Effect of Ag on the Microstructure of Sn-8.5Zn-xAg-0.01Al-0.1Ga Solders Under High-Temperature and High-Humidity Conditions

The effect of Ag on the microstructure and thermal behavior of Sn-Zn and Sn-8.5Zn-xAg-0.01Al-0.1Ga solders (x from 0.1 wt.% to 1 wt.%) under high-temperature/relative humidity conditions (85°C/85% RH) for various exposure times was investigated. Scanning electron microscopy (SEM) studies revealed that, in all the investigated solders, the primary α-Zn phases were surrounded by eutectic β-Sn/α-Zn phases, in which fine Zn platelets were dispersed in the β-Sn matrix. SEM micrographs revealed that increase of the Ag content to 1 wt.% resulted in coarsening of the dendritic plates and diminished the Sn-9Zn eutectic phase in the microstructure. Differential scanning calorimetry (DSC) studies revealed that the melting temperature of Sn-8.5Zn-xAg-0.01Al-0.1Ga solder decreased from 199.6°C to 199.2°C with increase of the Ag content in the solder alloy. Both ZnO and SnO₂ along with Ag-Zn intermetallic compound (IMC) were formed on the surface when Sn-8.5Zn-0.5Ag-0.01Al-0.1Ga solder was exposed to high-temperature/high-humidity conditions (85°C/85% RH) for 100 h. The thickness of the ZnO phase increased as the Ag content and exposure time were increased. Sn whiskers of various shapes and lengths varying from 2 μm to 5 μm were extruded from the surface when the investigated five-element solder with Ag content varying from 0.5 wt.% to 1 wt.% was exposed to similar temperature/humidity conditions for 250 h. The length and density of the whiskers increased with further increase of the exposure time to 500 h and the Ag content in the solder to 1 wt.%. The Sn whisker growth was driven by the compressive stress in the solder, which was generated due to the volume expansion caused by ZnO and Ag-Zn intermetallic compound formation at the grain boundaries of Sn.     

Effect of reaction time on mechanical strength of the interface formed between the Sn-Zn(-Bi) solder and the Au/Ni/Cu bond pad

In this work, the shear strengths and the interfacial reactions of Sn-9Zn, Sn-8Zn-1Bi, and Sn-8Zn-3Bi (wt.%) solders with Au/Ni/Cu ball grid array (BGA) pad metallization were systematically investigated after extended reflows. Zn-containing Pb-free solder alloys were kept in molten condition (240°C) on the Au/electrolytic Ni/Cu bond pads for different time periods ranging from 1 min. to 60 min. to render the ultimate interfacial reaction and to observe the consecutive shear strength. After the shear test, fracture surfaces were investigated by scanning electron microscopy equipped with an energy dispersive x-ray spectrometer. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. The solder ball shear load for all the solders during extended reflow increased with the increase of reflow time up to a certain stage and then decreased. It was found that the formation of thick Ni-Zn intermetallic compound (IMC) layers at the solder interface of the Au/electrolytic Ni/Cu bond pad with Sn-Zn(-Bi) alloys deteriorated the mechanical strength of the joints. It was also noticed that the Ni-Zn IMC layer was larger in the Sn-Zn solder system than that in the other two Bi-containing solder systems.     

Superplastic creep of low melting point solder joints

In prior work, we showed that eutectic Sn-Pb solder joints exhibit superplastic behavior after rapid solidification. Further examples of superplasticity in nominally air-cooled solder joints are reported in this study of three low-melting point alloys: 40In-40Sn-20Pb (wt. %), eutectic 52In-48Sn, and 43Sn-43Pb-14Bi, which were creep-tested in shear at 20°, 65°, and 90° C. The test results indicate that above 65° C, the indium-containing solders have stress exponents between 2.4 to 2.9, a possible overall shear strains of 500%, and an absence of primary creep; at 90° C, 43Sn-43Pb-14Bi solder has a stress exponent close to 2.3. Optical microstructures of the three solders are presented; they help to explain the superplastic behavior.     

Dissolution behavior of Cu and Ag substrates in molten solders

This study investigated the dissolution behavior of Cu and Ag substrates in molten Sn, Sn-3.5Ag, Sn-4.0Ag-0.5Cu, Sn-8.6Zn and Sn-8.55Zn-0.5Ag-0.1Al-0.5Ga lead-free solders as well as in Sn-37Pb solder for comparison at 300, 350, and 400°C. Results show that Sn-Zn alloys have a substantially lower dissolution rate of both Cu and Ag substrates than the other solders. Differences in interfacial intermetallic compounds formed during reaction and the morphology of these compounds strongly affected the substrate dissolution behavior. Soldering temperature and the corresponding solubility limit of the substrate elements in the liquid solder also played important roles in the interfacial morphology and dissolution rate of substrate.     

Characterization of eutectic Sn-Bi solder joints

This report presents experimental results on 58Bi-42Sn solder joints, optical and SEM microstructures of their matrix and of their interface with copper, solidification behavior studied by differential scanning calorimetry, wettability to copper, creep, and low cycle fatigue. These results are discussed in comparison with 60Sn-40Pb solder, and with three low temperature solders, 52In-48Sn, 43Sn-43Pb-14Bi, and 40In-40Sn-20Pb. The 58Bi-42Sn solder paste with RMA flux wets Cu matrix with a wetting angle of 35° and had a 15° C undercooling during solidification. The constitutive equation of the steady state shear strain rate, and the Coffin-Manson relation constants for the low cycle shear fatigue life at 65° C have been determined. The test results show that this solder has the best creep resistance but the poorest fatigue strength compared with the other four solders.     

Influences of Substrate Alloying and Reflow Temperature on Bi Segregation Behaviors at Sn-Bi/Cu Interface

In this study, the influences of substrate alloying and reflow temperature on the Bi segregation behaviors at the Sn-Bi/Cu interface were investigated. Cu and Cu-Ag alloys with different Ag contents were reflowed with Sn-Bi solder at 180°C, 200°C or 220°C, and then aged at 120°C for different times. The evolution of their interfacial morphologies during the aging process was observed, and tensile tests of some solder joints were conducted. The experimental results reveal that the Bi atoms that dissolved in the Cu₆Sn₅ during the reflow process are expelled when the Cu₆Sn₅ transforms into Cu₃Sn and then segregate around the Cu₃Sn/Cu interface, inducing interfacial embrittlement. Alloying the Cu substrate with Ag can alleviate the Bi segregation by suppressing Cu₃Sn formation and dissolving the Bi atoms in the Cu-Ag substrate; the critical Ag content to eliminate the Bi segregation is about 1 at.% for the interface reflowed at 200°C. For interfaces reflowed at 180°C, the Bi segregation is less serious because less Bi is dissolved in the Cu₆Sn₅, and 0.6 at.% Ag can eliminate it. Tensile tests demonstrate that the embrittlement will not occur at Sn-Bi/Cu-Ag joints once the Bi segregation is eliminated. Based on this understanding, aging embrittlement of Sn-Bi/Cu solder joints can be prevented by decreasing the reflow temperature and adding a small amount of Ag to the Cu substrate.     

Effect of small additions of alloying elements on the properties of Sn-Zn eutectic alloy

Sn-Zn alloys have been considered for use as lead-free solders. Their poor wetting and oxidation resistance properties are the main obstacles that prevent them from becoming commercially viable solders. The effects of alloying elements, such as lanthanum, titanium, aluminum, and chromium, on oxidation resistance, wetting properties, and tensile properties of eutectic Sn-Zn solder are described herein. Results show the addition of alloying Ti, Al, and Cr can improve the oxidation resistance of Sn-9Zn solder. La, Ti, and Cr do not have much effect on the wetting properties, whereas Al worsens the wetting. Differential scanning calorimetry investigations reveal the solidus temperature of these solders to be ∼200°C. Addition of Cr improves ductility while maintaining tensile strength.     

Metallurgical reactions in composite 90Pb10Sn/lead-free solder joints and their effect on reliability of LTCC/PWB assembly

Use of 90Pb10Sn solder as a noncollapsible sphere material with 95.5Sn 4Ag0.5Cu and SnInAgCu lead-free solders is investigated. Practical reflow conditions led to strong Pb dissolution into liquid solder, resulting in >20 at.% Pb content in the original lead-free solders. The failure mechanism of the test joints is solder cracking due to thermal fatigue, but the characteristic lifetime of 90Pb10Sn/SnInAgCu joints is almost double that of 90Pb10Sn/95.5Sn4Ag0.5Cu in a thermal cycling test (TCT) over the temperature range from −40°C to 125°C. It is predicted that this is mainly a consequence of the better fatigue resistance of the SnPbInAgCu alloy compared with the SnPbAgCu alloy. Indium accelerates the growth of the intermetallic compound (IMC) layer at the low temperature co-fired ceramic (LTCC) metallization/solder interface and causes coarsening of IMC particles during the TCT, but these phenomena do not have a major effect on the creep/fatigue endurance of the test joints.     

Interfacial Microstructure Evolution Between Sn-Zn Solders and Ag Substrate During Solid-State Annealing

In this study, solid-state interfacial reactions between Ag and Sn-Zn alloys with varying Zn content (0.1 wt.% to 9 wt.%) were investigated at 170°C. The reaction couples were prepared by electroplating Ag on the Sn-Zn alloy to avoid dissolution of Ag into the molten solder during soldering. The Zn content greatly influenced the reaction products and the interfacial microstructures. When the Zn content was less than 4 wt.%, Ag₃Sn and AgZn layers were simultaneously formed. Notably, Zn could actively diffuse through the Ag₃Sn layer and react with Ag to form the AgZn phase. With the proceeding reaction, small α-Ag particulates were produced within the AgZn phase. With 9 wt.% Zn, the dominant reactions formed Ag₅Zn₈ and AgZn layers. The interfacial microstructure evolved significantly with reaction time. Interface instability due to Zn depletion in the solder resulted in massive spalling of the Ag₅Zn₈ layer. The Ag₃Sn phase was then produced next to the AgZn layer. Moreover, another reaction couple, Sn-9 wt.%Zn/Sn(15 μm)/Ag, was prepared, in which fast interdiffusion between Zn and Ag across the Sn layer was demonstrated due to the strong chemical affinity of Zn.     

Contact resistance and shear strength of the solder joints formed using Cu bumps capped with Sn or Ag/Sn layer

Solder joints were successfully bonded by joining Ag/Sn/Cu bumps and Ag/Sn/Cu layers at 200°C for 30 sec under 20 MPa, 40 MPa, and 80 MPa using thermo-compression bonder. The solder joints were aged at 150°C up to 1000 h. The strength of the solder joints was measured by the shear test and the contact resistance was measured using four-point probe method. The microstructure of the solder joints and the fracture modes after shear test were analyzed by scanning electron microscopy (SEM) with the energy-dispersive spectrometry (EDS). Results showed that the electrical resistance of the solder joints decreased, and the shear strength of the solder joints increased after aging treatment. The fracture modes were observed to move from the interfacial failure between solder and intermetallic compounds (IMCs) to the interfacial failure between IMCs. It was considered that the transition of fracture modes was closely related with the microstructure evolution of the solder joints, especially the transformation of IMC phases during the aging treatment.     

Properties of low temperature Sn–Ag–Bi–In solder systems

The metallurgical and mechanical properties of Sn–3.5 wt%Ag–0.5 wt%Bi–xwt%In (x = 0–16) alloys and of their joints during 85 °C/85% relative humidity (RH) exposure and heat cycle test (−40–125 °C) were evaluated by microstructure observation, high temperature X-ray diffraction analysis, shear and peeling tests. The exposure of Sn–Ag–Bi–In joints to 85 °C/85%RH for up to 1000 h promotes In–O formation along the free surfaces of the solder fillets. The 85°C/85%RH exposure, however, does not influence the joint strength for 1000 h. Comparing with Sn–Zn–Bi solders, Sn–Ag–Bi–In solders are much stable against moisture, i.e. even at 85 °C/85%RH. Sn–Ag–Bi–In alloys with middle In content show severe deformation under a heat cycles between −40 °C and 125 °C after 2500 cycles, due to the phase transformation from β-Sn to β-Sn + γ-InSn₄ or γ-InSn₄ at 125 °C. Even though such deformation, high joint strength can be maintained for 1000 heat cycles.     

Microstructure, creep properties, and failure mechanism of SnAgCu solder joints

The effect of microstructure on the creep properties and the failure mechanism of SnAgCu solder joints was studied. Single overlap shear specimens made of FR-4 printed circuit boards (PCBs) with organic solderability preservative (OSP), NiAu, and immersion Sn surface finish were reflow-soldered with hypoeutectic, eutectic, and hypereutectic SnAgCu solder paste. Creep tests of the solder joints were performed at 85°C and 105°C under constant load. The effect of microstructure on the creep behavior of the joints was studied by examining the fracture surfaces and cross-sectional samples of the tested joints. Results show that the intermetallic compound at the interface between the PCB and solder affects the fracture behavior of SnAgCu solder joints, thus creating a significant difference in the creep properties of solder joints on different surface finishes. Composition of SnAgCu solder was also found to affect the creep properties of the joints.     

Mechanical response of solder joints in flip-chip type structures

The mechanical response of PbSn solder joints of two different solder alloys (37 wt.% Pb - 63 wt.% Sn and 95 wt.% Pb - 5 wt.% Sn) used as flip-chip type interconnects is measured through mechanical testing (in tension and in shear). The influence of solder pad composition (Au and Ni) upon the behaviour of the solder joints is examined. Fatigue testing performed upon flipchip samples demonstrates the difference in mechanical comportment between Pb37Sn63 and Pb95Sn5 solders. A model for predicting fatigue life is put forward.     

Diffusion Behavior of Zn During Reflow of Sn-9Zn Solder on Ni/Cu Substrate

Reflow behavior of Sn-Zn solder on Ni/Cu substrate was investigated in an infrared (IR) reflow furnace using different heating rates from 180 °C/min to 36 °C/min. Energy dispersive spectroscopy (EDX) studies of the reflowed samples showed increased diffusion of Zn from the solder to the intermetallic compound (IMC) layer with a decrease in heating rate. Higher interfacial activity and affinity of the Zn in the formation of the IMC with the lowering of the heating rate were further demonstrated by electron probe microanalyzer (EPMA) results. The Zn diffusion behavior was related to its higher reactivity and smaller size in comparison with Sn, phase separation during heating, and affinity of Zn to form Ni-Sn-Zn and Ni-Zn intermetallic compounds.     

Effects of Ce and La Additions on the Microstructure and Mechanical Properties of Sn-9Zn Solder Joints

The effects of rare-earth elements on the microstructure and mechanical properties of Sn-9Zn alloys and solder joints in ball grid array packages with Ni/Au(ENIG) surface finishes have been investigated. Metallographic observations showed that (Ce_0.8Zn_0.2)Sn₃ and (La_0.9Zn_0.1)Sn₃ intermetallic compounds appeared in the solder matrix of Sn-9Zn-0.5Ce and Sn-9Zn-0.5La alloys, respectively. Both fiber- and hillock-shaped tin whiskers were inhibited in the Sn-9Zn-0.5Ce solder, while tin fibers were still observed on the surface of oxidized (La_0.9Zn_0.1)Sn₃ intermetallics in Sn-9Zn-0.5La after air exposure at room temperature. Mechanical testing indicated that the tensile strength of Sn-9Zn alloys doped with Ce and La increased significantly, and the elongation decreased, in comparison with the undoped Sn-9Zn. The bonding strengths of the as-reflowed Sn-9Zn-0.5Ce and Sn-9Zn-0.5La solder joints were also improved. However, aging treatment at 100°C and 150°C caused degradation of ball shear strength in all specimens. During the reflowing and aging processes, AuZn₈ intermetallic phases appeared at the interfaces of all solder joints. In addition, Zn-rich phases were observed to migrate from the solder matrix to the solder/pad interfaces of the aged specimens.     

Elevated temperature aging of solder joints based on Sn-Ag-Cu: Effects on joint microstructure and shear strength

The shear strength behavior and microstructural effects after aging for 100 h and 1,000 h at 150°C are reported for near-eutectic Sn-Ag-Cu (SAC) solder joints (joining to Cu) made from Sn-3.5Ag (wt.%) and a set of SAC alloys (including Co- and Fe-modified SAC alloys). All joints in the as-soldered and 100-h aged condition experienced shear failure in a ductile manner by either uniform shear of the solder matrix (in the strongest solders) or by a more localized shear of the solder matrix adjacent to the Cu₆Sn₅ interfacial layer, consistent with other observations. After 1,000 h of aging, a level of embrittlement of the Cu₃Sn/Cu interface can be detected in some solder joints made with all of the SAC alloys and with Sn-3.5Ag, which can lead to partial debonding during shear testing. However, only ductile failure was observed in all solder joints made from the Co- and Fe-modified SAC alloys after aging for 1,000 h. Thus, the strategy of modifying a strong (high Cu content) SAC solder alloy with a substitutional alloy addition for Cu seems to be effective for producing a solder joint that retains both strength and ductility for extended isothermal aging at high temperatures.     

Comparison of Extensive Thermal Cycling Effects on Microstructure Development in Micro-alloyed Sn-Ag-Cu Solder Joints

Pb-free solder alloys based on the Sn-Ag-Cu (SAC) ternary eutectic have promise for widespread adoption across assembly conditions and operating environments, but enhanced microstructural control is needed. Micro-alloying with elements such as Zn was demonstrated for promoting a preferred solidification path and joint microstructure earlier in simple (Cu/Cu) solder joints studies for different cooling rates. This beneficial behavior now has been verified in reworked ball grid array (BGA) joints, using dissimilar SAC305 (Sn-3.0Ag-0.5Cu, wt.%) solder paste. After industrial assembly, BGA components joined with Sn-3.5Ag-0.74Cu-0.21Zn solder were tested in thermal cycling (−55°C/+125°C) along with baseline SAC305 BGA joints beyond 3000 cycles with continuous failure monitoring. Weibull analysis of the results demonstrated that BGA components joined with SAC + Zn/SAC305 have less joint integrity than SAC305 joints, but their lifetime is sufficient for severe applications in consumer, defense, and avionics electronic product field environments. Failure analysis of the BGA joints revealed that cracking did not deviate from the typical top area (BGA component side) of each joint, in spite of different Ag₃Sn blade content. Thus, SAC + Zn solder has not shown any advantage over SAC305 solder in these thermal cycling trials, but other characteristics of SAC + Zn solder may make it more attractive for use across the full range of harsh conditions of avionics or defense applications.     

Microstructure and shear strength of Sn37Pb/Cu solder joints subjected to isothermal aging

The effects of isothermal aging on the microstructure and shear strength of Sn37Pb/Cu solder joints were investigated. Single-lap shear solder joints of eutectic Sn37Pb solder were aged for 1–10 days at 120 °C and 170 °C, respectively, and then loaded to failure in shear with a constant loading speed of 5 × 10⁻³ mm/s. The growth of the interfacial Cu–Sn intermetallic compounds (IMC) layer (Cu₆Sn₅ + Cu₃Sn) of Sn37Pb/Cu solder joints subjected to isothermal aging exhibited a linear function of the square root of aging time, indicating that the formation of Cu–Sn IMC was mainly controlled by the diffusion mechanism. And the diffusion coefficient (D) values of IMC layer were 1.07 × 10⁻¹⁷ and 3.72 × 10⁻¹⁷ m²/s for aged solder joints at 120 °C and 170 °C, respectively. Shear tests results revealed that as-reflowed solder joint had better shear strength than the aged solder joints and the shear strength of all aged solder joints decreased with increasing aging time. The presence of elongated dimple-like structures on the fracture surfaces of these as-reflowed or aged for short time solder joints were indicative of a ductile failure mode. As aging time further increased, the solder joints fractured in the mixed solder/IMC mode at the solder/IMC interface.     

A microstructural study of the thermal fatigue failures of 60sn-40Pb solder joints

When an electronic package encounters thermal fluctuations, cyclical shear strain is imposed on the solder joint interconnections. The thermal cycling leads to a condition of thermal fatigue and eventual solder joint failure. This study was performed in order to understand the microstructural mechanisms that lead to solder joint failures in thermal fatigue. Thermal cycling tests were performed on 60Sn-40Pb joints using a -55° C to 125° C cycle and 19% imposed shear strain. A heterogeneously coarsened region of both Pb and Sn-rich phases develops within the 60Sn-40Pb solder joints. Cracks initiate in the heterogeneously coarsened Sn-rich phase at the Sn-Sn grain boundaries. Heterogeneous coarsening and failure occurs in both high (35 to 125° C) and low (-55 to 35° C) thermal cycles. The elevated temperature portion of the thermal cycle was found to be the most significant factor in the heterogeneous coarsening and failure of the solder joints.     

Formation of a Metastable Phase at the Interface Between Sn and Ag–Pd Substrates During Liquid-State Reaction

Ag–Pd alloys are widely used as thick-film conductors and are potential alternatives to the expensive Au bump. In this work, because Sn is the primary element in solders, we investigated Sn/Ag–Pd interfacial reactions at 250°C as a means of assessing the reliability and evaluating reflow reactions at joints between solder and Ag–Pd conductor contacts, and in the Ag bump combined with the solder cap. The experimental results showed that Sn/Ag–Pd interfacial reactions at 250°C are different from those of Sn/Ag and Sn/Pd. A metastable Sn–Ag–Pd ternary phase is formed when the amount of Pd added is 20–40 at.%. Because, in commercial applications, at least 20 wt.% Pd (~20 at.% Pd) is used in Ag–Pd alloys to eliminate the silver-migration phenomenon, assessment of the reliability of Ag bumps and the soldered joints of Ag–Pd thick film hybrid circuits must be based on Sn/Ag–Pd interfacial reactions, not those of Sn/Pd and Sn/Ag.