Depressing effect of 0.2wt.%Zn addition into Sn-3.0Ag-0.5Cu solder alloy on the intermetallic growth with Cu substrate during isothermal aging

The formation and growth of intermetallic compounds (IMCs) in lead-free solder joints, during soldering or subsequent aging, have a significant effect on the thermal and mechanical behavior of solder joints. In this study, the effects of a 0.2wt.%Zn addition into Sn-3.0Ag-0.5Cu (SAC) lead-free solder alloys on the growth of IMCs with Cu substrates during soldering and subsequent isothermal aging were investigated. During soldering, it was found that a 0.2wt.%Zn addition did not contribute to forming the IMC, which was verified as the same phase structure as the IMC for Sn-3.0Ag-0.5Cu/Cu. However, during solid-state isothermal aging, the IMC growth was remarkably depressed by the 0.2 wt.% Zn addition in the SAC solder matrix, and this effect tended to be more prominent at higher aging temperature. The activation energy for the overall IMC growth was determined as 61.460 and 106.903 kJ/mol for Sn-Ag-Cu/Cu and Sn-Ag-Cu-0.2Zn/Cu, respectively. The reduced diffusion coefficient was confirmed for the 0.2Zn-containing solder/Cu system. Also, thermodynamic analysis showed the reduced driving force for the Cu₆Sn₅ IMC with the addition of Zn. These may provide the evidence to demonstrate the depressing effect of IMC growth due to the 0.2wt.%Zn addition in the Sn-Ag-Cu solder matrix.     

Cross-Interaction of Interfacial Reactions in Ni (Au/Ni/Cu)-SnAg-Cu Solder Joints during Reflow Soldering and Thermal Aging

Interfacial reactions in Ni-SnAg-Cu and Au/Ni/Cu-SnAg-Cu solder joints were investigated to understand the coupling effect between different pads during soldering and thermal aging processes. Scanning electron microscopy (SEM) was used to characterize the microstructures and phases. The element distributions in the joints were identified using the x-ray mapping technique. The thickness variation of intermetallic compounds (IMCs) with aging time was also measured. The results showed that interfacial reactions were not only affected by the compositions of solders and the local metallizations but the remote pads as well. The Au surface finish had an effect on the growth of IMCs at the interfaces. No redeposition of (Au, Ni)Sn₄ was found in the Au/Ni/Cu-SnAg-Cu solder joint. The effect of Cu on the formation of IMCs and redeposition of (Au, Ni)Sn₄ was also discussed. An erratum to this article can be found at     

Microstructure characterization of SnAgCu solder bearing Ce for electronic packaging

The creep-rupture lives of Sn3.8Ag0.7Cu and Sn3.8Ag0.7Cu0.03Ce lead-free solder joints for electronic packaging were investigated, respectively. And the relationship between creep behavior and intermetallic compound (IMC: Ag₃Sn, Cu₆Sn₅, CeSn₃) particles in SnAgCu/SnAgCuCe solder joints has been obtained. Meanwhile, rare earth Ce concentration gradient and retardation effect of Ce on the IMC layer have been observed at the solder/Cu interface. Moreover, aging reaction of Sn and Cu, and the effect mechanism of rare earth Ce on two IMCs (Cu₆Sn₅ and Cu₃Sn) are reported.     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Effects of Solder Volume on Formation and Redeposition of Au-Containing Intermetallics in Ni/Au-SnAgCu-Ni(P) Solder Joints

SnAgCu is one of the most promising candidates for lead-free solders to replace conventional eutectic SnPb solders. The effects of solder volume on interfacial reactions and microstructure evolution in Ni/Au-SnAgCu-Ni(P) solder joints have been investigated under soldering and thermal aging conditions. The results show that solder volume has a strong effect on the formation of Au-containing intermetallic compounds (IMCs) and their redeposition at the interfaces. The size and volume fraction of Au-containing IMCs dispersed in the solder bulk increased with decreasing solder joint dimensions. For the small solder joint with 300-μm solder ball, the (Au,Ni)Sn₄ IMCs redeposited to the interfaces after thermal aging at 150°C for 9 days, but this was not observed for the other two solder joints with large solder volume. These results also indicate that the redeposition of (Au,Ni)Sn₄ is closely associated with the migration of Cu toward the interfaces.     

Properties of Sn3.8Ag0.7Cu Solder Alloy with Trace Rare Earth Element Y Additions

In the current research, trace rare earth (RE) element Y was incorporated into a promising lead-free solder, Sn3.8Ag0.7Cu, in an effort to improve the comprehensive properties of Sn3.8Ag0.7Cu solder. The range of Y content in Sn3.8Ag0.7Cu solder alloys varied from 0 wt.% to 1.0 wt.%. As an illustration of the advantage of Y doping, the melting temperature, wettability, mechanical properties, and microstructures of Sn3.8Ag0.7CuY solder were studied. Trace Y additions had little influence on the melting behavior, but the solder showed better wettability and mechanical properties, as well as finer microstructures, than found in Y-free Sn3.8Ag0.7Cu solder. The Sn3.8Ag0.7Cu0.15Y solder alloy exhibited the best comprehensive properties compared to other solders with different Y content. Furthermore, interfacial and microstructural studies were conducted on Sn3.8Ag0.7Cu0.15Y solder alloys, and notable changes in microstructure were found compared to the Y-free alloy. The thickness of an intermetallic compound layer (IML) was decreased during soldering, and the growth of the IML was suppressed during aging. At the same time, the growth of intermetallic compounds (IMCs) inside the solder was reduced. In particular, some bigger IMC plates were replaced by fine, granular IMCs.     

Effect of Interfacial Reaction on the Tensile Strength of Sn-3.5Ag/Ni-P and Sn-37Pb/Ni-P Solder Joints

This work investigates the effect of interfacial reaction on the mechanical strength of two types of solder joints, Sn-3.5Ag/Ni-P and Sn-37Pb/Ni-P. The tensile strength and fracture behavior of the joints under different thermal aging conditions have been studied. It is observed that the tensile strength decreases with increasing aging temperature and duration. Associated with the tensile strength decrease is the transition of failure modes from within the bulk solder in the as-soldered condition toward failures at the interface between the solder and the intermetallic compounds (IMCs). For the same aging treatment, the strength of the Sn-3.5Ag/Ni-P joint degrades faster than that of Sn-37Pb/Ni-P. The difference between the two types of joints can be explained by the difference in their interfacial reaction and growth kinetics. An empirical relation is established between the solder joint strength and the Ni₃Sn₄ intermetallic compound thickness.     

Comparative study of microstructures and properties of three valuable SnAgCuRE lead-free solder alloys

Lead-free solders with excellent material properties and low cost are essential for the electronics industry. It has been proved that mechanical properties of SnAgCu alloys can be remarkably improved with a minute addition of rare earth (RE) elements. For comparison and optimization, three valuable solder candidates, Sn3.8Ag0.7Cu0.05RE, Sn3Ag0.5Cu0.05RE, and Sn2.9Ag1.2Cu0.05RE, were chosen due to the excellent properties of their own SnAgCu basic alloys. Wetting properties, melting temperature, bulk tensile properties, and joint tensile and shear properties were investigated. In addition, the microstructures of solder joints were observed and the effects of microstructure on mechanical properties were analyzed. Experimental results indicated that the tensile and shear strengths of solder joints were decreased from Sn3.8Ag0.7Cu0.05RE, Sn2.9Ag1.2Cu0.05RE, to Sn3Ag0.5Cu0.05RE, in order. Such difference in mechanical properties could be attributed to the influence of slightly coarse or strong Cu₆Sn₅ scallops in the reaction layer as well as superior eutectic network and large volume percentage of large primary intermetallic compounds (IMCs) inside the solder joints. It is also suggested that the size and volume percentage of large primary IMCs inside the solder be controlled. In addition, serration morphology was observed at the edge of large primary and eutectic IMCs in the three solder joints, which could be related to the content of Ag, Cu, and RE. The serration morphology was proved to be beneficial to mechanical properties theoretically. Furthermore, the three alloys investigated possessed similar wetting properties, melting temperatures, and bulk tensile properties.     

Effect of rare earth element additions on the microstructure and mechanical properties of tin-silver-bismuth solder

There are numerous intermetallic compounds (IMCs) with various shapes in a tin-silver-bismuth solder alloy. These IMCs can affect the mechanical properties of the solder and, therefore, the reliability of the joints. In this study, minimal rare earth elements added into the solder were adsorbed at the grain boundary of IMCs. This adsorption behavior changed the relationship between growth velocities of the various crystalline directions of the IMC polycrystalline, which resulted in finer particles and more uniform distribution of the IMC phase. The average size of IMC particles decreased from 0.20 μm to 0.12 μm, while their average distance decreased from 1.25 μm to 0.65 μm. These fine IMC particles made the alloy stronger and more ductile through dispersion strengthening.     

Microstructure evolution of eutectic Sn-Ag solder joints

Laser and infrared reflow soldering methods were used to make Sn-Ag eutectic solder joints for surface-mount components on printed wiring boards. The microstructures of the joints were evaluated and related to process parameters. Aging tests were conducted on these joints for times up to 300 days and at temperature up to 190°C. The evolution of microstructure during aging was examined. The results showed that Sn-Ag solder microstructure is unstable at high temperature, and microstructural evolution can cause solder joint failure. Cu-Sn intermetallics in the solder and at copper-solder interfaces played an important role in both the microstructure evolution and failure of solder joints. Void and crack formation in the aged joints was also observed.     

Impact strength of Sn–3.0Ag–0.5Cu solder bumps during isothermal aging

In order to investigate the fracture behavior of Sn–3.0Ag–0.5Cu solder bump, solder balls with the diameter of 0.76 mm were soldered on Cu pad in this study, then high speed impact test and static shear test of solder bumps were carried out to measure the joint strength of the soldering interface. The effect of isothermal aging on joint strength as well as fracture behavior of solder bumps was investigated, and the composition of the fracture surface was identified by means of EPMA. The results indicate that the fracture is inside the bulk solder in low speed shear test regardless of the aging effect, thus the maximum load reflects the solder strength rather than the interfacial strength. It is also found that under 1 m/s impact loading, the crack initiation position is changed from solder/Cu₆Sn₅ interface to Cu₃Sn/Cu interface after long time isothermal aging, and the fracture occurs inside the bulk solder accompanying with intermetallic compound in both of the as-soldered and aged joints. The thickened multiple IMC layers during isothermal aging account for the degraded impact resistance, and the change of the solder matrix is another factor for reduced impact resistance owing to Sn residue on the fracture surface.     

Sn-Ag-Cu焊料中金属间化合物对焊接性能的影响

讨论了Sn-Ag-Cu焊料与Cu焊盘间在回流焊过程中形成的金属间化合物(IMC)的种类、形态,Ag含量和Cu含量对IMC的影响,IMC在老化过程中的生长演变及其对焊接性能的影响。结果表明:Sn-Ag-Cu焊料与Cu焊盘之间的金属间化合物主要是Cu6Sn5和长针状的Ag3Sn,Ag和Cu的添加对组织有明显细化作用,但过量添加会影响IMC的性能。IMC的演变主要是与老化温度、老化时间有关,较厚的IMC不利于焊接性能的提高。     

Interfacial reactions of Cu-containing lead-free solders with Au/NiP metallization

Cu-containing solder alloys have been used to identify their interfacial reactions with electroless NiP. As-reflowed, AuSn₄ intermetallic compounds (IMCs) are formed in the Sn-Cu and Sn-Ag-Cu solders, but in the cases of Sn-Ag-Cu-In, In-Sn-Au IMCs are formed and are uniformly distributed in the solder. Different types of IMCs such as high-Cu (>30 at.%), medium-Cu (30-15 at.%), and low-Cu (<15 at.%) containing IMCs are formed at the interface. High-Cu and medium-Cu containing ternary intermetallic compounds (TIMCs) are found in the Sn-Cu and Sn-Ag-Cu solder joints, respectively. Medium-Cu containing quaternary intermetallic compounds (QIMCs) are found in the Sn-Ag-Cu-In joints. Initially, TIMCs and QIMCs have higher growth rates, resulting in the entrapment of some Pb-rich phase in the high-Cu containing TIMCs and some In-Sn-Au phase in the QIMCs. High-Cu containing TIMCs have a lower growth rate and consume less of the NiP layer. The spalling of medium-Cu containing TIMCs in the Sn-Ag-Cu solder increases both the growth rate of TIMCs and the consumption rate of the NiP layer. Low-Cu containing QIMCs in the Sn-Ag-Cu-In solder are stable on P-rich Ni and reduce the dissolution rate of the NiP layer. Consumption of the NiP layer can be reduced by adding Cu or In, because of the changes of the interfacial IMCs phases, which are stable and adhere well to the P-rich Ni layer during reflow.     

Investigation of soldering process and interfacial microstructure evolution for the formation of full Cu3Sn joints in electronic packaging

Within electronic products, solder joints with common interfacial structure of Cu/IMCs/Sn-based solders/IMCs/Cu cannot be used under high temperature for relatively low melting points of Sn-based solders (200–300 °C). However, there is a trend for solder joints to service under high temperature because of the objective for achieving multi-functionality of electronic products.With the purpose of ensuring that solder joints can service under high temperature, full Cu₃Sn solder joints with the interfacial structure of Cu/Cu₃Sn/Cu can be a substitute due to the high melting point of Cu₃Sn (676 °C). In this investigation, soldering process parameters were optimized systematically in order to obtain such joints. Further, interfacial microstructure evolution during soldering was analyzed. The soldering temperature of 260 °C, the soldering pressure of 1 N and the soldering time of 5 h were found to be the optimal parameter combination. During soldering of 260 °C and 1 N, the Cu₆Sn₅ precipitated first in a planar shape at Cu-Sn interfaces, which was followed by the appearance of planar Cu₃Sn between Cu and Cu₆Sn₅. Then, the Cu₆Sn₅ at opposite sides continued to grow with a transition from a planar shape to a scallop-like shape until residual Sn was consumed totally. Meanwhile, the Cu₃Sn grew with a round-trip shift from a planar shape to a wave-like shape until the full Cu₃Sn solder joint was eventually formed at 5 h. The detailed reasons for the shape transformation in both Cu₆Sn₅ and Cu₃Sn during soldering were given. Afterwards, a microstructure evolution model for Cu-Sn-Cu sandwich structure during soldering was proposed. Besides, it was found that no void appeared in the interfacial region during the entire soldering process, and a discuss about what led to the formation of void-free joints was conducted.     

Effect of rare earth element addition on the microstructure of Sn-Ag-Cu solder joint

The effects of minimal rare earth (RE) element additions on the microstructure of Sn-Ag-Cu solder joint, especially the intermetallic compounds (IMCs), were investigated. The range of RE content in Sn-Ag-Cu alloys varied from 0 wt.% to 0.25 wt.%. Experimental results showed that IMCs could be dramatically repressed with the appropriate addition of RE, resulting in a fine microstructure. However, there existed an effective range for the RE addition. The best RE content was found to be 0.1 wt.% in the current study. In addition to the typical morphology of Ag₃Sn and Cu₆Sn₅ IMCs, other types of IMCs that have irregular morphology and uncertain constituents were also observed. The IMCs with large plate shape mainly contained Ag and Sn, but the content of Ag was much lower than that of Ag₃Sn. The cross sections of Cu₆Sn₅ IMCs whiskers showed various morphologies. Furthermore, some eutectic-like structures, including lamellar-, rod-, and needle-like phases, were observed. The morphology of eutectic-like structure was related to the RE content in solder alloys. When the content of RE is 0.1 wt.%, the needle-like phase was dominant, while the lamellar structure prevailed when the RE content was 0.05 wt.% or 0.25 wt.%. It is suggested that the morphology change of the eutectic-like structure directly affects the creep properties of the solder joint.     

Effect of Zn Addition on Interfacial Reactions Between Sn-4Ag Solder and Ag Substrates

In this study, the effect of Zn (Zn = 1 wt.%, 3 wt.%, and 7 wt.%) additions to Sn-4Ag solder reacting with Ag substrates was investigated under solid-state and liquid-state conditions. The composition and microstructure of the intermetallic compounds (IMCs) significantly changed due to the introduction of different Zn contents. In the case of Sn-4Ag solder with 1 wt.% Zn, a continuous Ag-Sn IMC layer formed on the Ag substrates; discontinuous Ag-Zn layers and Sn-rich regions formed on the Ag substrates under liquid-state conditions when the Sn-4Ag solders contained 3 wt.% and 7 wt.% Zn. If 3 wt.% Zn was added to Sn-4Ag solder, the Ag-Sn IMC would be transformed into a Ag-Zn IMC with increasing aging time. Rough interfaces between the IMCs and the Ag substrates were observed in Sn-4Ag-7Zn/Ag joints after reflowing at 260°C for 15 min; however, the interfaces between the IMCs and the Ag substrates became smooth for Sn-4Ag-1Zn/Ag and Sn-4Ag-3Zn/Ag joints. The nonparabolic growth mechanism of IMCs was probed in the Sn-4Ag-3Zn/Ag joints during liquid-state reaction, and can be attributed to the detachment of IMCs. On the other hand, the effect of gravity was also taken into account to explain the formation of IMCs at the three different interfaces (bottom, top, and vertical) during the reflow procedure.     

Study of Solidification Cracks in Sn-Ag-Cu Lead-Free Solder Joints

In the present work, solidification cracks in Sn-Ag-Cu solder joints were investigated. Experimental results indicate that solidification cracks existed in significant numbers in the miniature Sn-Ag-Cu solder joints. In order to create solidification cracks in the miniature solder joints during solidification and evaluate the susceptibility of Sn-Ag-Cu alloys to solidification cracking, a copper self-restraint specimen was designed, which can simulate the process of solidification crack formation. The solidification crack susceptibility of the Sn-Ag-Cu solder alloy was evaluated using the total crack length of the solder joint. In addition, the effect of trace amounts of elemental additions on solidification cracking of Sn-Ag-Cu solder joints was studied. It was found that adding trace amounts of Ni or Ce could depress the solidification cracks in Sn-3.0Ag-0.5Cu solder joints. However, P additions aggravated the formation of solidification cracks.     

Mechanical properties of intermetallic compounds associated with Pb-free solder joints using nanoindentation

Mechanical properties of intermetallic compound (IMC) phases in Pb-free solder joints were obtained using nanoindentation testing (NIT). The elastic modulus and hardness were determined for IMC phases associated with insitu FeSn particle reinforced and mechanically added, Cu particle-reinforced, composite solder joints. The IMC layers that formed around Cu particle reinforcement and at the Cu substrate/solder matrix interface were probed with NIT. Moduli and hardness values obtained by NIT revealed were noticeably higher for Cu-rich Cu₃Sn than those of Cu₆Sn₅. The Ag₃Sn platelets that formed during reflow were also examined for eutectic Sn-Ag solder column joints. The indentation modulus of Ag₃Sn platelets was significantly lower than that of FeSn, SnCuNi, and CuSn IMCs. Indentation creep properties were assessed in localized microstructure regions of the as-cast, eutectic Sn-Ag solder. The stress exponent, n, associated with secondary creep differed widely depending on the microstructure feature probed by the indenter tip.     

Effects of Trace Amounts of Rare Earth Additions on Microstructure and Properties of Sn-Bi-Based Solder Alloy

The effect of trace amounts of rare earth additions on the microstructure and properties were studied for the Sn-58Bi and Sn-58Bi-Ag solder alloys. At the same time, the intermetallic compounds (IMCs) in the solder alloys and intermetallic layer (IML) thickness at the solder/Cu substrate interface were investigated, both as-reflowed and after high-temperature aging. The results indicate that adding trace amounts of rare earth (RE) elements has little influence on the melting temperature and microhardness of the solders investigated, but adding RE elements improves the wettability and shear strength of the Sn-58Bi and Sn-58Bi-Ag solder alloys. In addition, it was found that the addition of RE elements not only refines the microstructure and size of the IMC particles, but also decreases the IML thickness and shear strength of the Sn-58Bi solder joint after high-temperature aging. Adding trace amounts of RE elements is superior to adding trace amounts of Ag for improving the properties of the Sn-58Bi solder. The reason may be related to the modification of the microstructure of the solder alloys due to the addition of trace amounts of RE elements.     

A numerical method for predicting intermetallic layer thickness developed during the formation of solder joints

A numerical, method has been developed for calculating the thickness of intermetallic layers formed in substrate-solder systems during the soldering process. As input, the method requires the temperature-time profile for the soldering process and the isothermal liquid state growth rate parameters for the growth of the intermetallic layer. These usually consist of a growth constant, k_o, and an activation energy, Q. The method allows one to predict the thickness of a layer at any time during the soldering process. As such, it can be used in industrial solder processing to enhance the reliability and lifetime of solder joints by allowing control of the thickness of intermetallic layers. The validity of the method is demonstrated for intermetallic growth between copper and 62Sn-36Pb-2Ag solder. The kinetic parameters for the chosen model system were experimentally determined and isothermal intermetallic layer growth between molten solder and copper was found to follow a t^0.25 dependence on time t. The growth rate increased with increasing temperature according to an Arrhenius dependence in the temperature range 187 to 258°C with Q = 7.04 kJ/mol and k_o = 7.75 μm/min^0.25.