Decrease in fatigue life of Sn - 3.8 wt-%Ag - 1.2 wt-%Cu alloy solder joints due to thermal cycling

Abstract

Copper plates joined with a thin solder layer (60 μm thick) of Sn - 3.8 wt-%Ag - 1.2 wt-% Cu alloy were subjected to heat treatments: a thermal cycling of a temperature range between 321 K and 381 K (Δ T = 60 K) and an isothermal heating at 357 K, and then subjected to a fatigue test at 6 MPa stress amplitude. Solder joints made with a thin solder layer of Sn - Pb eutectic alloy were also examined for comparison. After heat treatments, the η phase developed and dispersed at the bonding interface of the solder joints with increasing numbers of thermal cycling and with increasing time of isothermal heating. Small voids also appeared in the η phase after heat treatments. Fine cracks appeared in the η phase after thermal cycling for 2000 cycles and higher, but no cracks were observed after isothermal heating. There was no large difference in fatigue lifetime after thermal cycling between Sn - Ag - Cu alloy solder joints and Sn - Pb eutectic alloy solder joints. The fatigue lifetime of Sn - Ag - Cu alloy solder joints and Sn - Pb eutectic alloy solder joints was 2 - 3 × 105 with no thermal cycling and was greatly reduced to 0.1 - 0.6 × 10⁵ after 8000 thermal cycles. The fatigue lifetime was also decreased to 0.6 - 1.0 × 10⁵ after isothermal heating for 16 000 min, but the decrease in fatigue lifetime was gradual compared to that after thermal cycling. The decrease in fatigue lifetime after smaller numbers of thermal cycles is explained by coarsening of the η phase, and the large decrease in fatigue lifetime after a large number of thermal cycles is explained by the appearance of cracks in the η phase during thermal cycling.     

Thermal cycling test in Sn-Bi and Sn-Bi-Cu solder joints

The eutectic SnBi solder alloy is a candidate for Pb-free replacement of the conventional eutectic SnPb solders. This study presents series of results on the binary eutectic SnBi and ternary SnBi-1 wt % Cu a solder joints. Compositional analysis and wettability of the as-fabricated solder alloys are reported. In addition, microstructure, adhesion strength, fracture surface and contact resistance of the solder joints are also evaluated. The results of the wetting balance show that the addition of 1 wt % Cu has little effect on the contact angle of the eutectic SnBi solder alloy with various metallization layers. The adhesion strength of solder joints degrades abruptly after 2000 thermal cycles. In addition, thermal cycling would result in cracking in the solder joints, which is due to the mismatch in thermal expansion coefficients. Portions of the thermal fatigue cracks nucleate at the edge of the solder fillet, and then propagate along the solder/conductor interface. Some cracks are, however, through the Al₂O₃ substrate. The contact resistance of the solder/Cu joint does not increase after thermal cycling since the resistivity of Cu₆Sn₅ is lower than that of the solder. The solder joints of 42Sn-58Bi/Cu, SnBi-1Cu/Cu, 42Sn-58Bi/PtAg, and SnBi-1Cu/PtAg assemblies maintain their integrity after 2000 thermal cycles since the increase in contact resistance is rather small (ΔR<0.5 mΩ).     

Intermetallic growth study on SnAgCu/Cu solder joint interface during thermal aging

The intermetallic compound (IMC) growth behavior at SnAgCu/Cu solder joint interface under different thermal aging conditions was investigated, in order to develop a framework for correlating IMC layer growth behavior between isothermal and thermomechanical cycling (TMC) effects. Based upon an analysis of displacements for actual flip-chip solder joint during temperature cycling, a special bimetallic loading frame with single joint-shear sample as well as TMC tests were designed and used to research the interfacial IMC growth behavior in SnAgCu/Cu solder joint, with a focus on the influence of stress–strain cycling on the growth kinetics. An equivalent model for IMC growth was derived to describe the interfacial Cu-Sn IMC growth behavior subjected to TMC aging as well as isothermal aging based on the proposed “equivalent aging time” and “effective aging time”. Isothermal aging, thermal cycling (TC) and TMC tests were conducted for parameter determination of the IMC growth model as well as the growth kinetic analysis. The SnAgCu/Cu solder joints were isothermally aged at 125, 150 and 175 °C, while the TC and TMC tests were performed within the temperature range from ?40 to 125 °C. The statistical results of IMC layer thickness showed that the IMC growth for TMC was accelerated compared to that of isothermal aging based on the same “effective aging time”. The IMC growth model proposed here is fit for predicting the IMC layer thickness for SnAgCu/Cu solder joint after any isothermal aging time or thermomechanical cycles. In addition, the results of microstructure evolution observation of SnAgCu/Cu solder joint subjected to TMC revealed that the interfacial zone was the weak link of the solder joint, and the interfacial IMC growth had important influence on the thermomechanical fatigue fracture of the solder joint.     

热循环条件下SnAgCu/Cu焊点金属间化合物生长及焊点失效行为

研究了热循环过程中SnAgCu/Cu焊点界面金属间化合物的生长规律及焊点疲劳失效行为。提出了热循环条件下金属间化合物生长的等效方程以及焊点界面区不均匀体模型,并用有限元模拟的方法分析了热循环条件下焊点界面区的应力应变场分布及焊点失效模式。研究结果表明:低温极限较低的热循环,对应焊点的寿命较低。焊点的失效表现为钎料与金属间化合物的界面失效,且金属间化合物厚度越大,焊点中的累加塑性功密度越大,焊点越容易失效。     

Effect of thermal cycles on interface and mechanical property of low-Ag Sn1.0Ag0.5Cu(nano-Al)/Cu solder joints

Low-Ag content SnAgCu solder has drawn more and more researchers’ attention due to the low cost. In this paper, the effect of 0.1 wt% nano-Al particles on interface reaction between Sn1.0Ag0.5Cu and Cu substrate was investigated, and the growth of intermetallic compounds (IMC) and mechanical property of solder joints during ??55 to 125 °C thermal cycling were also analyzed. The results show that the Cu₆Sn₅ IMC formed at the as-soldered interface and grow obviously with the increase of thermal cycling. The growth rate of IMC in the SnAgCu–0.1Al/Cu is lower than that of SnAgCu/Cu, which indicates that the nano-Al particles can inhibit the diffusion coefficient of IMC layers. Moreover, the shear force of two kinds of solder joints decrease during thermal cycling, but the shear force of SnAgCu–0.1Al is higher than that of SnAgCu.     

Formation of Cu Nanodots on Diamond Surface to Improve Heat Transfer in Cu/D Composites

Diamond‐dispersed copper matrix (Cu/D) composite materials with different interfacial configurations are fabricated through powder metallurgy and their thermal performances are evaluated. An innovative solution to chemically bond copper (Cu) to diamond (D) has been investigated and compared to the traditional Cu/D bonding process involving carbide‐forming additives such as boron (B) or chromium (Cr). The proposed solution consists of coating diamond reinforcements with Cu particles through a gas–solid nucleation and growth process. The Cu particle‐coating acts as a chemical bonding agent at the Cu–D interface during hot pressing, leading to cohesive and thermally conductive Cu/D composites with no carbide‐forming additives. Investigation of the microstructure of the Cu/D materials through scanning electron microscopy, transmission electron microscopy, and atomic force microscopy analyses is coupled with thermal performance evaluations through thermal diffusivity, dilatometry, and thermal cycling. Cu/D composites fabricated with 40 vol% of Cu‐coated diamonds exhibit a thermal conductivity of 475 W m^?1 K^?1 and a thermal expansion coefficient of 12 × 10^?6 °C^?1. These promising thermal performances are superior to that of B‐carbide‐bonded Cu/D composites and similar to that of Cr‐carbide‐bonded Cu/D composites fabricated in this study. Moreover, the Cu/D composites fabricated with Cu‐coated diamonds exhibit higher thermal cycling resistance than carbide‐bonded materials, which are affected by the brittleness of the carbide interphase upon repeated heating and cooling cycles. The as‐developed materials can be applicable as heat spreaders for thermal management of power electronic packages. The copper‐carbon chemical bonding solution proposed in this article may also be found interesting to other areas of electronic packaging, such as brazing solders, direct bonded copper substrates, and polymer coatings.

     

Influence of Thermal Cycling on the Microstructure and Shear Strength of Sn3.5Ag0.75Cu and Sn63Pb37 Solder Joints on Au/Ni Metallization

The influence of thermal cycling on the microstructure and joint strength of Sn3.5Ag0.75Cu （SAC） and Sn63Pb37 （SnPb） solder joints was investigated. SAC and SnPb solder balls were soldered on 0.1 and 0.9 μm Au finished metallization, respectively. After 1000 thermal cycles between -40℃ and 125℃, a very thin intermetallic compound （IMC） layer containing Au, Sn, Ni, and Cu formed at the interface between SAC solder joints and underneath metallization with 0.1 μm Au finish, and （Au, Ni, Cu）Sn4 and a very thin AuSn-Ni-Cu IMC layer formed between SAC solder joints and underneath metallization with 0.9 μm Au finish. For SnPb solder joints with 0.1 μm Au finish, a thin （Ni, Cu, Au）3Sn4 IMC layer and a Pb-rich layer formed below and above the （Au, Ni）Sn4 IMC, respectively. Cu diffused through Ni layer and was involved into the IMC formation process. Similar interfacial microstructure was also found for SnPb solder joints with 0.9μm Au finish. The results of shear test show that the shear strength of SAC solder joints is consistently higher than that of SnPb eutectic solder joints during thermal cycling.     

Influence of solder thickness on interfacial structures and fatigue properties of Cu/Sn/Cu joints

Abstract

Copper plates were soldered with tin foil of different thicknesses to examine the influence of the solder thickness on formation of secondary phases at the interface and fatigue properties. In the case of an initial solder thickness of 60 μm, the thickness of the η (Cu₆ Sn₅) phase and the ? (Cu₃ Sn) phase linearly increased with the square root of the bonding time. The fatigue strength was 3 MPa and the fatigue life decreased with increasing stress amplitude and had a low scatter. In the fatigue process, fine cracks appeared in the η phase and propagated in the solder layer. This process was different from the case of static shear fracture. In the case of a 5 μm solder thickness, the solder was replaced by secondary phases in a short time, and only the ? phase remained at the interface after a bonding time of more than 300 min. The fatigue strength was 13 MPa and was independent of the interfacial structure, but the fatigue life showed a large scatter. In the fatigue test, unstable fracture occurred along the interfaces, similar to that observed in static shear fracture. From these results, it was concluded that solder joints become brittle with decreasing solder thickness.     

Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density

Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate current density of 6.0 × 10² A/cm² at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement, microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating, which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening, volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing.     

Investigation on high temperature mechanical fatigue failure behavior of SnAgCu/Cu solder joint

In this paper, high temperature mechanical fatigue tests on SnAgCu/Cu solder joints were carried out under three test temperatures (100, 125, 150 °C). Failure mechanism was analyzed through observation of micro-crack evolution and fracture morphology. The results show that the deformation curve of solder joint under high temperature mechanical fatigue tests can be divided into three stages: strain hardening stage, stable deformation stage and accelerated failure stage, which is similar to the curve under creep test condition. In addition, the cyclic life decreases rapidly with increasing temperature. Deformation field in the solder joint is non-uniform and shear strain concentration occurs in solder close to the intermetallic compound (IMC) layer. Micro-crack initiates at the corner of the solder joint and then tend to propagate along interface between Cu substrate and solder. The fracture morphology under three temperatures all exhibits ductile fracture mode and the failure path transforms from cutting through the top of Cu₆Sn₅ to propagation in solder matrix close to IMC layer with increasing temperature.     

Effect of composition and thermal cycling on the adhesion strength of Sn-Zn-Al solder hot-dipped on Cu substrate

Reliability losses in many electronic systems were identified with the failure of solder joints rather than device malfunctions. The adhesion strength is an important factor for assessing the reliability of the solder joints. In this work, a pull-off test was used to investigate the adhesion strength at the interface of the (100 – x)Sn-x(5Al-Zn) lead-free solders on Cu substrate as-soldered and after thermal cycling, respectively. For the (100 – x)Sn-x(5Al-Zn) solders with the x value increased up to 40 wt%, the adhesion strength decreased from 11.8 ± 1.5 to 3.3 ± 0.9 MPa. After thermal cycling (–20–120°C) for 40 cycles, the adhesion strength of 95Sn-5(5Al-Zn) and 91Sn-9(5Al-Zn) solders decreased from 11.2 ± 1.7 to 8.2 ± 1.3, 7.6 ± 0.7 to 5.0 ± 0.8 MPa, respectively. However, the adhesion strength for the solders of 80Sn-20(5Al-Zn), 70Sn-30(5Al-Zn) and 60Sn-40(5Al-Zn) increased from 5.7 ± 1.7 to 13.3 ± 1.9 MPa, 4.8 ± 2.0 to 12.2 ± 1.8 MPa, and 3.3 ± 1.5 to 16.2 ± 1.2 MPa, respectively. The formation of intermetallic compound (IMC) is proposed for the enhancement of the strength after thermal cycling in this study.     

Fracture mechanics analysis of the effect of substrate flexibility on solder joint reliability

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue life significantly. The reliability of solder joints in real flip chip assembly with both rigid and compliant substrates was evaluated by the accelerated temperature cycling test and thermal mechanical analysis. The mechanism of substrate flexibility on improving solder joint thermal fatigue lifetime was investigated by fracture mechanics methods. Two different methods (crack tip opening displacement, CTOD and virtual crack closure technique, VCCT) are used to determine the crack tip parameters which are considered as the indices of reliability of solder joints, including the strain energy release rate and phase angle for the different crack lengths and temperatures. It was found that the thermal fatigue lifetime of solder joints in flip chip on flex assembly (FCOF) was much longer than that of flip chip on rigid board assembly (FCOB). The flex substrates could dissipate energy that otherwise would be absorbed by solder joints, that is, substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during thermal cycling.     

Microstructural evolutions of the Ag nano-particle reinforced SnBiCu-xAg/Cu solder joints during liquid aging

Ag nano-particle reinforced Sn30Bi0.5Cu-xAg (x = 1 wt%, 2 wt% and 5 wt%) solder pastes were prepared and reflowed on Cu substrates at 523 K. Then, the solder joints were liquid aged at 473 K for 6 and 12 h. Microstructural evolutions of these solder joints were observed by scanning electron microscopy (SEM). The results show that Bi-rich phase was refined in the as-reflowed Sn30Bi0.5Cu-xAg composite solder matrices. With the increase of the liquid aging time, Bi-rich phase was refined both in the Sn30Bi0.5Cu solder and in the Sn30Bi0.5Cu-xAg composite solders. The addition of Ag nano-particles changed the growth rate of the IMC layers during liquid aging due to the absorption effect of the Ag₃Sn micro-particles.     

Effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu and SnPb solders with Au/Ni/Cu substrate during aging

The effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu (SAC) and SnPb solders with Au/Ni/Cu substrate during aging was investigated in this study. AuSn₄ needle-like intermetallic compound (IMC) formed at the interfaces of both SAC and SnPb solder joints after laser reflow. In SnPb solder joints, a layer-type ternary IMC, (Au,Ni)Sn₄, formed over a thin layer quaternary IMC (Au,Ni,Cu)₃Sn₄ after aging at 150 °C for 4 days, in which Cu came from the Cu pad below the Ni layer through diffusion. This (Au,Ni,Cu)₃Sn₄ IMC evolved into a new kind of quaternary IMC (Au,Ni,Cu)₆Sn₅ with further aging at 150 °C for 8 days because more Cu diffused through Ni and was involved into the IMC formation. For SAC solder joints, needle-like binary AuSn₄ IMC in as-soldered state changed into a layer-type quaternary IMC (Au,Ni,Cu)Sn₄. The quaternary IMC (Au,Ni,Cu)₆Sn₅, which has the same composition as those found in SnPb solder joints, was developed below the (Au,Ni,Cu)Sn₄ layer after aging at 150 °C for 8 days. It was found that Cu diffusion played a key role in the interfacial reaction and compound formation.     

Effect of strain rate on interfacial fracture behaviors of Sn-58Bi/Cu solder joints

The interfacial reactions and mechanical properties of Sn-58Bi/Cu solder joints reflowed at different temperatures ranging from 180 to 220 °C for constant time of 10 min were investigated with various strain rates. Only a continuous Cu₆Sn₅ intermetallic compound (IMC) layer was formed at the interface between the Sn-58Bi solder and the Cu substrate during reflow. The equivalent thickness of the Cu₆Sn₅ layer increased with increasing reflow temperature, and the relationship between Cu₆Sn₅ layer equivalent thickness (X) and reflow temperature (T) is obtained by using method of linear regression and presented as $ X = 0.01 \times T + 0.187 $ . For the tensile property, the tensile strength of solder joint gradually decreased as the reflow temperature it increased, whereas it increased with increasing strain rate. Moreover, the fracture behavior of Sn-58Bi/Cu solder joint indicated the ductile fracture with low strain rate (5 × 10^?4 and 1 × 10^?3 s^?1), while toward brittle fracture with high strain rate (2 × 10^?3 and 1 × 10^?2 s^?1). The strain rate sensitivities of the solder joints fractured with various modes were also investigated, and it is found that the tensile strength of the solder is more sensitive to the strain rate than that of the IMC layer.     

Torsional fatigue with axial constant stress for Sn–3Ag–0.5Cu lead-free solder

A series of multiaxial ratcheting–fatigue interaction tests have been carried out on Sn–3Ag–0.5Cu lead-free solder specimens. All tests were conducted under cyclic shear strain with the constant axial stress at the room temperature with the shear strain rate of 5 × 10⁻³ s⁻¹. It was found that the ratcheting strain increased with increasing axial stress and shear strain amplitude while the fatigue life decreased at the same time. The ratcheting strain rate was linear with axial stress in double logarithmic coordinate. The Ohno–Wang II constitutive model was employed to simulate the stress–strain responses. Several fatigue life prediction models were applied to predict the multiaxial ratcheting–fatigue life of the Sn–3Ag–0.5Cu lead-free solder. The Gao–Chen model which adopted the maximum shear strain and the ratcheting strain rate as the damage parameter predicted the multiaxial ratcheting fatigue life well.     

Critical studies to improve service reliability of Sn–Ag–Cu solder joints by thermal treatments

Lead-free electronic packages intended for use in applications such as aerospace, military, and other highly demanding service conditions, necessitate exceptional mechanical reliability of lead-free electronic solder joints under realistic service conditions. Most current design strategies employed for improving the reliability of lead-free electronic solder joints are aimed at developing suitable alloying additions and reinforcements to the solder itself. At present there exists no suitable methodology to minimize the effects of service conditions while the solder joint is in service. Since thermomechanical fatigue reliability of electronic solder joints is closely related to the crack nucleation that occurs during very early stages of repeated thermal excursions, this study is based on subjecting solder joints to a limited number of thermal shock (TS) cycles in a chosen temperature regime to nucleate cracks, then evaluating their effectiveness in improving reliability when the solder joints are subjected to additional TS cycles in a different temperature regime. This study is a preliminary investigation, aimed at developing suitable methodology to minimize the effects of damage to lead-free solder joint specimens subjected to repeated thermal excursions during service, by imposing appropriate thermal treatments. These thermal treatments can be automatically implemented at programmed intervals during the service life of the electronic packages. Methods employed in these studies may also be useful to enhance long-term service reliability and to obtain a conservative estimate of long-term service reliability.     

Effect of solder filler thickness on the mechanical stability of fiber-solder-ferrule joint under temperature cyclic loading

The base materials of package and ferrule are often gold-coated Kovar and Invar, they both have relatively low coefficient of thermal expansion (CTE). Solder 63Sn37Pb dissolves Au substantially and forms brittle AuSn₄, which may cause catastrophic failure in the fiber-solder-ferrule (FSF) joint in the long-term application. It is well known that thermal fatigue creep is one of the crucial factors affecting the life and reliability of a solder joint in electronic and optoelectronic assemblies. Therefore, it is important to understand the behavior of the FSF joint under thermal cyclic loading. In this study, four different thicknesses of solder filler in a FSF joint were examined. By using the finite element method (FEM), the equivalent creep strains of eutectic lead-tin solder were compared. The joints were subjected to 5 cycles of temperature cycling test, i.e., −65 to 150^∘C. It was found that the thicker solder filler is subjected to a larger equivalent creep strain than the thinner solder filler. It is discussed the vertical shift of the optical fiber, which is sensitive to temperature and has effects on the power loss coupling. Modeling and experimental results show that 0.5 mm is the best inner diameter of ferrule that provides the lowest displacement and, thus, the lowest power loss under temperature cycle.     

Microstructural Evolution and Cracking of Pb-free Ball Grid Array Assemblies under Thermal Cycling

Two kinds of CBGA （ceramic ball grid array） assemblies were made by reflow soldering process using two different Pb-free solders. Microstructural evolution and cracks induced by thermal cycling in CBGA assemblies were examined by scanning electron microscopy （SEM） and finite element method （FEM）. Before thermal cycling, intermetallic compounds （IMCs） Cu6Sn5 and Ag3Sn were observed at the solder interface between Cu and Ag metallizations, respectively. After thermal cycling, another IMC Cu3Sn was observed near the Cu pad in both two assemblies and the layers of Cu6Sn5 and Ag3Sn became thicker. As a result of thermal cycling, cyclic stress and strain were accumulated in the solder joint leading to fatigue cracking. Both experiments and FEM revealed that cracks preferred to initiate at the corner of each solder joint. Multi-modes of the crack propagation were found in the two assemblies. Based on Coffin-Manson equation, the thermal fatigue life was calculated and the predicted life showed good agreement with the experimental results.     

The effects of Mn powder additions on the microstructures and tensile property of SnAgCu/Cu solder joints

In this paper, the effects of Mn powder on fusion property of Sn3.0Ag0.5Cu solder alloy and microstructures as well as tensile property of the solder joints of Sn3.0Ag0.5Cu/Cu were investigated by differential scanning calorimetry analysis, scanning electron microscopy and tensile tests. The results showed that the addition of Mn dramatically suppressed under cooling of SnAgCu solder alloy. Mn addition contributed to the growth of Cu₆Sn₅ intermetallic compound layers since it provided nucleation sites for Cu₆Sn₅ at the solder joints. Moreover, Mn addition increased the hardness of the solder alloys and reduced the tensile strength of SnAgCu/Cu solder joints. During aging, the growth of IMC layers of SnAgCuMn/Cu solder joints was slower than that of SnAgCu/Cu solder joints, and the tensile strength of all the solder joints increased after aging.