temperature applications in both the energy and aerospace industries. The various bonding parameters for Al-Ge eutectic bonding and Pt-In transient liquid-phase (TLP) bonding have been optimized, and their influence on the bond quality is reported. Optimization of bonding parameters has been carried out with the objective of ensuring void-free bonds. A new configuration for stacking Al-Ge thin films has been demonstrated to tackle the issue of loss of Ge prior to bonding, since native Ge oxides are soluble in deionized water. The impact of solid-state aging prior to Al-Ge eutectic bonding has been investigated. The method of tailoring the phases in the Pt-In joint is also discussed. The prospects and constraints of eutectic and TLP bonding from the hermeticity perspective are discussed in detail. Furthermore, changes in the microstructure under aging at 300°C up to 500 h and the resulting influence on the mechanical properties are presented. The overall finding of this work is that Al-Ge can achieve better mechanical and hermetic performance for high-temperature applications.

IMC growth of Sn-3.5Ag/Cu system: Combined chemical reaction and diffusion mechanisms

The growth kinetics of intermetallic (IMC) compound layers formed between Sn-3.5Ag solders and Cu substrate in soldering process are investigated experimentally and analytically. In order to determine the governing mechanisms of IMC formation, an experiment is designed in which samples are fabricated with bonding temperature varied at three levels of 260, 310 and 360°C while time is varied at seven levels between 1 and 240 min. Microstructural analysis is conducted to analyze the IMC thickness and morphology. A physics-based analytical model (Dybkov), in which net growth of each IMC phase is modeled as result of diffusion and partial chemical reaction, is modified for a case of liquid–solid state for two IMC layers. Dissolution of solid IMCs into liquid solder is included in the model. The model is then used to evaluate governing mechanisms of IMC formation and growth and determine the activation energies for formation and growth of different IMC compounds. The chemical reaction and diffusion rate constants have also been evaluated for different intermetallic compounds of Sn-3.5Ag/-Cu-substrate soldering system. Two intermetallic phases were observed during soldering at the interface: η-phase (Cu6Sn5) and ε-phase (Cu3Sn) IMC layers. The thickness of the η and ε IMC phases increase with increasing the soldering time and/or soldering temperature. The increase in the IMC layer thickness during the process is found to obey linear relationship with time during the chemical reaction-controlled stage of formation of Cu3Sn IMC layer and it obeys a parabolic relationship with time during the diffusion-controlled growth stage of formation of both IMC layers. It was shown that as temperature increases, the chemical reaction and diffusion rate constants monotonically increase. Modeling results indicate clear distinction of governing mechanisms of growth at different IMC growth stages with chemical reaction-controlled growth at initial stage and diffusion-controlled growth at final stage of IMC formation. The apparent activation energy calculated for the chemical reaction-controlled growth stage is 20.59 (KJ/mol) for ε IMC phase. For diffusion-controlled growth stage, the activation energy values obtained are 41.98 (KJ/mol) for η IMC phase and 50.10 (KJ/mol) for ε IMC phase. These values slightly differ from values reported in literature where a simple power-law fit is used to extract those quantities. This is expected because unlike present study, the power-law fit does not explicitly incorporate the interaction between the chemical reaction and diffusion mechanisms in IMC growth. The values obtained in the current study are still within the range of the values found in the literature.

Multi-physics modelling of a vapour phase soldering (VPS) system

This paper presents a novel multi-physics modelling approach of the Vapour Phase Soldering (VPS) process. The model applies combined transport mechanisms including heat transport by heat conduction; mass transport by diffusion in the fluids and by phase change during the evaporation and condensation of the Galden liquid; and energy transport caused by mass transport. The model can describe the temperature and vapour concentration relations in the oven – which are the main parameters of the vapour phase soldering process – from the start of the Galden evaporation until the saturation of the vapour space (prepared for soldering). The results calculated by the model show proper conformation with the experimental results of temperature and pressure measurements. The model was tested on a batch type VPS system where the position of the processing volume was optimized in the soldering tank. With the information obtained from the model the whole VPS process can be investigated.

Modeling of Multi-Phase Solid State Reactions-Case of IMC Growth

The formation of intermetallic compounds (IMC) at the solder-substrate interface is required to initiate the metallurgical bond. However, rapid growth of IMCs may degrade joint strength through i) the increased presence of a low toughness phase, ii) the consumption of the solderable surface (void formation) and iii) generation of primary and secondary stresses. Knowledge of mass transport and reaction processes during joint formation and service life are essential for solder system design. The mathematical description of inter-and reactive diffusion in open system presented here is based on Darken method (bi-velocity), involving the different molar volumes in the system and Wagner boundary conditions. It combines the interdiffusion, reactive diffusion and the effective flux constraints to couple processes occurring at different time scales. The rCADiff software serves as a tool to simulate simultaneous growth of the two Cu-Sn IMCs.

Diffusion of Cu and interfacial reactions during reflow of Sn–8.5Zn–0.5Ag–0.01Al–0.1Ga alloy on Ni/Cu substrate

Abstract

Rehm delivers its 3,000th soldering system

Rehm delivers its 3,000th soldering system

Bismuth-Antimony as an Alternative for High Temperature Lead Free Solder

The development works on high temperature lead free solder are mostly discussed nowadays. To replace the current high temperature lead free solders, further research need to be done. A great deal of effort has been put into the development of lead free solder alloys. Bi (Bismuth) and Sb (Antimony) solder system proved as one of the promising candidates for electronic assembly. Melting temperature of three Bi-Sb solder alloys studied in this research enhanced their potential as the alternative solder candidates for high temperature lead free solder. At interface, Cu3Sb IMC layer was formed for 95Bi-5Sb solder alloy. Spallation of Cu3Sb IMC layer took placed with the results of Cu3Sb IMC also found in the solder bulk. Analysis of 97.5Bi-2.5Sb solder alloy classified as no metallurgical reaction at the interface and only the mechanical joining existed at the interface. The dissolution of Cu from subtrate affected the formation of Cu rich phase and the unstable Bi-Cu rich phase phenomena act as the isothermal product found in solder bulk. Mechanical grain boundary grooving observed in 98.5Bi-1.5Sb solder alloys at interface. Different compositions of Bi-Sb solder alloys resulted in different types of microstructures at interface and in solder bulk after reflow.

Premelting behavior and interfacial reaction of the Sn/Cu and Sn/Ag soldering systems during the reflow process

The early interfacial reaction and premelting characteristics of the Sn/Cu and Sn/Ag soldering systems, and the formation and morphology transition of interfacial intermetallic compounds in the two systems during the reflow soldering process were investigated by using a differential scanning calorimeter. Results show that the initial interfacial eutectic reaction arising from atomic interdiffusion in solid-state Sn/Cu and Sn/Ag systems results in the premelting at each interface of the two systems at a temperature 4.9 and 10.6 °C respectively lower than the actual melting point of pure tin, and consequently both of the Sn/Cu and Sn/Ag soldering systems exhibit morphology change of the intermetallic compound (IMC) as the solder experiences a transition from solid-state to liquid-state in a very small temperature range. The change in the interfacial energy between the solid (or liquid) Sn-rich phase and IMC phase is the essential factor leading to the morphology transition of the interfacial IMC in the Sn/Cu and Sn/Ag soldering systems.