Microstructure Of Solder Joints Research Articles

The Evolution of IMCs in Single Crystal Sn3.0Ag0.5Cu and Sn3.0Ag3.0Bi3.0In BGA Solder Joints with Au/Ni/Cu Pads Under Current Stressing

The growth behavior of intermetallic compound (IMC) in single crystal Sn3.0Ag0.5Cu (SAC305) and Sn3.0Ag3.0Bi3.0In (SABI333) ball grid array solder joints with Au/Ni/Cu pads under 104 A/cm2 current stressing was investigated. Characterization by scanning electron microscopy and electron backscattered diffraction mapping were utilized to identify the microstructure and crystal orientation of solder joints. The AuSn4 IMC particles in the SAC305 solder matrix were formed along the electron flow direction and c-axis direction of Sn. On the other hand, it was observed that the Au(Sn0.83, In0.17)4 IMC particles generated in SABI333 solder matrix decomposed from needle-like morphology into small pieces of Au(Sn0.23, In0.77)2 during current stressing. This phenomenon depends on the different diffusion rate and opposite migration direction of Sn and In atoms, which have different driving forces. Moreover, the results showed that the growth behavior of IMC particles in SAC305 solder joints was significantly dependent on the c-axis direction of Sn, while that of IMCs in SABI333 was almost maintained without polarization effect. This study indicated that SABI333 solder joints under EM service could exhibit a better performance than that of SAC305 solder joints.

Critical Review of Size Effects on Microstructure and Mechanical Properties of Solder Joints for Electronic Packaging

With the miniaturization of electronic packaging and devices, the size of solder joints in electronics is also decreasing from bulk solder joints to micro-bumps. Both the microstructure and mechanical properties of the solder joints are also evolving with the decreasing size, which brings great concern for the reliability of different sizes of solder interconnections. In this paper, the effect of solder size on the microstructure (i.e., interfacial intermetallic compound (IMC) growth, precipitation in the solder matrix, dendrite arms, and undercooling) and mechanical properties (i.e., tensile property, shear and compression strength, fracture toughness, and creep deformation) are reviewed from the mechanical point of view. In addition, some areas for further researches about size effects on solder joints are discussed.

Effect of Ni concentration on solderability, microstructure and hardness of SAC0705-xNi solder joints on Cu and graphene-coated Cu substrates

In this paper, solderability, microstructure and hardness of SAC0705-xNi solder joints on Cu and graphene-coated Cu (G-Cu) substrates were studied. As Ni content increases in the solder, the solderability improves gradually on both the Cu and G-Cu substrates. The solderability of SAC0705-xNi is better on G-Cu substrate than that on Cu substrate. The increasing Ni content in the solder has a positive effect on the microstructure refinement of both the kinds of substrates. Such effect is more significant on G-Cu substrate than that on Cu substrate. With the increase of Ni content, the thickness of the interfacial intermetallic compound (IMC) shows an increasing trend first and then decreasing trend on the two kinds of substrates. Since the graphene layer works as a diffusion barrier, the IMC on G-Cu is thinner than that on Cu substrate. The addition of Ni leads to the strengthening of the microstructure and thus increases the hardness of the solder bulks.

The Effect of Solder Joint Microstructure on the Drop Test Failure—A Peridynamic Analysis

We investigate the drop reliability of different solder joint microstructures using a coupled board-level finite-element (FE) analysis and the microscale peridynamic (PD) simulations that can capture fracture in the heterogeneous solder joint. A new PD model for elastic behavior across a material interface is introduced and used in computing the damage and failure in the two-phase microstructure of a solder joint. This new model eliminates oscillations in strains at an interface. The microstructural geometry is digitized from the scanning electron microscopy images of experimentally tested samples. To simulate the drop test conditions, we employ the input acceleration method in the board-level FE model and calculate nodal velocities for the boundary conditions imposed on the microscale PD model. We test two different microstructures of intermetallic component (IMC) (AuSn4) and SAC305: flash and thick Au content. The results show significantly more fracture in the samples with larger Au content compared to the flash samples, which correlate well with experimental observations. The computed results show that the failure mechanism for solder joints with high Au content is a fracture through the AuSn4 IMC as well as along the interface between the inclusions and the matrix. A newly introduced measure, the quasi-damage index, estimates the locations in the microstructure of the solder joint with the highest risk for initiation of cracks and damage.

Microstructure, IMCs layer and reliability of Sn-58Bi solder joint reinforced by Mo nanoparticles during thermal cycling

The microstructure, IMCs layer and reliability of Sn-58Bi and Sn-58Bi-0.25Mo solder joints during thermal cycling were investigated. The results showed that the microstructure of Sn-58Bi and Sn-58Bi-0.25Mo solder joints is coarsened with increasing thermal cycling times and it is refined by adding Mo nanoparticles. The IMCs morphologies of Sn-58Bi and Sn-58Bi-0.25Mo solder joints are both changed to be flat and thick with increasing thermal cycling times and fewer voids are found in the IMCs layer of Sn-58Bi-0.25Mo solder joint than that of Sn-58Bi solder joint. The IMCs layer thickness of Sn-58Bi and Sn-58Bi-0.25Mo solder joints is increased with increasing thermal cycling times due to the formation of Cu6Sn5 and Cu3Sn compounds, and slow IMCs layer growth is obtained by Mo-reinforced solder joint during thermal cycling. The shear and tensile strength of Sn-58Bi-xMo (x = 0, 0.25) solder joints are both decreased with increasing thermal cycling times and the mechanical properties of Sn-58Bi solder joint are improved by adding Mo nanoparticles. The fracture mode of Mo contained composite solder joint is mixed with ductile and brittle fracture with lots of dimples, and it is changed to brittle fracture after 1200 thermal cycles with the reduction in dimples.

Effect of Ni addition into the Cu substrate on the interfacial IMC growth during the liquid-state reaction with Sn–58Bi solder

The interfacial reactions between Sn–58Bi solder and Cu–xNi (x = 0, 0.5, 1.5, 5 and 10 wt.%) substrates at 200 °C with different liquid-state reaction durations were investigated to reveal the effect of the Ni addition into the Cu substrate on the growth of intermetallic compound (IMC), and grains evolution in this study. The results of this research indicated that addition of Ni significantly changed the interfacial microstructure of solder joints. Moreover, the formation of Cu3Sn was suppressed with the addition of Ni element, the Cu3Sn was only observed on Sn–58Bi/Cu–0.5Ni and Sn–58Bi/Cu–1.5Ni interfaces after liquid-state reaction for 150 min while Cu3Sn was completely eliminated when 5 or 10 wt.% Ni was added to Cu substrate regardless of the reaction time, and one new single phase (Cu,Ni)6Sn5 was formed between solder and Cu–Ni substrate. The total thickness increased as the liquid-state reaction time increased and the line relationship existed between total thickness and reaction time. The total thickness and growth rate of IMC gradually increased with the mass percentage of Ni increasing from 0 to 5 wt.% for the same liquid-state reaction time. In contrast, when the Ni content increased to 10%, the thickness and growth rate decreased slightly. The grain size became fine and the prismatic grain perpendicular to the interface tended to be dominant as the Ni element was added.

Mechanical Property of Sn-58Bi Solder Paste Strengthened by Resin

Sn-58Bi solder has been widely used for microelectronics packaging due to its low melting point temperature, good wetting performance, good mechanical properties, and low cost. Compared with Sn-Bi solder alloy and Sn-Pb solder alloy, the strength and plasticity of Sn-Bi solder are not enough, due to the higher brittleness of bismuth, which thus limits the application of Sn-Bi solder. In order to improve the properties of Sn-Bi solder, a novel solder paste strengthened with resin was developed by mixing epoxy resin (ER) with Sn-58Bi solder, which enhanced the joint strength at a low cost. Aimed at the electronic industry, in this study, the spreadability of the novel solder paste was investigated, and the mechanical properties and microstructure of solder joints after reflow soldering were tested and analyzed. The results showed that when the content of epoxy resin was in the optimum range, the shear strength was significantly higher, reaching nearly twice that of Sn-58Bi solder alone.

Effect of gamma-ray irradiation on microstructure and mechanical property of Sn63Pb37 solder joints

In order to meet the reliability requirement of solder joints for future small satellite, the effect of γ-ray irradiation on the microstructure and mechanical property of Sn63Pb37 solder joints was investigated by scanning electron microscope observation and mechanical testing. The results showed that γ-ray irradiation possessed few influence on the thickness and morphology of intermetallic compound layer of Sn63Pb37/Cu. But the mechanical properties of solder joints were reduced with micro-voids and micro-cracks appearing in Pb-based solid solution during irradiation process. Energetic electrons created by γ-ray through Compton Effect, generated dislocated atoms which eventually resulted in the formation of microscopic defects and the decrement of mechanical property. After 960 h γ-ray irradiation at the dose rate of 0.25 Gy(Si)/s, the shear force of Sn63Pb37 solder joints was decreased by 11.47%. Compared with as-soldered joint, the ductility of irradiated solder joints was reduced with more unobvious shear dimples and lower impact toughness.

Microstructure growth and tensile strength of Cu/Sn3.0Ag0.5Cu/Cu solder joints

The reliabilities of solder joints, which are closely related to the microstructure and mechanical properties of solder joints, are important for electronic packaging. In this work, the growth behaviors of microstructure in Sn3.0Ag0.5Cu/Cu solder joints, including intermetallic compounds and Sn dendrites, were investigated when different cooling methods—quenched in water, cooling in air, and cooling in a furnace after reflow—were used. Local enrichment of element was observed with the quadrant backscattering diffraction and energy dispersive spectrometer in the sample quenched in water. The growth mechanism of microstructure in Sn3.0Ag0.5Cu/Cu solder joint was discussed. The results of tensile tests indicated that the enrichment of element induced a hardening effect. Moreover, plate-like Ag3Sn compounds and oversize Sn dendrites were apt to reduce the tensile strength of solder joint.

Analysis of high-temperature aging and microstructure of FC LED solder joints of a silver conductive layer bonded using SAC solder

Flip-chip (FC) light-emitting diode (LED) filaments with a silver conductive layer are impeded by heat dissipation, with the flip-chip junction temperature of reaching up to 423.15 K. To overcome this problem, we simulated aging of SAC305 solder joints at a high temperature of 433.15 K. Scanning electron microscopy was performed to analyze the changes in the microstructure of the solder joints with aging time. Moreover, energy-dispersive X-ray spectroscopy was performed to analyze changes in the element content of the solder joints with aging time. The shear force of solder joints was measured using a strength tester. Results revealed that the Ag/Au content of the solder joints increased under the high-temperature conditions, resulting in the formation of cracks and holes in the solder joints. Furthermore, the reliability of the FC LED filaments was affected by the maximum shear stress that can be tolerated by the solder joints.

Effect of Epoxy on Mechanical Property of SAC305 Solder Joint with Various Surface Finishes Under 3-Point Bend Test.

Microstructures and mechanical property of Sn-3.0Ag-0.5Cu (SAC305) and epoxy Sn-3.0Ag-0.5Cu (epoxy SAC) solder joints were investigated with various surface finishes; organic solderability preservative (OSP), electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG). Bending property of solder joints was evaluated by 3-point bend test method. Microstructure and chemical composition of solder joints was characterized by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), respectively. Epoxy did not effect on intermetallic compound (IMC) morphology. Scalloped shaped Cu6Sn5 IMC was observed at OSP surface finish. Chunky-like shaped and needle-like shaped (Ni,Cu)6Sn5 IMC were observed at the solder/ENIG joint and solder/ENEPIG joint, respectively. The bending cycles of SAC305/OSP joint, SAC305/ENIG joints and SAC305/ENEPIG joints were 720, 440 and 481 cycle numbers. The bending cycles of epoxy SAC and three types surface finished solder joints were over 1000 bending cycles. Under OSP surface finish, bending cycles of epoxy SAC solder was approximately 1.5 times higher than those of SAC305 solder joint. Bending cycles of epoxy SAC solder was over twice times higher than those of SAC305 solder with ENIG and ENEPIG surface finishes. The bending property of epoxy solder joint was enhanced due to epoxy fillet held the solder joint.

Effect of Long-Term Room Temperature Aging on the Fatigue Properties of SnAgCu Solder Joint

Solder joints in electronic assemblies are subjected to mechanical and thermal cycling. These cyclic loadings lead to the fatigue failure of solder joints involving damage accumulation, crack initiation, crack propagation, and failure. Aging leads to significant changes on the microstructure and mechanical behavior of solder joints. While the effect of thermal aging on solder behavior has been examined, no prior studies have focused on the effect of long-term room temperature aging (25 °C) on the solder failure and fatigue behavior. In this paper, the effects of long-term room temperature aging on the fatigue behavior of five common lead-free solder alloys, i.e., SAC305, SAC105, SAC-Ni, SAC-X-Plus, and Innolot, have been investigated. Several individual lead-free solder joints on printed circuited boards with two aging conditions (no aging and 4 years of aging) have been prepared and subjected to shear cyclic stress–strain loadings until the complete failure. Fatigue life was recorded for each solder alloy. From the stress–strain hysteresis loop, inelastic work and plastic strain ranges were measured and empirically modeled with the fatigue life. The results indicated that 4 years of room temperature aging significantly decreases the fatigue life of the solder joints. Also, inelastic work per cycle and plastic strain range are increased after 4 years of room temperature aging. The fatigue life degradation for the solder alloys with doped elements (Ni, Bi, Sb) was relatively less compared to the traditional SAC105 and SAC305.

Effect of aging on mechanical properties of high temperature Pb-rich solder joints

The effect of aging on the evolution of interfacial microstructure and mechanical properties of Pb-rich PbSnAg solder joints with different Sn content was investigated. Tensile samples were prepared by soldering two pieces of Ni or Cu strips resulting in joints with gap sizes of about 250 μm. Multi-layered structures composed of Ni coated Si-chips soldered onto Ni/Cu metallized ceramic substrates were used for fatigue testing. All samples were subjected to thermal aging at 250 °C up to 500 h. Microstructural investigations revealed that independent from the substrate material, increased Sn content of the solder alloy results in improvement of the tensile and fatigue properties of the joints and a higher growth rate of the interfacial intermetallic compound (IMC) layers. It was found that the thickness of the wettable substrate especially in the case of low-Sn alloys, also affects the interfacial properties of high temperature PbSnAg solder joints. In this case, a reduced Sn content in solder joints results in weakening of the interface during the reflow process and subsequent thermal aging. The dominant failure mode of the solder joints subjected to cyclic loading was delamination of the interfacial IMC layer from the substrate. Finite element simulations were conducted by using a strain rate and pressure dependent material model for PbSnAg solder in order to analyse the states of stress and strain during static and cyclic loading.

Nucleation and twinning in tin droplet solidification on single crystal intermetallic compounds

βSn nucleation is a key step in the formation of microstructure in electronic solder joints. Here, the heterogeneous nucleation of βSn is studied in undercooled tin droplets spread on the facets of various intermetallic compounds (IMCs). Nucleation undercoolings are measured in solidifying droplets and are linked to orientation relationships (ORs) measured by electron backscatter diffraction (EBSD). Preferred ORs developed on all IMCs studied. For the more potent nucleants (αCoSn3, IrSn4, PtSn4, PdSn4) the ORs represent relatively simple atomic matches. ORs with lower potency nucleants (Cu6Sn5, Ag3Sn, Ni3Sn4) had more complex atomic matches that are explored based on matching of the closest packed atomic rows. βSn solidification twinning is shown to be more complex than has been reported previously: both nucleation on an IMC facet and cyclic twinning of that grain occurred in many droplets on Cu6Sn5, Ag3Sn, Ni3Sn4; in all twinned droplets the <100>Sn twinning axis occurred along a direction on the IMC with the lowest linear atomic disregistry; and interrelated cyclic twins formed consisting of up to five rings of cyclic twins all related by shared <100>Sn axes.

A method to determine the slip systems in BGA lead-free solder joints during thermal fatigue

The ball grid array (BGA) component with cross-sectioned edge row was thermally shocked to investigate the activated slip systems in the Sn–3.0Ag–0.5Cu lead-free solder joint. The microstructure and crystal orientations of Sn-based solder joints in as-reflowed and thermally shocked conditions were obtained by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD), respectively. The sample was reexamined after 200, 300 and 400 thermal shock cycles without further polishing. In this paper, one single-crystal solder joint in a BGA component was selected to analyze the activated slip systems during recrystallization under the thermally shocked cycled condition. Two steps were used to determine the activated slip systems in this study. Step one, subgrain rotation angles and axes were calculated by the Euler angles obtained by EBSD before and after thermal shock. Then several slip systems were obtained according to the calculated subgrain rotation axes. Step two, in order to further determine the accurate slip system which could cause the subgrain rotation along the axes in step one, the theoretical slip traces were obtained by calculating Euler angles. Because the results contained many kinds of theoretical slip traces, so the slip bands observed by SEM were used and assisted to select the theoretical slip traces. Based on the comprehensive analysis of the two steps, slip systems that causes the serious deformation were determined. The results showed that recrystallization occurred at different areas of the solder joint and the slip bands appeared at the corners of the solder joint. The subgrain rotation behavior was variable at the recrystallization area by analyzing the subgrain orientation after 200, 300 and 400 thermal shock cycles. By compared the as-calculated slip traces with the slip bands characterized by SEM and the rotation axes calculated by the Euler angles, (0 \(\bar {1}\) 0) [0 0 1], (1 \(\bar {1}\) 0) [0 0 1] and (1 1 0) [0 0 \(\bar {1}\)] were the slip systems that caused the subgrain rotation along a certain axis in this solder joint.

Microstructural evolution and IMCs growth behavior of Sn-58Bi-0.25Mo solder joint during aging treatment

The microstructural evolution and IMCs growth behavior of Sn-58Bi and Sn-58Bi-0.25Mo solder joints were investigated. The results showed that the microstructure is coarsened, the IMCs layer thickness is increased and the tensile strength of Sn-58Bi and Sn-58Bi-0.25Mo solder joints is decreased with increasing aging time and temperature. Aging temperature is the key factor that causes the excessive IMCs growth of the solder joint compared with aging time, and the activation energy of IMCs layer growth of Sn-58Bi and Sn-58Bi-0.25Mo solder joints is 48.94 kJ mol−1 and 53.79 kJ mol−1, respectively. During the aging treatment, the microstructure of Sn-58Bi solder joint is refined by adding Mo nanoparticles, and the appropriate IMCs layer thickness and improved mechanical properties are obtained by Sn-58Bi-0.25Mo solder joint.

SAC–xTiO2 nano-reinforced lead-free solder joint characterizations in ultra-fine package assembly

PurposeThis paper aims to investigate the characteristics of ultra-fine lead-free solder joints reinforced with TiO2 nanoparticles in an electronic assembly.Design/methodology/approachThis study focused on the microstructure and quality of solder joints. Various percentages of TiO2 nanoparticles were mixed with a lead-free Sn-3.5Ag-0.7Cu solder paste. This new form of nano-reinforced lead-free solder paste was used to assemble a miniature package consisting of an ultra-fine capacitor on a printed circuit board by means of a reflow soldering process. The microstructure and the fillet height were investigated using a focused ion beam, a high-resolution transmission electron microscope system equipped with an energy dispersive X-ray spectrometer (EDS), and a field emission scanning electron microscope coupled with an EDS and X-ray diffraction machine.FindingsThe experimental results revealed that the intermetallic compound with the lowest thickness was produced by the nano-reinforced solder with a TiO2 content of 0.05 Wt.%. Increasing the TiO2 content to 0.15 Wt.% led to an improvement in the fillet height. The characteristics of the solder joint fulfilled the reliability requirements of the IPC standards.Practical implicationsThis study provides engineers with a profound understanding of the characteristics of ultra-fine nano-reinforced solder joint packages in the microelectronics industry.Originality/valueThe findings are expected to provide proper guidelines and references with regard to the manufacture of miniaturized electronic packages. This study also explored the effects of TiO2 on the microstructure and the fillet height of ultra-fine capacitors.

Study on the Tensile Creep Behavior of Carbon Nanotubes-Reinforced Sn-58Bi Solder Joints

The microstructure and tensile creep behavior of plain Sn-58Bi solder and carbon nanotubes (CNTs)-reinforced composite solder joints were investigated. The stress exponent n under different stresses and the creep activation energy Q c under different temperatures of solder joints were obtained by an empirical equation. The results reveal that the microstructure of the composite solder joint is refined and the tensile creep resistance is improved by CNTs. The improvement of creep behavior is due to the microstructural change of the composite solder joints, since the CNTs could provide more obstacles for dislocation pile-up, which enhances the values of the stress exponent and the creep activation energy. The steady-state tensile creep rates of plain solder and composite solder joints are increased with increasing temperature and applied stress. The tensile creep constitutive equations of plain solder and composite solder joints are written as $$ \dot{\varepsilon }_{s1} = 14.94\left( {\sigma /G} \right)^{3.7} \exp \left( { - \frac{81444}{RT}} \right) $$ and $$ \dot{\varepsilon }_{s2} = 2.5\left( {\sigma /G} \right)^{4.38} \exp \left( { - \frac{101582}{RT}} \right) $$ , respectively. The tensile creep mechanism of the solder joints is the effects of lattice diffusion determined by dislocation climbing.

Reflow of tiny 01005 capacitor/SAC305 solder joints in protective atmosphere

PurposeThis paper aims to study the influence of reflow atmosphere and placement accuracy on the solderability of 01005 capacitor/SAC305 solder joints.Design/methodology/approachThe 01005 capacitors were mounted on OSP-coated pads, and the samples were fabricated in four different atmospheres, i.e. 200 ppm O2/N2, 1,000 ppm O2/N2, 3,000 ppm O2/N2 and air. After the reflow process, visual inspection and X-ray detection were carried out to examine the solder joint shapes and possible defects. Some of the samples fabricated in different conditions were cross-sectioned and the solder joint microstructures were analyzed. On the other hand, besides placing the components on their normal pad positions, a 50 per cent offset of the x-axis (long axis) or y-axis (short axis) was introduced into the chip mounter programs to evaluate the 01005 capacitor’s assembly sensitivity to placement accuracy. The process-induced defects were investigated.FindingsExperimental results indicated that an N2-based protective atmosphere was necessary for 01005 type assembly, as it could obviously improve the 01005 solder joint quality, compared with the air condition. The protective atmosphere had little effect on the appearance, quality and microstructure of solder joints when the oxygen concentration was below 3,000 ppm. But a very low oxygen concentration could increase the risk of tombstoning defects for the assembly process. The N2-based protective atmosphere containing 1,000-2000 ppm O2 was acceptable and appropriate for the assembly of tiny components.Originality/valueThe results of this work provide a set of reflow process parameters and recommendations for 01005 size component assembly in manufacturing.

Geometric size effect on IMC growth and elements diffusion in Cu/Sn/Cu solder joints

PurposeThe purpose of this paper is to investigate the effect of geometric size on intermetallic compound (IMC) growth and elements diffusion of Cu/Sn/Cu solder joint and establish the correlation model between the thickness of the IMC layer and size of the solder joint on the dozens of microns scale.Design/methodology/approachThe sandwich-structured Cu/Sn/Cu solder joints with different gaps between two copper-clad plates (δ) are fabricated using a reflow process. The microstructure and composition of solder joints are observed and analyzed by scanning electron microscopy.FindingsAfter reflow, the thickness of the IMC and Cu concentration in solder layers increase with the reduction of δ from 50, 40, 30, 20 to 10 μm. During isothermal aging, the thickness of the IMC fails to increase according to the traditional parabolic rule due to changes in Cu concentration. The reduction of δ is the root cause of changes in Cu concentration and the growth rule of the IMC layer. A correlation model between the thickness of the IMC layer and δ is established. It is found that the thickness of the IMC layer is the function of aging time and δ. With δ reducing, the main control element of IMC growth transfers from Cu to Sn.Originality/valueThis paper shows the changes of IMC thickness and elements concentration with the reduction of the size of solder joints on the dozens of microns scale. A correlation model is established to calculate the thickness of the IMC layer during aging.