Electronic Solder Research Articles

Dendrite Growth in Single-Grain and Cyclic-Twinned Sn–3Ag–0.5Cu Solder Joints

The microstructure of electronic solder joints is generated by the solidification of a small volume of bulk undercooled liquid. Here, we study β-Sn dendrite growth in Sn–3Ag–0.5Cu in the specific geometry and nucleation conditions of ball grid array (BGA) solder joints by combining electron backscatter diffraction and imaging of microstructures. It is shown that, while ⟨110⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 110\\rangle$$\\end{document} is the preferred dendrite growth direction, out-of-plane branching and growth with ⟨11W⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 11W\\rangle$$\\end{document} directions are important for allowing dendrites to fan out into the spheroidal volume of BGA joints due to the low symmetry of β-Sn. We find that the crystallographic orientation of β-Sn at the nucleation point plays a strong role in subsequent dendrite growth. In single-grain joints, dendrites are often unfavorably oriented for growth, resulting in different types of zig-zag dendrite growth. In cyclic-twinned joints, it is shown how competitive out-of-plane trunk growth between three dendrite orientations produces {101} boundaries and the characteristic beachball microstructure.

Chapter 9. Ultrasonic Soldering and Metallization in Electronics

Chapter 9. Ultrasonic Soldering and Metallization in Electronics

The role of microstructure in the thermal fatigue of solder joints

Thermal fatigue is a common failure mode in electronic solder joints, yet the role of microstructure is incompletely understood. Here, we quantify the evolution of microstructure and damage in Sn-3Ag-0.5Cu joints throughout a ball grid array (BGA) package using EBSD mapping of localised subgrains, recrystallisation and heavily coarsened Ag3Sn. We then interpret the results with a multi-scale modelling approach that links from a continuum model at the package/board scale through to a crystal plasticity finite element model at the microstructure scale. We measure and explain the dependence of damage evolution on (i) the β-Sn crystal orientation(s) in single and multigrain joints, and (ii) the coefficient of thermal expansion (CTE) mismatch between tin grains in cyclic twinned multigrain joints. We further explore the relative importance of the solder microstructure versus the joint location in the array. The results provide a basis for designing optimum solder joint microstructures for thermal fatigue resistance.

Strong composite SnAgCu solder joints with internal aligned carbon fibers by magnetic field

Lightweight carbon fibers with high mechanical performance are referred to as an ideal reinforcement for an extensive variety of composite materials. However, the addition of carbon fibers into electronic interconnection solders is extremely limited so that the expected reinforcement in the mechanical properties of the composite solder joints has not been completely realized. Here, we used magnetic fields to align Ni-coated carbon fibers in the Sn-3.0Ag-0.5Cu solder vertically with the Cu substrate and prepared the oriented composite solder joints. We achieved a further improvement in the shear performance of composite solder joints (∼58 MPa) with a small addition of Ni-coated carbon fibers (0.5 wt%), which is 25% higher than that of pristine joints. It is found that these vertically oriented carbon fibers changed the propagation path of shear cracks and thus increased the shear resistance. Ni-coated carbon fibers also promoted nucleation of interfacial Cu6Sn5 in the solder joints during reflow, but prevented the formation of brittle Cu3Sn phase during isothermal aging, thus ensuring the high shear strength of composite solder joints after isothermal aging. This success provides a new pathway to fabricate a strong composite solder joint with a limited addition of reinforcements by aligning their orientation.

Precipitation processes and phase interface structure of Ag3Sn during solidification of SnAg3Cu0.5/Cu solder joint

Ag-containing solder alloys are widely used in high-end electronic soldering processes to improve the reliability of electronic components. This study investigates the precipitation process of Ag3Sn during solidification. The precipitation of β-Sn, Ag3Sn, and η′-Cu6Sn5 in SnAg3Cu0.5/Cu solder joints during solidification was examined using Thermo-Calc (2020a). Furthermore, the crystallographic structure of Ag3Sn/β-Sn and Ag3Sn/η′-Cu6Sn5 was analyzed by transmission electron microscopy. The results showed that, below 217.1 °C, the η-Cu6Sn5 phase begins to precipitate: β-Sn + Ag3Sn+η-Cu6Sn5. At the Ag3Sn/β-Sn interface, the angle between (1‾21‾)Ag3Sn and (101‾)β−Sn planes was measured to be 176.4°, with a mismatch of 26.47 %. The Ag3Sn/Cu6Sn5 interface exhibited a specific crystallographic relationship of [223]Ag3Sn// [1‾12]Cu6Sn5, with an angle between (212‾)Ag3Sn and [402]Cu6Sn5 planes of 101.5°. The growth patterns of β-Sn, Ag3Sn, and Cu6Sn5 phases during isothermal aging were further investigated according to their interface orientation relationship, providing a basis for improving the stability of the soldering process.

Novel hydrometallurgical process for the recovery of copper from end-of-life mobile phone printed circuit boards using ionic liquids

Printed circuit boards, which are embedded in mobile phones, are complex composite materials containing valuable components of strategic and critical importance. With the number of mobile phone devices operating worldwide expected to reach 18bn by 2025, their low recycling rate leads to high tonnages of waste printed circuit boards (WPCBs) at their end of life. Copper, representing 58% of the total metal content in an end-of-life mobile phone (EoL-MP) and the main metal present in the printed circuit board, with an average of 27.8 wt %, offers a resource-rich source of this strategic metal for recovery. To overcome the limitations associated with conventional technologies for recycling WPCBs, use of greener technologies (ionic liquids (ILs) as leaching agents), offers greater potential for the recovery of copper from this waste stream. Presented here for the first time is an optimised hydrometallurgical process for recovery of copper involving pre-treatment, leaching, and electrowinning and the use of two task-specific ionic liquids. The pre-treatment step, using the IL [Bmim]BF4 proved to be a clean, efficient, and non-polluting system for the separation of solder and electronic components (ECs) from WPCBs, at 210 °C, 150 rpm for 15 min, favouring the enrichment of copper for a subsequent recovery process. The IL, [Bmim]HSO4, successfully leaches >99% of copper from pretreated WPCBs under the optimal conditions of 30 %v/v [Bmim]HSO4, 10 %v/v H2O2, and 60 %v/v H2O at 60 °C, 1:15 solid to liquid ratio for 2 h. Recovery of copper is achieved through direct electrowinning from the [Bmim]HSO4-leach solution, as electrolyte. Under controlled deposition conditions of 100 mA (4.2 V) during 2 h, 100% of copper (>99% purity) is recovered with an average current efficiency of 78% and an energy consumption of 4.5 kWh/kg Cu. This work combines a dual leaching and electrowinning system that is clean, environmentally friendly, minimises reagent use and spent wastes, permits multiple reuses of both the ILs, and maximises resource conservation through recovery of copper metal and return of secondary streams (electronic components and solder) for reuse in new or refurbished components or products.

Machine Vision Based Defect Detection Method for Electronic Component Solder Pads

<p>This paper proposes a machine vision based solder pad detection method to improve the detection accuracy and efficiency of PCB solder pad defects in electronic components due to missed detection and low detection efficiency. Firstly, preprocess the electronic component pad images collected by the visual system, then use threshold segmentation method to perform preliminary segmentation of the pad images. Then, the coarse segmented images are finely segmented using mean clustering method, and the fine segmented images are pixel edge extracted. Finally, the matrix subpixel edge detection method is used to improve the edge detection accuracy. Simulation experiments have shown that the proposed method can significantly improve the accuracy and speed of defect recognition.</p> <p> </p>

Design of a Multicomponent Alloy for Application in Electronic Components Solder.

Design of a Multicomponent Alloy for Application in Electronic Components Solder.

Effect of Surface Roughness and Droplet Size on Solder Wetting Angles.

Reactive wetting is a process ubiquitous in modern electronic soldering processes. Nonetheless, the dependencies of reactive wetting angles on surface roughness and droplet size are not well understood but become increasingly important for reliable miniaturization of devices. In this study, advancing contact angles of the Sn(l)/Cu(s) reactive solder system are investigated and the aforementioned dependencies are evaluated. Several surface roughnesses and a large array of droplet sizes are measured. Clear size dependencies are observed for both smooth and rough substrates, yet interestingly, the trends are inverted. This inversion of the size dependence of the wetting angle is discussed on the basis of the triple line pinning phenomenon, Wenzel's wetting model, and the consumption of Sn by a solder reaction. Quantitative models are proposed, with which size dependencies on rough and smooth surfaces can be better understood. Triple line pinning barrier energies are calculated. Larger pinning barriers are determined for rougher substrates.

First-Principles Study on IMC Formation and Interface Failure of Electronic Packaging Solder Joints

At present, Sn-3.0Ag-0.5Cu (SAC305) solder is one of the commonly used lead-free solders, and copper-clad laminate is the commonly used PCB substrate. SAC solder and copper substrate mainly generate two intermetallic compounds (IMCs) of Cu6Sn5 and Cu3Sn during the soldering process. Based on the first principles, this paper analyzes the formation difficulty of the two kinds of IMCs generated in the soldering, and the failure mechanism of the Sn/IMC and IMC/Cu interfaces. Firstly, based on the density functional theory (DFT), the “CASTEP” module in Material Studio (MS) is used to analyze the formation difficulty of the two IMCs from the point of the density of states, population analysis, and energy. The results show that Cu6Sn5 is easier to generate, which also explains the reason for the first generation of Cu6Sn5 in the initial stage of soldering. Then, Sn / IMC and IMC / Cu interfaces were established in MS, and the reason why solder joint failure mainly occurred at the Sn / IMC interface was explained by energy analysis.

Optimization And Analysis of Furnace Temperature Profile for The Processing Technology of Electronic Components

The soldering furnace is an important piece of equipment for processing and soldering electronic components in the integrated circuit design process. After the printed circuit boards are mounted with various electronic components, they are placed in the soldering furnace for heating so that the electronic components are automatically soldered to the circuit boards. In this process, the furnace temperature profile, which records temperature data of the electronic components, is an important reference indicator for measuring the effectiveness of electronic component processing. In the context of electronic component soldering, the transfer speed of the furnace and the setting of the temperature in each temperature zone are decisive in ensuring the quality of the component soldering. Based on the Newton's law of cooling and heat equation, this paper establishes several mathematical models to study the effect of the temperature setting of each temperature zone and the transfer speed of the soldering furnace on the furnace temperature profile with the help of multi-objective planning, circular search algorithm and other mathematical methods. Finally, the corresponding conclusions and optimization methods of furnace temperature profile are drawn, which have certain significance for the processing of electronic components.

Study on Microstructure and Mechanical Properties at Constant Electromigration Temperature of Sn2.5Ag0.7Cu0.1RE0.05Ni-GNSs/Cu Solder Joints

To solve the electromigration problem of micro-electronic connection solder joints, an ideal electromigration tester was designed, and the thickness of the intermetallic compounds (IMCs), average void diameter, grain orientation, failure, shear strength, and fracture path of Sn2.5Ag0.7Cu0.1RE0.05Ni-GNSs/Cu solder joints under constant-temperature electromigration were studied. The results indicate that the solder joints show evidence of typical electromigration polarity in the asymmetric growth of interfacial IMCs on the anode and cathode sides under the conditions of a current density ≥7 × 103 A/cm2 and an included angle between the c-axis of the β-Sn grains and the current direction θ ≤ 53.2°. The anode-side interfacial IMC is dominated by a Cu6Sn5 phase with a gradually increasing thickness, forming a Cu3Sn phase and showing evidence of microcracks. The Cu6Sn5 phase of the cathode-side interfacial IMC is gradually completely dissolved, and the growth of the Cu3Sn phase is accompanied by the formation of Kirkendall voids. The anisotropic diffusion of Cu atoms in the β-Sn of the micro-solder joints causes increased solder joint resistance and reduced shear strength. The shear fracture path of the solder joints moves from the cathode side near the IMC solder seam to the Cu3Sn interface. The shear fracture mechanism changes from ductile transgranular fracture dominated by β-Sn dimples to brittle fracture dominated by interfacial IMC cleavage and slip steps.

Multi-scale plasticity homogenization of Sn–3Ag-0.5Cu: From β-Sn micropillars to polycrystals with intermetallics

The mechanical properties of β-Sn single crystals have been systematically investigated using a combined methodology of micropillar tests and rate-dependent crystal plasticity modelling. The slip strength and rate sensitivity of several key slip systems within β-Sn single crystals have been determined. Consistency between the numerically predicted and experimentally observed slip traces has been shown for pillars oriented to activate single and double slip. Subsequently, the temperature-dependent, intermetallic-size-governing behaviour of a polycrystal β-Sn-rich alloy SAC305 (96.5Sn–3Ag-0.5Cu wt%) is predicted through a multi-scale homogenization approach, and the predicted temperature- and rate-sensitivity reproduce independent experimental results. The integrated experimental and numerical approaches provide mechanistic understanding and fundamental material properties of microstructure-sensitive behaviour of electronic solders subject to thermomechanical loading, including thermal fatigue.

A review on nanodispersed lead-free solders in electronics: synthesis, microstructure and intermetallic growth characteristics

A review on nanodispersed lead-free solders in electronics: synthesis, microstructure and intermetallic growth characteristics

An influence of substrate thickness on electrical conductivity of dip-soldered copper joints

Tin (Sn) metal is used as a major ingredient in various levels of solders (both lead bearing and lead free) to join electronic components whereas, copper is the most common substrate material to be in direct contact with electronic solders. In this regard, study on the reaction between tin and copper (Cu) is of great practical interest. Copper substrates of two thicknesses (0.3 and 3 mm) were immersed in liquid tin (maintained at 350°C) for the duration of 3mins. An immersion and redrawn speed maintained was 2.5mm/s. An interfacial reaction between substrate metals of different thickness and liquid tin vice versa was investigated. Effect of substrate thickness on electrical resistivity and electrical conductivity was accessed. It was observed that as the thickness of Cu substrate increases both electrical conductivity decreases.

Innovation Ultrasonic Assistant Soldering in Electronics

Innovative approaches in ultrasonic assistant soldering consist consists of increasing the activity of cavitation and accelerating diffusion processes at the interface between the solder and the soldering material. Besides that, it improves the effectiveness of cavitation processes in melts by saturating gas cavities with diamete rs that are smaller than the resonant sizes of cavitation germs. Gas saturation of liquids and melts raises level of cavitation pressure by 20 25%25%, that intensifies US processing of cleaning, soldering and metallization. Modelling diffusion process showed that the US activation increased the concentration of diffusing elements of Zn and Al in the interface depth by 15 20% on average, and the combined activation by the US and electric field increased it by 30 45%. Furthermore, as the energy quantity adsorbed by melt increases, increased amplitude and frequency of US vibrations induces concentration rise. The heat energy was also boosted by combining the activation of the melt–soldered material system with US vibrations energy and high current pulses. This allows for a faster increase in soldering temperature, as well as improved solder wettability.

Metallurgical, physical, mechanical and oxidation behavior of lead-free chromium dissolved Sn–Cu–Bi solders

According to the European Union's (EU) legislation in 2006, the Restriction of Hazardous Substances (RoHS) led to eliminating Sn–Pb conventional solders from electronic assemblies. Despite this elimination, the use of lead-bearing solders in car electronics was persisted, and eventually, in 2014 the lead usage prohibition was introduced for the automotive industry. The reliability and safety issues are still arguable in this industry, notwithstanding the lead-free solders substitution. The Sn–Cu solder is one of the alternatives modified by alloying elements to improve its moderate mechanical properties. In order to fabricate a cost-effective solder that has simultaneously a good level of mechanical properties and oxidation behavior, the Cr along with Bi was added to the Sn–Cu base solder. In this research, a comparative study has been conducted on the Sn-0.8Cu-1.8Bi-xCr (x = 0, 0.01, 0.1 wt.%). The effect of adding these alloying elements on the physical and mechanical properties is studied compared to Sn-0.8Cu and Sn-0.8Cu-1.8Bi solders. The physical properties of synthesized solders such as microstructure, XRD, thermal properties, wettability, oxidation behavior, and density were investigated. Also, these experienced mechanical tests such as uniaxial tensile test, microhardness test, and fractography to evaluate the effect of Cr and Bi on mechanical properties. The results showed that the addition of the minor amount of Cr along with Bi can consequently improve the microstructure, add some kinds of new IMCs, modifies the oxidation behavior and the wettability. The mechanical properties of the base solder got better by the adding these alloying elements.

Transient Numerical Modelling of the Pin-in-Paste Technology

The pin-in-paste technology is an advancing soldering technology for assembling complex electronic products, which include both surface-mounted and through-hole components. A computational fluid dynamics model was established to investigate the stencil printing step of this technology, where the hole-filling by the solder pastes is the most critical factor for acquiring reliable solder joints. The geometry of the transient numeric model included the printing squeegee, the stencil, and the through-holes of a printed circuit board with different geometries and arrangements. A two-phase fluid model (solder paste + air) was applied, utilizing the Volume of Fluid method (VoF). The rheological properties of the solder paste were addressed by an exhaustive viscosity model. It was found that the set of through-holes affected the flow-field and yielded a decrease in the hole-filling if they were arranged in parallel with the travelling direction of the printing squeegee. Similar disturbance on the flow-field was found for oblong-shaped through-holes if they were arranged in parallel with the squeegee movement. The findings imply that the arrangement of a set of through-holes and the orientation of oblong-shaped through-holes should be optimized even in the early design phase of electronic products and during the set of assembly processes. The soldering failures in pin-in-paste technology can be reduced by these early design-phase considerations, and the first-pass yield of electronic soldering technologies can be enhanced.

Effects of impurities on void formation at the interface between Sn-3.0Ag-0.5Cu and Cu electroplated films

Void formation is a critical reliability concern for solder joints in electronic packaging. The control of microstructures and the quantity of impurities in Cu electroplated films significantly affect void formation at the joint interface, but studies investigating these factors are rare. In this study, three Cu films (denoted as A, B, and C) are fabricated using an electroplating process. The Cu A film has a faceted grain texture embedded with twin boundaries, while Cu B and C films have similar columnar textures. After thermal aging at 200 °C for 1000 h, the SAC 305 (Sn-3.0Ag-0.5Cu) solder joints with Cu A and B films exhibit robust interfacial structures without voids. However, microstructural collapse is observed in the solder joint of SAC 305/Cu C, where many crevices are formed parallel to the interface. Based on the microanalysis, the concentration of impurities is higher in Cu C than in Cu A and B. Moreover, discrete voids rather than continuous crevices are present in the SAC305/Cu C system when the impurity concentration in Cu C is reduced. The findings demonstrate that impurity control in Cu electroplated film is critical for the control of void/crevice formation in electronic solder joints.

Soldered joints of Ag electrode to ultra-thin metallic Au layer on ceramic substrate

Among various methods of microjoining applied to electronic equipment soldering technologies are of great importance. In the construction of sensors a technique of a reflow soldering, common for microelectronic devices assembly, is rather not applicable because of the unique design of the sensor. Connecting external leads to thin metal layer of contact pads is a particular challenge. Special requirements are placed on devices and joints operating at cryogenic temperatures. The joints operating in cryo-temperatures should be characterized by good and stable electrical conductivity, tightness and the required mechanical strength. In the paper the new method of soldering of Ag leads to thin Au layer on ceramic substrate is presented. A special feature of the new soldering method is the very short time (2-3 seconds) in which the solder (In) is in the liquid state. The manufactured joints are characterized by a very low resistance as well as a linear and continuous current-voltage characteristic in the temperature range of 15 to 300 K.