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.     

Microstructural and tensile properties of Fe and Bi added Sn-1Ag-0.5Cu solder alloy under high temperature environment

In this work, the iron (Fe) and bismuth (Bi) added (0.05 wt% Fe and 1 wt% or 2 wt% Bi) Sn-1Ag-0.5Cu (SAC105) lead-free solder alloys were prepared and their microstructure and tensile properties under severe thermal environments were extensively investigated and compared with the base alloy SAC105. The isothermal aging was done at 200 °C for 100 h, 200 h, and 300 h. Fe/Bi added SAC105 showed a significant reduction in the IMCs size (Ag₃Sn and Cu₆Sn₅), especially the Cu₆Sn₅ IMCs and a refinement in the microstructure, which is due to the existence of Bi in the alloys. Moreover, the existence of Fe and Bi gives the microstructure better stability under severe thermal aging conditions. The tensile testing results showed that the addition of Fe and Bi to SAC105 greatly improves yield stress and tensile strength, but decreases ductility level, which is because of the Bi solid solution strengthening mechanism. Under severe thermal aging, the Fe/Bi added SAC105 showed more stable tensile properties, because of the existence of both Fe and Bi in the alloys.     

Effect of Ag Content and the Minor Alloying Element Fe on the Mechanical Properties and Microstructural Stability of Sn-Ag-Cu Solder Alloy Under High-Temperature Annealing

This study compares the high-Ag-content Sn-3Ag-0.5Cu with the low- Ag-content Sn-1Ag-0.5Cu solder alloy and the three quaternary solder alloys Sn-1Ag-0.5Cu-0.1Fe, Sn-1Ag-0.5Cu-0.3Fe, and Sn-1Ag-0.5Cu-0.5Fe to understand the beneficial effects of Fe on the microstructural stability, mechanical properties, and thermal behavior of the low-Ag-content Sn-1Ag-0.5Cu solder alloy. The results indicate that the Sn-3Ag-0.5Cu solder alloy possesses small primary β-Sn dendrites and wide interdendritic regions consisting of a large number of fine Ag₃Sn intermetallic compound (IMC) particles. However, the Sn-1Ag-0.5Cu solder alloy possesses large primary β-Sn dendrites and narrow interdendritic regions of sparsely distributed Ag₃Sn IMC particles. The Fe-bearing SAC105 solder alloys possess large primary β-Sn dendrites and narrow interdendritic regions of sparsely distributed Ag₃Sn IMC particles containing a small amount of Fe. Moreover, the addition of Fe leads to the formation of large circular FeSn₂ IMC particles located in the interdendritic regions. On the one hand, tensile tests indicate that the elastic modulus, yield strength, and ultimate tensile strength (UTS) increase with increasing Ag content. On the other hand, increasing the Ag content reduces the total elongation. The addition of Fe decreases the elastic modulus, yield strength, and UTS, while the total elongation is still maintained at the Sn-1Ag-0.5Cu level. The effect of aging on the mechanical behavior was studied. After 720 h and 24 h of aging at 100°C and 180°C, respectively, the Sn-1Ag-0.5Cu solder alloy experienced a large degradation in its mechanical properties after both of the aging conditions, whereas the mechanical properties of the Sn-3Ag-0.5Cu solder alloy degraded more dramatically after 24 h of aging at 180°C. However, the Fe-bearing SAC105 solder alloys exhibited only slight changes in their mechanical properties after both aging procedures. The inclusion of Fe in the Ag₃Sn IMC particles suppresses their IMC coarsening, which stabilizes the mechanical properties of the Fe-bearing SAC105 solder alloys after aging. The results from differential scanning calorimetry (DSC) tests indicate that the addition of Fe has a negligible effect on the melting behavior. However, the addition of Fe significantly reduces the solidification onset temperature and consequently increases the degree of undercooling. In addition, fracture surface analysis indicates that the addition of Fe to the Sn-1Ag-0.5Cu alloy does not affect the mode of fracture, and all tested alloys exhibited large ductile dimples on the fracture surface.     

Isothermal aging of near-eutectic Sn-Ag-Cu solder alloys and its effect on electrical resistivity

Solder joints were prepared from seven eutectic and near-eutectic Sn-based compositions and characterized for electrical resistivity after 100 h and 1,000 h of isothermal aging at 423 K. The solder joint samples were prepared by hand soldering to copper substrates, and the post-heat treatment resistivity was measured at room temperature in a specially designed, four-point probe fixture. Compositions tested included Sn-3.5Ag, Sn-3.7Ag-0.9Cu, Sn-3.0Ag-0.5Cu, Sn-3.6Ag-1.0Cu, and Sn-3.9Ag-0.6Cu. In addition, the effect of a minor addition of a fourth element, designed to improve high-temperature shear strength, was also evaluated in the compositions Sn-3.7Ag-0.6Cu-0.3Co and Sn-3.7Ag-0.7Cu-0.2Fe. The observed changes in electrical resistivity are discussed in terms of microstructural coarsening, diffusional transport from the substrate, and nucleation of precipitate phases.     

Kinetics of Intermetallic Compound Formation at the Interface Between Sn-3.0Ag-0.5Cu Solder and Cu-Zn Alloy Substrates

The growth kinetics of an intermetallic compound (IMC) layer formed between Sn-3.5Ag-0.5Cu (SAC) solders and Cu-Zn alloy substrates was investigated for samples aged at different temperatures. Scallop-shaped Cu₆Sn₅ formed after soldering by dipping Cu or Cu-10 wt.%Zn wires into the molten solder at 260°C. Isothermal aging was performed at 120°C, 150°C, and 180°C for up to 2000 h. During the aging process, the morphology of Cu₆Sn₅ changed to a planar type in both specimens. Typical bilayer of Cu₆Sn₅ and Cu₃Sn and numerous microvoids were formed at the SAC/Cu interfaces after aging, while Cu₃Sn and microvoids were not observed at the SAC/Cu-Zn interfaces. IMC growth on the Cu substrate was controlled by volume diffusion in all conditions. In contrast, IMC growth on Cu-Zn specimens was controlled by interfacial reaction for a short aging time and volume diffusion kinetics for a long aging time. The growth rate of IMCs on Cu-Zn substrates was much slower due to the larger activation energy and the lower layer growth coefficient for the growth of Cu-Sn IMCs. This effect was more prominent at higher aging temperatures.     

Microstructural modifications and properties of Sn-Ag-Cu solder joints induced by alloying

Slow cooling (1–3°C/sec) of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens, made by hand soldering, simulated reflow in a surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shear-strength testing, using Sn-3.5Ag (wt.%) as a baseline. Consistent with predictions from a recent Sn-Ag-Cu ternary phase-diagram study, either Sn dendrites, Ag₃Sn primary phase, or Cu₆Sn₅ primary phase were formed during solidification of joint samples made from the selected near-eutectic Sn-Ag-Cu alloys. Minor substitution of Co for Cu in Sn-3.7Ag-0.9Cu refined the joint-matrix microstructure by an apparent catalysis effect on the Cu₆Sn₅ phase, whereas Fe substitution promoted extreme refinement of the Sn-dendritic phase. Ambient-temperature shear strength was reduced by Sn dendrites in the joint microstructure, especially coarse dendrites in solute poor Sn-Ag-Cu, e.g., Sn-3.0Ag-0.5Cu, while Sn-3.7Ag-0.9Cu with Co and Fe additions have increased shear strength. At elevated (150°C) temperature, no significant difference exists between the maximum shear-strength values of all of the alloys studied.     

Effects of fourth alloying additive on microstructures and tensile properties of Sn–Ag–Cu alloy and joints with Cu

The effects of the fourth elements, i.e., Fe, Ni, Co, Mn and Ti, on microstructural features, undercooling characteristics, and monotonic tensile properties of Sn–3 wt.%Ag–0.5 wt.%Cu lead-free solder were investigated. All quaternary alloys basically form third intermetallic compounds in addition to fine Ag₃Sn and Cu₆Sn₅ and exhibit improved solder structure. The precipitates of Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Ni, Ti and Mn) alloy are very fine comparing with the other alloys. The effective elements for suppressing undercooling in solidification are Ti, Mn, Co and Ni. All quaternary bulk alloys exhibit similar or slightly larger tensile strengths; especially Mn and Ni can improve elongation without degrading strength. The interfacial phases of Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Fe, Mn and Ti)/Cu joints are typical Cu₆Sn₅ scallops. Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Ni and Co)/Cu joints form very fine Sn–Cu–Ni and Sn–Cu–Co scallops at interface. The Cu/Sn–3Ag–0.5Cu–0.1Ni/Cu joint exhibits improved tensile strength prior to thermal aging at 125 and 150 °C. The fracture surface of Cu/Sn–3Ag–0.5Cu/Cu joint exhibits mixture of ductile and brittle fractures, while Cu/Sn–3Ag–0.5Cu (–0.1X; X=Ni and Co)/Cu joints exhibit only brittle fracture at interface. The Sn–3Ag–0.5Cu–0.1Ni alloy is more reliable solder alloy with improved properties for all tests in the present work.     

Reliability of adhesion strength of the Sn-9Zn-1.5Ag-0.5Bi/Cu during isothermal aging

The reliability of adhesion strength of the Sn-9Zn-1.5Ag-0.5Bi/Cu during isothermal aging has been investigated. Due to the growth and decomposition of the intermetallic compound (IMC), the adhesion strength varies with aging at 150°C from 100, 400, and 700–1,000 h as wetted at 250°C for 60 sec. The IMC layers are determined at the Sn-9Zn-1.5Ag-0.5Bi/Cu interface by an x-ray diffractometer (XRD), an optical microscope (OM), a scanning electron microscope (SEM), an energy-dispersive spectroscope (EDS), and a transmission electron microscope (TEM). The adhesion strength has been investigated by the pull-off test. The results show that the Cu₆Sn₅, Cu₅Zn₈, and Ag₃Sn IMCs are identified at the Sn-9Zn-1.5Ag-0.5Bi/Cu interface as aging. The adhesion strengths are 12.44±0.58, 8.57±0.43, 5.50±0.78, 4.32±0.78, and 3.32±0.43 MPa for aging times of 0 h, 100 h, 400 h, 700 h, and 1,000 h, respectively.     

Interfacial reaction and shear strength of SnAgCu-xNi/Ni solder joints during aging at 150 °C

The interfacial microstructure and shear strength of Sn3.8Ag0.7Cu-xNi (SAC-xNi, x = 0.5, 1, and 2) composite solders on Ni/Au finished Cu pads were investigated in detail after aging at 150 °C for up to 1000 h. The interfacial characteristics of composite solder joints were affected significantly by the weight percentages of added Ni micro-particles and aging time. After aging for 200 h, the solder joints of SAC, SAC-0.5Ni and -1Ni presented duplex intermetallic compound (IMC) layers regardless of the initial interfacial structure on as-reflowed joints, whose upper and lower IMC layers were comprised of (CuNi)₆Sn₅ and (NiCu)₃Sn₄, respectively. Only a single (NiCu)₃Sn₄ IMC layer was ever observed at the SAC-2Ni/Ni interface on whole aging process. Based on the compositional analysis, the amount of Ni within the IMC regions increased as the proportion of Ni addition increased. The IMC (NiCu)₃Sn₄ layer thickness on the interface of SAC and SAC-0.5Ni grew more slowly when compared to that of SAC-1Ni and -2Ni, while for the (CuNi)₆Sn₅ layer the reverse is true. Except the IMCs sizes are increased with increased aging time, the interfacial IMCs tended to transfer their morphologies to polyhedra. In all composite joints testing, the shear strengths were approximately equal to non-composite joints. The fracturing observed during shear testing of composite joints occurred in the bulk solder, indicating that the SAC-xNi/Ni solder joints had a desirable joint reliability.     

Nucleation Control and Thermal Aging Resistance of Near-Eutectic Sn-Ag-Cu-X Solder Joints by Alloy Design

Elemental (X) additions to Sn-3.5Ag-0.95Cu (SAC3595) solder were developed with minimal (<0.25 wt.%) concentration to avoid pro-eutectic Ag₃Sn blades by reducing undercooling (ΔT) and to eliminate thermal-aging-induced embrittlement. Calorimetry and microstructure results on simple Cu/Cu joints identified 0.21Zn, 0.10Mn, and 0.05Al as sufficient to reduce undercooling below that for SAC3595 and to eliminate Ag₃Sn blades. A 211°C melting onset for the X = Mn alloys also suggested the discovery of a new quaternary eutectic. Shear testing and microstructure analysis of larger joints showed that 0.05Al and 0.21Zn additions resulted in reduced as-soldered strength (30 MPa), like Sn-0.95Cu, but all joints showed ductile failure at about 30 MPa after 1000 h at 150°C.     

Effect of aging on the microstructure and shear strength of SnPbAg/Ni-P/Cu and SnAg/Ni-P/Cu solder joints

The effect of aging on the microstructure and shear strength of 62Sn36Pb2Ag/Ni-P/Cu and SnAg/Ni-P/Cu surface mount solder joints was investigated. An intermetallic (IMC) layer of Ni₃Sn₄ forms at the interface between both solders and the Ni-P barrier layer and it thickens with aging time, with a decrease in the thickness of remaining Ni-P layer. The SnAg solder joint initially has a greater shear force than that of SnPbAg, but it drops dramatically after 250 h aging, and fracture occurs at the Ni-P/Cu interface afterwards, although it initiates in the solder in the initial stage of aging. The fracture in SnAg solder joint may arise from the excessive depletion of Ni characterized by a rapid accumulation of P in the remaining Ni-P layer, which results in a poor adhesion between the Ni-P layer and the Cu substrate. However, for the SnPbAg solder joint, the shear force initially decreases rapidly then asymptotically approaches a minimum, and fracture occurs from inside solder toward the solder/Ni-P interface. SnPbAg solder joint keeps relatively higher shear strength compared to SnAg solder joint after long term aging even though it decreases with aging time.     

Experimental determination of fatigue behavior of lead free solder joints in microelectronic packaging subjected to isothermal aging

The effects of aging on the cyclic shear stress–strain and fatigue behavior of lead-free solders have been explored experimentally and have been presented in this paper. An experimental procedure has been developed for preparing Iosipescu shear specimens of SAC105 (Sn–1.0Ag–0.5Cu) lead-free solder, and the resulting solder joint specimens have been subjected to cyclic shear stress/strain loading at different aging conditions. A combination of four-parameter hyperbolic tangent empirical models has been used for the empirical fit of the entire cyclic stress strain curve. The fatigue life data were then fit using popular empirical failure criteria such as the strain-based Coffin–Manson model and the energy-based Morrow model. Evolution of shear hysteresis loop of SAC 105 with aging has been studied. Degradation of isothermal fatigue life due to aging has also been studied in this paper. A comparison between uniaxial fatigue data and shear fatigue data is shown and a good qualitative agreement has been found. Subsequent microstructure analysis has also been presented in the paper in support of isothermal aging effects.     

Chip to Chip Bonding Using Cu Bumps Capped with Thin Sn Layers and the Effect of Microstructure on the Shear Strength of Joints

Chip to chip bonding techniques using Cu bumps capped with thin solder layers have been frequently applied to 3D chip stacking technology. We studied the effect of joint microstructure on shear strength. Joints were formed by joining Sn/Cu bumps on a Si die and Sn/Cu layers on another Si die at 245–330°C using a thermo-compression bonder. Three different types of microstructures were fabricated in the joints by controlling the bonding temperature and time, (1) a Sn-rich phase with a Cu₆Sn₅ phase at the Cu interfaces, (2) a Cu₆Sn₅ phase in the interior with a Cu₃Sn phase at the Cu interfaces, and (3) one single Cu₃Sn phase throughout the whole joint. The joint having a single Cu₃Sn phase had the highest shear strength. Specimens were aged up to 2000 h at 150°C and 180°C. During aging, the microstructures of all joints were transformed in a single Cu₃Sn phase. The shear strength of the joints was very sensitive to the formation of Cu₃Sn and microvoids. Microvoids formed in the solder joints with a Cu₆Sn₅ phase with and without a Sn-rich phase during aging and decreased the shear strength of the joints. Conversely, aging did not induce the formation of microvoids in the joints which originally had only a Cu₃Sn phase and the shear strength was not decreased.     

Effect of Ni-Coated Carbon Nanotubes on Interfacial Reaction and Shear Strength of Sn-Ag-Cu Solder Joints

In this study, varying weight percentages of Ni-coated carbon nanotubes (Ni-CNTs) were incorporated into Sn-Ag-Cu (SAC) solder matrix, to form composite solder. Up to 0.05% of Ni-CNTs were successfully incorporated. The interfacial microstructure and shear strength of solders on Ni/Au-finished Cu substrates were investigated after aging at 150°C for up to 42?days. Results revealed that, after soldering and aging for various lengths of time, the interfacial intermetallic compound (IMC) thickness of the unreinforced solder joint was observed to grow more significantly than that of the composite solder joints. Furthermore, the composite solder joints also showed lower diffusion coefficients (2.5?×?10^?15?cm²/s to 3.2?×?10^?15?cm²/s) compared with that of the SAC solder joints (4.9?×?10^?15?cm²/s). Shear test results revealed that as-soldered and aged composite solder joints had better ultimate shear strength than their monolithic counterparts and the shear strength of all aged solder joints decreased with increasing aging time.     

Microstructure and mechanical properties evolution of intermetallics between Cu and Sn-3.5Ag solder doped by Ni-Co additives

The evolution of intermetallic compounds (IMCs) generated between Sn-3.5Ag solder doped by additive couples (namely, 0.2mass%Co and 0.1mass%Ni) and Cu substrate was characterized. After soldering, the additive couples, Co-Ni, were all detected at the intermetallic region. The microstructure of intermetallic was identified as (Cu, Ni, Co)₆Sn₅ by electron probe microanalysis (EPMA) and x-ray diffraction (XRD). However, the morphology of (Cu, Ni, Co)₆Sn₅ was converted to columnar like and was not as dense as the typical scallop-like Cu₆Sn₅. A duplex structure of (Cu, Ni, Co)₆Sn₅, namely, two distinct regions bearing different concentrations of Ni and Co, was observed. Much higher Ni and Co concentrations were probed in the outer intermetallic region adjacent to the solder matrix, while lower concentration at the inner region was verified. After aging, the intermetallic (Cu, Ni, Co)₆Sn₅ tended to be dense, while the growth rate was depressed at the early stage. In addition, the Cu₃Sn phase was not detected after aging at 110°C, while it appeared at 130°C and 150°C for 504 h. Using the nanoindentation technique, some mechanical properties of (Cu, Ni, Co)₆Sn₅ were investigated. The lower hardness and Young’s modulus of the outer intermetallic region was revealed. After aging treatment, both the hardness and Young’s modulus values were elevated.     

Reliability testing of WLCSP lead-free solder joints

The interfacial reactions of solder joints between Sn-4Ag-0.5Cu solder ball and a couple of presoldered pastes (Sn-7Zn-Al(30ppm) and Sn-3Ag-0.5Cu) were investigated in wafer-level chip-scale package (WLCSP). After appropriate surface mount technology reflow processes on printed circuit boards with a Cu/OSP (organic solderability preservative) surface finish, samples were subjected to 150°C high-temperature storage (HTS) for 1,000 h of aging or 1,000 cycles of a thermal cycling test (TCT). Sequentially, cross-section analysis is scrutinized by scanning electron microscopy/energy dispersive spectrometry and energy probe microanalysis to observe metallurgical evolution in the interface and solder buck itself. It was found that the degradation of the joint shear strength after TCT is more pronounced than that of the shear strength after HTS. Fracture surface analyses of the shear tests show that the degradation of the joint strength for HTS is solely due to the influence of the interfacial IMC grain growth, while the shear strength degradation for TCT is mainly due to the coefficient thermal expansion mismatch from the thermal cycling at the chip-solder interface and can lead to the occurrence of the crack.     

Aging study on interfacial microstructure and solder-ball shear strength of a wafer-level chip-size package with Au/Ni metallization on a Cu pad

Eutectic solder balls (63Sn-37Pb) joined to Cu pads with an Au/Ni metallization have been widely used in wafer-level chip-size package (WLCSP) technology for providing electrical and mechanical interconnections between components. However, some reliability issues must be addressed regarding the intermetallic compounds (IMCs). The formation of a brittle IMC layer between the solder/Cu pad interface impacts considerably upon the solder-ball shear strength. In addition, it will degrade the long-term operating reliability of the WLCSP. This study investigates, by means of experiments, the growth of the IMC layer under isothermal aging for the eutectic Sn-Pb solder reflowed on a Cu pad with an Au/Ni metallization. Forming the Cu pad with an Au/Ni metallization was achieved by a simple semiconductor-manufacturing process. The effects of the intermetallic layer on solder-ball shear strength were examined for various parameters, including the thickness of the Au layer, solder-ball size, and the diameter of the Cu pad. Experimental results indicate that two IMC layers, Au_0.5Ni_0.5Sn₄ and Ni₃Sn₄, form at the solder/Cu pad interface after aging. The Au_0.5Ni_0.5Sn₄ intermetallic layer dominates the total thickness of the IMC layer and grows with aging time while the solder-ball shear strength decreases after aging. The degradation of the solder-ball shear strength was found to be caused mainly by the formation of the Au_0.5Ni_0.5Sn₄ layer. The experimental results established that a thinner Au layer on Cu pad can effectively control the degradation of solder-ball shear strength, and this is especially true for smaller ball sizes.     

Intermetallic compounds formed during the reflow and aging of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder ball grid array packages

After reflow of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder balls on Au/Ni surface finishes in ball grid array (BGA) packages, scallop-shaped intermetallic compounds (Cu_0.70Ni_0.28Au_0.02)₆Sn₅ (IM1a) and (Cu_0.76Ni_0.24)₆(Sn_0.86In_0.14)₅ (IM1b), respectively, appear at the interfaces. Aging at 100°C and 150°C for Sn-3.8Ag-0.7Cu results in the formation of a new intermetallic phase (Cu_0.70Ni_0.14Au_0.16)₆Sn₅ (IM2a) ahead of the former IM1a intermetallics. The growth of the newly appeared intermetallic compound, IM2a, is governed by a parabolic relation with an increase in aging time, with a slight diminution of the former IM1a intermetallics. After prolonged aging at 150°C, the IM2a intermetallics partially spall off and float into the solder matrix. Throughout the aging of Sn-20In-2Ag-.5Cu solder joints at 75°C and 115°C, partial spalling of the IM1b interfacial intermetallics induces a very slow increase in thickness. During aging at 115°C for 700 h through 1,000 h, the spalled IM1b intermetallics in the solder matrix migrate back to the interfaces and join with the IM1b interfacial intermetallics to react with the Ni layers of the Au/Ni surface finishes, resulting in the formation and rapid growth of a new (Ni_0.85Cu_0.15)(Sn_0.71In_0.29)₂ intermetallic layer (IM2b). From ball shear tests, the strengths of the Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder joints after reflow are ascertained to be 10.4 N and 5.4 N, respectively, which drop to lower values after aging. An erratum to this article is available at .     

Characteristics of the Interfacial Microstructure of Chip-on-Plastic Joints under Thermal Cycling and Aging Treatment

This study presents the change of contact resistance and microstructure evolution of chip-on-plastic (COP) joints during the thermal cycling (T/C) test and aging treatment. Results showed that the average contact resistance of COP joints decreased in the early stage of the T/C test as well as during aging treatment. It was observed that after 1,000 h of aging, the (Cu, Au)₃Sn phase was completely consumed and transformed into (Cu, Au)₆Sn₅. The dependence of the growth of interfacial intermetallic compounds (IMCs) on the time of T/C and aging was investigated. The IMC growth had parabolic kinetics with time. The time exponent n under the condition of T/C was approximately 0.5, which indicated that the IMC growth was diffusion controlled. However, the fitting results on the aging treatment showed that n was only about 0.37. The reason for this finding is considered to be due to the fact that after 250 h, the Cu in some areas was depleted; thus, the diffusion in these areas was restrained.     

Creep Behavior of Bi-Containing Lead-Free Solder Alloys

The creep behavior of Sn-3.0Ag-0.5Cu (SAC305), Sn-3.4Ag-1.0Cu-3.3Bi (SAC-Bi), and Sn-3.4Ag-4.8Bi (SnAg-Bi, all wt.%) was studied in constant-stress creep tests from room temperature to 125°C. The alloys were tested in two microstructural conditions. As-cast alloys had a composite eutectic-primary Sn structure, while in aged alloys the eutectic regions were replaced by a continuous Sn matrix with coarsened intermetallic (Cu₆Sn₅ and Ag₃Sn) particles. After aging, Bi in SAC-Bi and SnAg-Bi was found as precipitates at grain boundaries and grain interiors. The creep resistance of of-cast SAC305 was higher than that of as-cast Bi-containing alloys, but after aging the SAC305 had the lowest creep resistance. The creep strain rates in SAC-Bi and SnAg-Bi were much less affected by aging. The apparent activation energy for creep was also changed more for SAC305 than for the other two alloys. The creep behavior of SAC-Bi and SnAg-Bi can be understood by considering the solubility of Bi in Sn. The difference in creep behavior between as-cast and aged SAC-Bi is greatly reduced when room-temperature test results are excluded from analysis. This suggests that the strongest influence on creep in these alloys is due to Bi solute interaction with moving dislocations during deformation.