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

Based on the size effect on solidification behavior of lead-free solders, we propose a novel method to prepare solder joints with the metallurgical bonding between the undercooled liquid solder and Cu metallization under the temperature lower than the solder's melting point. The interfacial reaction and growth behavior of intermetallic compound (IMC) between the undercooled liquid Sn-3.0Ag-0.5Cu solder and Cu metallization have been investigated. The results show that for Sn-3.0Ag-0.5Cu solder as the undercooled melt or eutectic melt or normal melt in the joints, both of the interfacial IMC thickness and Cu 6 Sn 5 grain size increase with prolonging time. The depth of the groove between scallop-like grains in the interfacial IMC layer increases with increasing either the dwelling time or temperature. However, under different liquid solder dwelling conditions, the change tendency of interfacial IMC thickness is different, which is directly related to the dynamic equilibrium between the deposition and dissolution of the interfacial Cu-Sn phase. Furthermore, when the joints are mainly formed at the undercooling temperature, the excessive growth of interface IMC and the consumption of UBM can be reduced. The proposed method has the potential to be a new technique for preparing solder joints with thin interfacial IMC layer.

A comparative study of solid/solid interfacial reactions of electroless Ni-P (15 at.% P) with lead-free solders, Sn-0.7Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn, was carried out by performing thermal aging at 150°C up to 1000 h. For pure Sn and Sn-3.5Ag solder, three distinctive layers, Ni3Sn4, SnNiP, and Ni3P, were observed in between the solder and electroless Ni-P; while for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders, two distinctive layers, (CuNi)6Sn5 and Ni3P, were observed. The differences in morphology and growth kinetics of the intermetallic compounds (IMCs) at the interfaces between electroless Ni-P and lead-free solders were investigated, as well as the growth kinetics of the P-enriched layers underneath the interfacial IMC layers. With increasing aging time, the coarsening of interfacial Ni3Sn4 IMC grains for pure Sn and Sn-3.5Ag solder was significantly greater than that of the interfacial (CuNi)6Sn5 IMC grains for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders. Furthermore, the Ni content in interfacial (CuNi)6Sn5 phase slightly increased during aging. A small addition of Cu (0.7 wt.%) resulted in differences in the type, morphology, and growth kinetics of interfacial IMCs. By comparing the metallurgical aspects and growth kinetics of the interfacial IMCs and the underneath P-enriched layers, the role of initial Cu and Ag in lead-free solders is better understood.

In the flip-chip (FC) solder joint, solder alloys usually connect with Ni and Cu based under bump metallurgies (UBMs) at chip-side and substrate-side, respectively. The material selection for solder alloys and UBM material is a critical issue to affect the microstructure, phase formation, and the reliability of the solder joints. In this study, the interfacial reaction, microstructure variation, and impact reliability of Sn-based Pb-free solder joints with Ni, Cu, and novel Cu-Zn UBMs are discussed and categorized into six topics. (1) Microstructural variation and interfacial reaction in Ni/Sn-Ag-Cu/Ni assemblies with and without Ni doping Ni and Cu elements cross-interacted between the Cu and Ni UBMs during thermal aging and affected the interfacial reactions at both Cu and Ni sides. Dual-phased (Cu,Ni)6Sn5 intermetallic compounds (IMCs), which has high and low Ni concentration, formed at the solder/Ni interface. In contrast, thicker low-Ni (Cu,Ni)6Sn5 and thin (Cu,Ni)3Sn layers formed at the Cu side. The Ni doping into solder varied the microstructure of solder alloys and the formation of interfacial IMCs. The re-distribution of Ni and Cu was correlated to the formation mechanism of interfacial (Cu,Ni)6Sn5. (2) Effect of dual-phased (Cu,Ni)6Sn5 IMCs on the impact reliability of Sn-Ag-Cu/Ni solder joints By employing the high-speed shear impact test, the impact fracture morphology reveals that the interface of high-Ni (H)/low-Ni (L) (Cu,Ni)6Sn5 facilitates the crack nucleation within the IMC. It is believed that the crack propagation depends on the fracture toughness of interfacial IMCs. The indentation data shows that bulk H-(Cu,Ni)6Sn5 exhibits distinctly lower fracture toughness than bulk L-(Cu,Ni)6Sn5. In correlating the impact fracture behavior and mechanical properties of two kinds of (Cu,Ni)6Sn5, cracks tend to propagate through H-(Cu,Ni)6Sn5 due to the relatively low fracture toughness of H-(Cu,Ni)6Sn5. (3) Development of a novel Cu-Zn UBM for Pb-free solder joints To suppress the thicker IMCs and voids at the solder/Cu interface, the Cu-Zn alloy was designed for a novel UBM material. The interfacial reactions of Sn/Cu-xZn (x = 0, 15 and 30 at.%) solder joints were investigated. Interestingly, the growth of Cu-Sn IMCs was significantly reduced and no void was found in the Sn/Cu-Zn solder joints after thermal aging. Transmission electron microscopy (TEM) images and the field emission electron probe microanalyzer (FE-EPMA) analysis show that there are two types of Zn-rich phases, i.e. CuZn and Cu-Zn-Sn phases, to form in Sn/Cu-Zn joints. The formation mechanisms of IMCs were probed and proposed with regard to the thermodynamics and kinetics. (4) Liquid-state reaction of Sn-Ag-Cu solders and the novel Cu-Zn UBM During the reflow process, Cu and Zn atoms would dissolve from the Cu-Zn UBM into the molten solders, leading to the variation of the composition in the solders. Then, the composition variation further altered the microstructure of the solders. In comparison with the Sn-Ag-Cu/Cu, it was found that the coarser eutectic region and smaller Cu6Sn5 IMCs inside the solder matrix of Sn-Ag-Cu/Cu-Zn. In addition, the interfacial reaction was also affected by Zn dissolution. In this study, it was demonstrated that the microstructural variation and the phase evolution in the solder joints were controlled by the reflow time and the Zn concentration in the Cu-Zn UBM. (5) Characterization of the Cu6(Sn,Zn)5 intermetallic compound Cu6Sn5 is a dominant IMC at the Sn-based solder/Cu joint interface. The crystal structure of Cu6Sn5 varies with temperature. After reflow at 250 oC, the interfacial Cu6Sn5 revealed hexagonal structure (η-Cu6Sn5). During aging at 150 oC, hexagonal η-Cu6Sn5 would transform into monoclinic η’-Cu6Sn5. According to literature, the phase transformation between η’ and η would induce crack easily propagating through the Cu6Sn5 at the solder joint interface. In the novel solder/Cu-Zn joints, and Zn would dissolve into Cu6Sn5 to form the Cu6(Sn,Zn)5 IMC at the interface. X-ray diffraction and differential scanning calorimetry analyses show that doping small amounts of Zn into Cu6(Sn,Zn)5 can stabilize the hexagonal structure during the thermal aging process. Thermodynamic calculation also demonstrates that Zn can stabilize the hexagonal Cu6(Sn,Zn)5. (6) Application of Cu-Zn UBM on the Ni/Sn-Ag-Cu/Cu-Zn assemblies The feasibility of novel Cu-Zn UBM applied for the Ni/solder/Cu-Zn assemblies was also evaluated. In comparison with the Ni/Sn-Ag-Cu/Cu solder joint, Ni/Sn-Ag-Cu/Cu-Zn solder joints revealed thinner Cu6Sn5-based IMCs at both Ni/Sn-Ag-Cu and Sn-Ag-Cu/Cu-Zn interfaces after aging. (Cu,Ni)6(Sn,Zn)5/(Cu,Ni)6Sn5 dual-phase formed at the Ni side while (Cu,Ni)6(Sn,Zn)5 single-phase at the Cu-Zn side. The interfacial IMCs grew very slowly, and no void formed in these Zn-contained solder joints during thermal aging. Additionally, the dissolved Zn in the solder alloy reduced the elemental cross-interaction between the Ni and Cu-Zn substrates. The noticeable thermal stability of Ni/Sn-Ag-Cu/Cu-Zn solder joints is attributed to the Zn re-distribution retarding the reaction of Ni, Cu and Sn. Phase formation and IMCs suppression mechanisms in Ni/Sn-Ag-Cu/Cu-Zn solder joints were probed and discussed. In summary, novel Cu-Zn UBM shows lots of advantages for soldering, including: (I) reduction of Cu-Sn IMCs, (II) suppression of voids at the interface, (III) formation of the a hexagonal Cu6(Sn,Zn)5, and (IV) retardation of the elemental cross-interaction in the Ni/solder/Cu-Zn assemblies. The Cu-Zn alloys could be a potential UBM material for the advanced electronic packaging.

Through ultrasonic wave assisted Sn2.5Ag0.7Cu0.1RExNi/Cu (x = 0, 0.05, 0.1) soldering test and − 40 to 125 °C thermal shock test, the microstructure and shear properties of Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints under thermal cycling were studied by the SEM, EDS and XRD. The results show that the Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints with high quality and high reliability can be obtained by ultrasonic assistance. When the ultrasonic vibration power is 88 W, the ultrasonic-assisted Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu solder joints exhibits the optimized performance. During the thermal cycling process, the shear strength of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints had a linear relationship with the thickness of interfacial intermetallic compound (IMC). Under the thermal cycling, the interfacial IMC layer of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints consisted of (Cu,Ni)6Sn5 and Cu3Sn. The thickness of interfacial IMC of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints was linearly related to the square root of equivalent time. The growth of interfacial IMC of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints had an incubation period, and the growth of IMC was slow within 300 cycles. And after 300 cycles, the IMC grew rapidly, the granular IMC began to merge, and the thickness and roughness of IMC increased obviously, which led to a sharp decrease in the shear strength of the solder joints. The 0.05 wt% Ni could inhibit the excessive growth of IMC, improve the shear strength of solder joints and improve the reliability of solder joints. The fracture mechanism of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints changed from the ductile–brittle mixed fracture in the solder/IMC transition zone to the brittle fracture in the interfacial IMC.

Interfacial IMC growth of SAC305/Cu joint with a novel dual-layer of Ni(P)/Cu plating during solid-state aging

Sn-Bi solder with different Bi content can realize a low-to-medium-to-high soldering process. To obtain the effect of Bi content in Sn-Bi solder on the microstructure of solder, interfacial behaviors in solder joints with Cu and the joints strength, five Sn-Bi solders including Sn-5Bi and Sn-15Bi solid solution, Sn-30Bi and Sn-45Bi hypoeutectic and Sn-58Bi eutectic were selected in this work. The microstructure, interfacial reaction under soldering and subsequent aging and the shear properties of Sn-Bi solder joints were studied. Bi content in Sn-Bi solder had an obvious effect on the microstructure and the distribution of Bi phases. Solid solution Sn-Bi solder was composed of the β-Sn phases embedded with fine Bi particles, while hypoeutectic Sn-Bi solder was composed of the primary β-Sn phases and Sn-Bi eutectic structure from networked Sn and Bi phases, and eutectic Sn-Bi solder was mainly composed of a eutectic structure from short striped Sn and Bi phases. During soldering with Cu, the increase on Bi content in Sn-Bi solder slightly increased the interfacial Cu6Sn5 intermetallic compound (IMC)thickness, gradually flattened the IMC morphology, and promoted the accumulation of more Bi atoms to interfacial Cu6Sn5 IMC. During the subsequent aging, the growth rate of the IMC layer at the interface of Sn-Bi solder/Cu rapidly increased from solid solution Sn-Bi solder to hypoeutectic Sn-Bi solder, and then slightly decreased for Sn-58Bi solder joints. The accumulation of Bi atoms at the interface promoted the rapid growth of interfacial Cu6Sn5 IMC layer in hypoeutectic or eutectic Sn-Bi solder through blocking the formation of Cu6Sn5 in solder matrix and the transition from Cu6Sn5 to Cu3Sn. Ball shear tests on Sn-Bi as-soldered joints showed that the increase of Bi content in Sn-Bi deteriorated the shear strength of solder joints. The addition of Bi into Sn solder was also inclined to produce brittle morphology with interfacial fracture, which suggests that the addition of Bi increased the shear resistance strength of Sn-Bi solder.

A solid-state reaction between the Sn–Ag–Cu solder and Ni metallization resulted in the formation of interfacial bilayer Cu6Sn5-based intermetallic compounds (IMCs) in a solder joint incorporating Au/Pd metallization. The layer near the Ni metallization was identified as (Cu,Ni,Au)6Sn5 containing 20.3 at.% of Ni and 1.7 at.% of Au, and the layer near the solder matrix was identified as (Cu,Au,Ni,Pd)6Sn5 containing 5.7 at.% of Au, 1.2 at.% of Ni, and 1.0 at.% of Pd. The electron diffraction analysis with high resolution transmission electron microscopy further characterized the interfacial bilayer IMCs as having the same hexagonal crystal structure with different crystal orientations. The kinetics study revealed that the (Cu,Ni,Au)6Sn5 in the interfacial bilayer reaction products was formed during the initial reflow process, while the (Cu,Au,Ni,Pd)6Sn5 layer was formed during subsequent solid-state aging. Predominant growth of the (Cu,Au,Ni,Pd)6Sn5 layer was observed during solid-state aging. In contrast, the growth of the initially formed (Cu,Ni,Au)6Sn5 layer was suppressed. The growth of the (Cu,Au,Ni,Pd)6Sn5 layer was governed by the dissolution of the (Au,Pd)Sn4 IMC that occurred during solid-state aging. The complete dissolution of (Au,Pd)Sn4 resulted in a reactant-limited chemical reaction from the conversion of (Au,Pd)Sn4 to (Cu,Au,Ni,Pd)6Sn5. The development of the interfacial bilayer IMCs in the solder joint incorporating Au/Pd metallization suppressed excessive IMC growth and unfavorable phase transformation during long-term solid-state aging. The detailed mechanism of the formation of the interfacial bilayer Cu6Sn5-based IMCs was investigated in this paper.

Objective of this work is to understand diffusion kinetics and mechanical properties of interfacial intermetallic compounds (IMC) in the microbump solder joints. Diffusion studies on different sizes of Sn/Cu joints are conducted to compare their interfacial diffusion kinetics with microbump solder joints. Growth rate constants and activation energy of interfacial IMC layer during multiple reflows in microbump joints are lower than 80μm solder joints. However, in the case of solid-state aging, activation energy for interfacial IMC growth increases with the reduction in the size of solder joints. Besides the diffusion kinetics, microstructural changes such as, morphology, kirkendall voids and grain size of IMCs also affect with reduction in the solder joint. These structural changes adversely affect the mechanical integrity of interconnect system. However, determination of mechanical properties of interfacial IMCs in microbump joints is difficult due to its thicknesses (∼ 0.5 to 2fjm). Tapper mounting technique for microbump joints is demonstrated to characterize the interfacial IMC layers thickness as low as 500nm. Substrate effects on measured elastic modulus and hardness are removed using post-indentation analysis i.e. S-h and tf-P analysis. Elastic modulus and hardness of interfacial IMCs are extracted from the harmonic contact stiffness-load-displacement data.

Electromigration effects on the solder joint formation of 99.3Sn-0.7Cu and 96.5Sn-3.0Ag-0.5Cu lead-free solder with Cu electroplated Ni layer wire were investigated. The electromigration effects on the solder joints were studied after current density stressing at 1 x 103 A/cm2 in room temperature for 0 h, 120 h, and 240 h. The research work found that intermetallic compound (IMC) formation on the joint is increases for both solders with longer period of current stress applied. Higher IMC thickness growth in 99.3Sn-0.7Cu solder joint compared to 99.3Sn-0.7Cu is detected and both anode regions of the solder joints show higher IMC thickness growth compared to cathode region. Experimental results show 99.3Sn-0.7Cu solder joint is more prone to failure under current stress compared to 96.5Sn-3.0Ag-0.5Cu solder joint with thicker IMC which translates to higher brittleness.

Interfacial intermetallic growth and shear strength of lead-free composite solder joints

The interfacial reaction and growth of intermetallic compounds (IMCs) between the eutectic Sn-0.7 wt.% Cu solder and Cu substrate during soldering process were investigated experimentally. The Sn-0.7Cu/Cu couples were fabricated with soldering temperature varied at four levels of 300, 320, 340 and 360°C, while soldering time is varied at five levels between 5 and 90 min. Microstructural analysis is conducted to analyze the IMCs thickness and morphology. Two intermetallic phases were observed during soldering at the interface: η-phase (Cu 6 Sn 5 ) and e-phase (Cu 3 Sn) IMC layers, except for the solder joints which were fabricated at low temperature and/or short soldering time. The thickness of the η and e IMC phases increase with increasing the soldering temperature and/or soldering time. It's found that the increase in total IMC layer thickness obeys a linear relationship with soldering temperature. And the relationship between X and T was given as X = 0.0866×T − 22.5 by means of linear fitting method.

Interfacial reaction and shear strength of ultrasonically-assisted Sn-Ag-Cu solder joint using composite flux

Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu halogen free solder joints were fabricated by an ultrasonic vibration (USV)-assisted soldering process. The effects of the USV power on the three-dimensional (3-D) morphology of the intermetallic compounds (IMC) and mechanical properties of the halogen free solder joints were characterized systematically. Results showed that the root-mean-square roughness (Rrms) and the mean spacing of adjacent peaks of the profile (λave) of the interfacial IMC were linearly correlated with shear strength of the halogen free solder joints. It was more comprehensive to adopt the λave and Rrms evaluation parameters to characterize the relationship between the interfacial IMC 3-D morphology and shear strength of the halogen free solder joints. With increasing USV power, the eutectic microstructure of the solder seam was refined, and the proportion of eutectic microstructure in the solder seam and microhardness increased. When the USV power increased to 130 W, the Rrms and average thickness of the interfacial IMC decreased (decreases of 37.6% and 56.4%, respectively) and the corresponding λave and shear strength of the solder joint increased (increases of 68.7% and 45.8%, respectively). With increasing USV power, the fracture mechanism of the halogen free solder joint changed from brittle fracture to the ductile–brittle mixed fracture, and the fracture pathway transferred from the interfacial transition zone consisting of the solder seam and the interfacial IMC layer to the solder seam.

The formation and evolution of the intermetallic compound (IMCs) between SnAgCu lead-free solder and Cu substrate, after isothermal aging at 150°C for 24, 48, 120, 240 and 480 hours, were studied. Scanning electron microscope (SEM) was used to observe the microstructure evolution of solder joint during aging. The IMC phases were identified by energy dispersive X-ray (EDX). The results showed that IMCs layer of Cu6Sn5 was formed at the interface of solder and Cu during reflowing. With the increase of aging time, the grain size of the interfacial Cu6Sn5 increased and the morphology of the interfacial Cu6Sn5 column was changed from scallop-like to needle-like and then to rod-like and finally to particles. At the same time, the rod-like Ag3Sn phase formed at the interface of solder and the IMCs layer of Cu6Sn5 with the aging time increased. In addition, large Cu6Sn5 formed in the solder with the aging time increased. The tensile strength was measured for the solder joints. The results showed that the tensile strength increases slightly at beginning and then decreases with the aging time. SEM was used to observe the fracture surface and it showed that the fracture position moved from solder matrix to the interfacial Cu6Sn5 IMCs layer with the aging time increased. The weakening of the solder matrix is caused by the coarsening of the eutectic solder structures. The weakening of the interfacial IMCs layer is caused by the evolution of morphology and size of the interface Cu6Sn5 layer.

In this paper, regarding the joint interfacial zone as a special region with serious heterogeneity on microstructure and mechanical property, a creep-fatigue interaction damage model was proposed, in which two parameters named as creep strain energy density and average equivalent stress were introduced, aiming to reflect the effect of interfacial microstructure evolution. Based on the proposed heterogeneous research model and damage evolution equation, the damage evolution behavior and the effect of interfacial intermetallic compound (IMC) growth were studied based on finite element modeling (FEM). The results showed that, the accumulated damage in the solder joint increased with the increase of thermal cycles. The failure mode exhibited as that failure site was close to solder/intermetallic compound (IMC) interface, and the thicker the IMC layer, the more accumulated damage per cycle in the solder joint. These results demonstrate that IMC impacts solder joint reliability, and the bond at interfacial IMC/solder interface is most vulnerable to thermal fatigue damage.