Solder Joint Reliability Research Articles

Numerical Investigation of the Evolving Inelastic Deformation Path of a Solder Ball Joint under Various Loading Conditions

Solder joints of ball grid arrays (BGA) have been widely used to connect electronic components to printed circuit boards (PCBs) and are often subjected to mechanical stress. Several studies have been conducted on the mechanical reliability of solder joints. While these studies have been useful in the industry, detailed studies on how the inelastic deformation path of the solder ball joints evolves under specific loading conditions have not been sufficiently reported. This study aims to understand how the inelastic deformation path evolves when a solder joint is subjected to a constant external force by utilizing the theory of mechanics. It has also been found that the mechanical failure is strongly influenced by the evolution history of the deformation modes in materials. For this study, an elastoplastic constitutive model and a ductile fracture criterion were implemented into the vectorized user-defined material (VUMAT) subroutine of the ABAQUS program for finite element (FE) analysis. With the model, the evolution of the inelastic deformation path of a single solder ball under different loading conditions was numerically analyzed. Three loadings (shear, compression, and bending) were chosen as the basic loading conditions. In addition, combinations of the basic loadings resulted in three dual loadings and one complex loading. The simulation results showed that the shear and bending caused the fracture for both single and dual loadings, but when combined with compression, the fracture was suppressed. The results indicate that fracture is not solely determined by the magnitude of equivalent plastic strain but also by the evolution of inelastic deformation mode. This research offers an improved understanding of the significance of the inelastic deformation path and fracture.

Comparative study of solder joint reliability assessment on fan-out CSP and flip-chip DCA package

Comparative study of solder joint reliability assessment on fan-out CSP and flip-chip DCA package

Bonding Property of Middle Temperature Sn-Ag-Cu Solder Ball Joint of Ball Grid Array Package

Alloying effects of Nickel and Bismuth in SAC solder to achieve mechanical and microstructural evolution were investigated by comparing three different chemical composition of solders. Sn-3.0Ag-0.5Cu (SAC305) is the most commonly used for Pb-free solder and solder balls are based on Sn-Ag-Cu alloy composition. Compositions of solder balls were Sn-3.0Ag-0.5Cu (SAC305), Sn-1.2Ag-0.5Cu-0.05Ni (SAC1205N) and Sn-Ag-Cu-Ni-Bi based solder (SACNB). Organic solderability preservative (OSP) surface finished substrate, ball grid array (BGA) package arranged by 300 ㎛ diameter solder ball and Type 4 solder paste were used. Solder ball shear test of the BGA package was conducted before the reflow soldering. Board level package bonding process was carried out by hot air reflow soldering. Void content of the BGA solder joint was investigated after soldering. The microstructure evolution before and after thermal cycle test (TCT) was observed using scanning eletron microscopy (SEM) and energy dispersive spectroscopy (EDS). After TCT, intermetallic compound (IMC) refinement and solid solution strengthening effects of Bi element addition were observed in the solder matrix, and also any segregation of Bi element was not founded. Based on these result, it was found that the solder joint reliability of the middle and low temperature solder joints was improved.

Current-induced solder evolution and mechanical property of Sn-3.0Ag-0.5Cu solder joints under thermal shock condition

Solder joint reliability suffers great challenges due to high current density and miniature solder bump diameter of electronic packages at cryogenic temperature. To tackle these issues, the solder microstructure evolution and the corresponding failure mechanism should be emphasized. In this study, the current-induced microstructure characteristics and mechanical behavior of Sn-3.0Ag-0.5Cu (SAC305) solder joints during thermal shock process was thoroughly addressed over cycles by combining diffusional, electrical, thermal and mechanical features. This result verified that the combining method of thermal shock and electromigration (EM) contributed to the atom diffusion and high thermal stress formation, further causing intermetallic compound (IMC) growth, the repaid dissolution of Cu pad and high thermal stress of the solder joints. High stress induced by either the thermal expansion mismatch of different component, large temperature change (ΔT =346 ℃) and severe lattice distortion became the direct reason for twins and cracks formation of SAC305 solder joints. The combination of crack propagated along the interface and the quick dissolution of Cu substrate at the corner accelerated the eventually failure of the solder joints. Moreover, as the thermal chock cycles was extended, the initiation and propagation of cracks at the cathode side weaken the cathodic shear strength. High stress-induced twin formation at the interface effectively moderated the shear strength degradation due to anode IMC growth after 9 cycles. This study contributed to thoroughly grasp the failure mechanism of the solder joints and design the twin-strengthened Sn-based solder joints under the coupling effects of extreme temperature variation and current stressing.

Thermal cycle reliability and creep behavior of nano-IMC mixed solder joints

Thermal cycle reliability and creep behavior of nano-IMC mixed solder joints

Enhanced size effects on shear performance and fracture behavior of BGA structure micro-scale Cu/Sn–3.0Ag–0.5Cu/Cu joints at low temperatures

To assess the serving reliability of shrinking-sized solder joints in space electronics (especially cryogenic electronics), the effects of joint size on the shear performance of Cu/Sn–3.0Ag–0.5Cu/Cu joints with ball grid array (BGA) structure at different temperatures (25 to −125 °C) were investigated. As the joint size decreased, shear strength of the solder joint increased, showing prominent size effect, which was enhanced at low temperatures. Moreover, a ductile-to-brittle transition (DBT) fracture occurred as the test temperature declined, and the critical temperature decreased as the joint size decreased. All of the results suggest that the small-sized joints are more robust at low temperatures under shear loading.

Dynamics of corrosion on mechanical and electrical reliability of SAC305 solder joints during salt spray test

This paper, concerns the impact of a corrosion evolution on mechanical and electrical reliability of solder joints during salt spray tests. Focus is made in test vehicle resistors (1210) soldered with a SAC305 lead-free solder. Solder joints resistors were electrically and mechanically tested and aged from 24 to 96 h in a salt spray chamber with one salinity of 5 % NaCl at five temperatures ranging from 25 °C to 45 °C. The shear strengths of SAC305 solder joints decreased with increasing temperature and corrosion time. No electrical failure was observed. However, the solder joints deterioration is visible by cross-section after 72 h at 25 °C, 60 h at 30 °C, 48 h at 35 °C, 36 h at 40 °C and 24 h at 45 °C. Moreover, the corrosion has a significant effect on the mechanical strength of lead-free solder joints. A relationship between the mechanical behaviour and the dynamics of corrosion of the solder joints has been established. In addition, although the corrosion product remains the same, the corrosion dynamics of the solder material (alloy) and the solder joint (assembly) are very different: the corrosion kinetics is significantly less in the solder material than in the solder joints.

Microstructure and Properties of Electromigration of Sn58Bi/Cu Solder Joints with Different Joule Thermal Properties

Electromigration is one of the most important research issues affecting the reliability of solder joints. Current-induced Joule heating affects the electromigration behavior of solder joints. Solder joints with different cross-sectional areas were designed to obtain different Joule heating properties. The effects of the interfacial intermetallic compound (IMC) and mechanical properties of Sn58Bi/Cu solder joints were studied for different Joule heating properties. The results showed that as the cross-sectional area of the Sn58Bi/Cu solder joints increased, the Joule heating of the joint increased. The anode IMC thickness of the joint thickened and transformed into a planar shape. The Bi migrated to the anode region to form a Bi-rich layer and gradually increased in thickness. The cathode IMC thickness first increased, then decreased, and gradually dissolved. The Sn-rich layer formed near the solder side and gradually increased in thickness, with microcracks occurring when the cross-sectional area of the joint increased to 0.75 mm2. The joint shear fracture path moved from the soldering zone near the cathode IMC layer to the interfacial IMC layer. The fracture mechanism of the joint changed from a mixed brittle/tough fracture, dominated by deconstruction and secondary cracking, to a brittle fracture dominated by deconstruction. The joint shear strength was reduced by 60.9% compared to that in the absence of electromigration.

Effect of nano-intermetallic compound on creep properties and β-Sn grain recrystallization of Cu/Sn-Ag-Cu/Ni solder joints

In this paper, the nanoindentation method was used to study the creep properties of nano-intermetallic compound (nano-IMC) mixed solder joints. Furthermore, the microstructure evolution, crack initiation, and β-Sn grain recrystallization during thermal cycles were discussed. The results revealed that the addition of nano-IMC increased the nano-(Cu,Ni)6Sn5 content in the mixed solder joints after reflow, which increased the stress index, reduced the stress sensitivity index, and improved the creep resistance. During thermal cycles, cracks appeared at the corner of solder joints and substrate under cyclic stress, expanding along the solder/IMC interface with the increase in thermal cycles. Compared with the mixed solder joints, the recrystallization of SAC305 solder joints aggravated during thermal cycles. This is because of the high nano-(Cu,Ni)6Sn5 content in the mixed solder joints, which can effectively pin dislocations and inhibit the dislocation slip and rotation of β-Sn grains, thereby preventing the recrystallization of β-Sn grains. This improved the reliability of the mixed solder joints.

Effect of Cu on the diffusion behavior of Bi in Sn matrix during electromigration

The segregation of Bi phase during current stressing deteriorates the mechanical property of Sn-58Bi solder joint. To improve the reliability of Sn-58Bi solder joint, a thorough understanding of the diffusion behavior of Bi elements is necessary. In this study, a Cu/SnBi/Sn-xCu/SnBi/Cu multi-layer solder joint structure was adopted to reveal the diffusion path of Bi element and the effect of Cu on the diffusion behavior of Bi during electromigration. The result showed that the Bi element diffused along the grain boundary. The presence of Cu had two competing mechanisms on the diffusion of Bi: it slowed the process by forming Cu6Sn5 intermetallic compounds that blocked and prolonged the diffusion path, yet also accelerated it by refining the Sn and creating more diffusion channels. A data processing method was used to estimate the diffusion rate of Bi and determine the dominant mechanism. The results indicated that the diffusion rate was accelerated by Cu doping, confirming that the dominant mechanism was the refining effect, which promoted the diffusion of Bi element.

Growth behavior of intermetallic layers at the interface between Cu and eutectic Sn–Bi by grain boundary diffusion with the grain growth at solid-state temperatures

The growth kinetics and rate-controlling processes of intermetallic layers formed in Cu/(Sn-58 wt% Bi) diffusion couples were investigated. Isothermal annealing of diffusion couples was conducted in the temperature range of 363–393 K for various times up to 384 h. In the diffusion couple, an intermetallic layer composed of Cu6Sn5 with irregular scallop shapes and Cu3Sn with relatively uniform thickness forms at the Cu/(Sn–Bi) interface. Isothermal sections of the equilibrium phase diagram in the ternary Bi–Cu–Sn system were calculated at the isothermal annealing temperatures and the peak temperature of the reflow process by a CALPHAD (calculation of phase diagrams) method. The diffusion path passes through the three-phase tie-triangle, including Cu6Sn5 and Cu3Sn. The obtained thicknesses were plotted against the annealing time, and a power function equation was found to describe their relationship. The values of the exponent in the power function were close to 0.25, indicating that the layer growth was predominantly controlled by boundary diffusion with the grain growth. Furthermore, the exponent values were relatively insensitive to the annealing temperature. The relationship between the exponent and the annealing temperature was analyzed, and it was concluded that the grain growth in the intermetallic layer followed a parabolic law. The growth and morphology evolution of the intermetallic compound and Sn–Bi eutectic microstructure at the interface were depicted, shedding light on the widening of the eutectic microstructure. Additionally, the activation enthalpy for the rate-controlling process of intermetallic layer growth at solid-state temperatures was evaluated using a least-squares method, providing insights into the energy barrier for the diffusion or transport of atoms across the interface. This study contributes to a better understanding of intermetallic layer formation and growth in Cu/(eutectic Sn–Bi) diffusion couples, offering valuable information for developing reliable solder joints and high-temperature solders.

Study of microstructure, growth orientations and shear performance of Cu/Sn-3.0Ag-0.5Cu/Cu solder joints by using thermal gradient bonding

In this study, the effects of the thermal gradient (TG) applied during the bonding process on the microstructure, grain orientations and shear properties of Cu/SAC305/Cu solder joints were investigated, and the magnitude of the TG was controlled by the thickness of the solder layer. The results indicated that the introduction of TG caused the asymmetric growth of intermetallic compounds (IMCs) and enabled the rapid generation of IMCs at the cold end. Cu6Sn5, as the main component of IMCs in Cu/Sn solder joints, developed a strongly preferred orientation in the (0001) ‖ RD direction. While for β-Sn grains, the preferred orientation was uncontrollable, which formed the preferred orientations of (010) ‖ TD, (031) ‖ RD and (114) ‖ RD at 20 um, 60 um and 100 um solder joints, respectively. In addition, the TG reduced the reliability of solder joints whose shear properties were lower than those of joints with isothermal bonding.

Research Progress of Electroplated Nanotwinned Copper in Microelectronic Packaging.

Copper is the most common interconnecting material in the field of microelectronic packaging, which is widely used in advanced electronic packaging technologies. However, with the trend of the miniaturization of electronic devices, the dimensions of interconnectors have decreased from hundreds of microns to tens of or even several microns, which has brought serious reliability issues. As a result, nanotwinned copper (nt-Cu) has been proposed as a potential candidate material and is being certified progressively. Firstly, the physical properties of nt-Cu have been widely studied. Notably, the higher thermal stability and oxidation resistance of the (111) texture causes nt-Cu to maintain excellent physical properties under high-temperature serving conditions. Secondly, recent works on the electrolyte and electroplating processes of nt-Cu on wafer substrates are summarized, focusing on how to reduce the thickness of the transition layer, improve the twin density, and achieve complicated pattern filling. Thirdly, nt-Cu can effectively eliminate Kirkendall voids when it serves as UBM or a CuP. Additionally, the high (111) texture can control the preferred orientation of interfacial intermetallic compounds (IMCs) at the Cu-Sn interface, which should be helpful to improve the reliability of solder joints. nt-Cu has superior electromigration resistance and antithermal cycling ability compared to ordinary copper RDLs and TSVs. Above all, nt-Cu has attracted much attention in the field of microelectronic packaging in recent years. The preparation-performance-reliability interrelationship of nt-Cu is summarized and displayed in this paper, which provides a solid theoretical basis for its practical applications.

Effects of Multiple Reflow on the Formation of Primary Crystals in Sn-3.5Ag and Solder Joint Strength: Experimental and Finite Element Analysis.

The growth and formation of primary intermetallics formed in Sn-3.5Ag soldered on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish after multiple reflows were systematically investigated. Real-time synchrotron imaging was used to investigate the microstructure, focusing on the in situ growth behavior of primary intermetallics during the solid-liquid-solid interactions. The high-speed shear test was conducted to observe the correlation of microstructure formation to the solder joint strength. Subsequently, the experimental results were correlated with the numerical Finite Element (FE) modeling using ANSYS software to investigate the effects of primary intermetallics on the reliability of solder joints. In the Sn-3.5Ag/Cu-OSP solder joint, the well-known Cu6Sn5 interfacial intermetallic compounds (IMCs) layer was observed in each reflow, where the thickness of the IMC layer increases with an increasing number of reflows due to the Cu diffusion from the substrate. Meanwhile, for the Sn-3.5Ag/ENIG solder joints, the Ni3Sn4 interfacial IMC layer was formed first, followed by the (Cu, Ni)6Sn5 IMC layer, where the formation was detected after five cycles of reflow. The results obtained from real-time imaging prove that the Ni layer from the ENIG surface finish possessed an effective barrier to suppress and control the Cu dissolution from the substrates, as there is no sizeable primary phase observed up to four cycles of reflow. Thus, this resulted in a thinner IMC layer and smaller primary intermetallics, producing a stronger solder joint for Sn-3.5Ag/ENIG even after the repeated reflow process relative to the Sn-3.5Ag/Cu-OSP joints.

Reliability Evaluation of Board-Level Flip-Chip Package under Coupled Mechanical Compression and Thermal Cycling Test Conditions.

Flip Chip Ball Grid Array (FCBGA) packages, together with many other heterogeneous integration packages, are widely used in high I/O (Input/Output) density and high-performance computing applications. The thermal dissipation efficiency of such packages is often improved through the use of an external heat sink. However, the heat sink increases the solder joint inelastic strain energy density, and thus reduces the board-level thermal cycling test reliability. The present study constructs a three-dimensional (3D) numerical model to investigate the solder joint reliability of a lidless on-board FCBGA package with heat sink effects under thermal cycling testing, in accordance with JEDEC standard test condition G (a thermal range of -40 to 125 °C and a dwell/ramp time of 15/15 min). The validity of the numerical model is confirmed by comparing the predicted warpage of the FCBGA package with the experimental measurements obtained using a shadow moiré system. The effects of the heat sink and loading distance on the solder joint reliability performance are then examined. It is shown that the addition of the heat sink and a longer loading distance increase the solder ball creep strain energy density (CSED) and degrade the package reliability performance accordingly.

A Contribution to Printed Circuit Boards' Miniaturization by the Vertical Embedding of Passive Components

Abstract This study describes a novel and unconventional approach for embedding passive surface mounted device (SMD) components into printed circuit boards. Therefore, passive components whose package size is 0201 are embedded in two types of vias. On the final quality of an embedded passive component, the effects of various technological factors, including tin-lead and lead-free solder pastes, various types and dimensions of vias, various soldering techniques, and sample positions during the reflow process, have been investigated and described. The results show the impact of tin-lead solder paste and polymeric solder paste on the creation of electrical shorts in the embedding of passive components. The application of microvias for the embedding of passive components eliminates the fundamental issues, such as electrical shorts, component dislocation, and the low success rate for creating a reliable solder joint. The proposed method for miniaturizing printed circuit boards by embedding passive components in microvias was verified by experimental results. The reliability of the proposed methodology is further supported by electrical measurements. This study describes an approach suitable to printed circuit board (PCB) prototyping that makes a negligible contribution to hardware design and electronic technologies.

Effect of twin grain boundary on the diffusion of Cu in bulk β-Sn

For Cu is the dominant diffusing species in the Cu/Sn diffusion couple, the diffusion behaviors of Cu in the β-Sn grain and at the twin grain boundary (TGB) have been studied in this paper by combining the first-principles calculation and molecular dynamics simulation, which has a significant impact on solder joint reliability. The results show that the energy barrier for Cu bulk diffusion in β-Sn depends on the diffusion path, ranging from 0.01 eV to 0.33 eV. Among the three possible diffusion paths at the (101) TGB, the primary diffusion path is along the [010] direction of the TGB plane. However, this diffusion path along the TGB still presents a relatively high energy barrier of 0.26 eV compared to the energy barrier for bulk diffusion. Furthermore, due to the segregation of Cu atoms at the β-Sn TGB and the inhibition effect of multiple energy barriers for diffusion crossing the TGB, long-range diffusion of Cu across the TGB between the two β-Sn bulk grains is difficult. As a result, the (101) TGB actually inhibits the diffusion of Cu in β-Sn. It can be concluded that increasing the number of TGBs inside a Cu/Sn solder joint can effectively suppress the interdiffusion between Cu and Sn, which will prevent the growth of Cu-Sn intermetallic compounds and improve the reliability of the solder joint.

Role of Cu percentage in the Sn-xCu alloy on the mechanical performance at Sn-xCu/ENIG interface produced by laser soldering

PurposePb-free solders have been developed to replace the standard Sn–Pb eutectic solder since the prohibition on Pb used in solders. The Sn–Ag–Cu series of lead-free solders is the most extensively used in the electronics industry. The Ag3Sn, which forms during isothermal ageing, can significantly degrade solder joint reliability. Sn–Ag–Cu solder’s high price further hindered its use in the electronics industry. This paper aims to investigate different copper percentages into Sn–xCu solder alloy to improve its microstructure and strength performance.Design/methodology/approachThe solder alloys used in this work were Sn–xCu, where x = 0.0, 0.3, 0.5, 0.7, 1.0 Wt.%, which was soldered onto electroless nickel immersion gold (ENIG) substrate using carbon dioxide (CO2) gas laser. Then these samples were subjected to isothermal aging for 0, 200, 500, 1,000 and 2,000 h. The Sn–xCu solder alloy was fabricated through a powder metallurgy process.FindingsMicrostructure characterization showed that Cu addition resulted in fine and rounded shape of Cu–Sn–Ni particles. Shear strength of Sn–xCu solder joints was increased with increasing Cu content, but at aging duration of 1,000 h, it dropped slightly. It is believed that the strength improved due to the increment of diffusion rate during isothermal aging.Practical implicationsIn a Cu–Sn solder, the recommended amount is 1.0 Wt.% of Cu. In extensive aging procedures, it was discovered that Sn1.0Cu solder improved the reliability of solder joints. The findings indicated that the innovative solder alloys might satisfy the needs of high-reliability applications.Originality/valueThe study shows that the right amount of Cu enhances the solidification of Sn–Cu solder, increasing the shear force of the Cu–Sn solder joint. The Sn1.0Cu exhibits a ductile fracture on the top microstructure, improving the joint’s average shear strength.

Mixed reaction gold as versatile plating solution for ENIG, ENEPIG and EPAG plating

The surface finish on a printed circuit board (PCB) or substrate serves as surface protection and at the same time enables a variety of assembly techniques. Depending on which final finish is selected it allows for e.g. soldering connections or wire bonding by Al, Au or even Cu-wires. Surface finishes are typically characterized by different metal layers, including tin, nickel, palladium or gold. In this context the electroless or immersion plating of gold has a long history in the electronics industry. The gold plating represents a key step in different finish systems such as electroless nickel/immersion gold (ENIG), electroless nickel/electroless palladium/immersion gold (ENEPIG) and electroless palladium/autocatalytic gold (EPAG). ENIG, ENEPIG and EPAG finishes are characterized by their capabilities to be used as solderable and wire-bondable finish with specific benefits for ENEPIG and EPAG such as enhanced solder joint reliability and gold-wire bondability. In this paper an overview is given on the different types of electroless gold plating electrolytes from fully immersion type and autocatalytic type to mixed reaction type gold electrolytes. The details of the various plating mechanisms are presented, and the different properties and capabilities of these electrolytes are compared and explained based on the respective mechanisms. The presented study investigates the performance of a gold electrolyte of a mixed reaction type looking into the layer properties as well as the soldering and bonding capabilities. The results show that a mixed reaction gold electrolyte can be capable for versatile application- ENIG, ENEPIG and EPAG. The capability of the process was tested with respect to the solder wetting performance, solder joint reliability, wire bonding performance, corrosive attack to the nickel and thickness distribution. It can be shown that this gold process passes or exceeds all related industry standards and target requirements for ENIG, ENEPIG and EPAG final finishes.

Study on IMC Growth Law of Solder Joints in Aerospace Electronic Products and Its Influence on Reliability

In order to better understand the influence degree of temperature curve parameters on the growth characteristics of intermetallic compounds (IMC) in solder joints during reflux welding, the influence of different temperature curve parameters on the thickness of IMC in Sn63Pb37 solder joints was explored based on the orthogonal test method. A multiple regression model was established for each parameter and the thickness of IMC. In addition, the results show that: (1) The influence degree of temperature curve parameters on IMC thickness in descending order is the highest temperature, liquidus time, cooling time and cooling rate. (2) The coefficient of the established multiple regression model is R2=0.70391. In this model, the IMC thickness increases with the increase of the maximum reflow temperature and liquidus time and decreases with the increase of the cooling time. (3) When the thickness of IMC is 1 μm- 4 μm, the thermal shock reliability of solder joints decreases with the increase of IMC thickness.