Interfacial behaviors of Sn-Pb,Sn-Ag-Cu Pb-free and mixed Sn-Ag-Cu/Sn-Pb solder joints during electromigration

SnPb-SnAgCu mixed solder joints with Sn-Pb soldering Sn-Ag-Cu Pb-free components are inevitably occurred in the high reliability applications. In this study, the interfacial behaviors in Sn-37Pb and Sn-3.0Ag-0.5Cu mixed solder joints was addressed and compared with Sn-37Pb solder joints and Sn-3.0Ag-0.5Cu solder joints with the influence from isothermal aging and electromigration. Considering the difference on the melting point between Sn-3.0Ag-0.5Cu and Sn-37Pb solder, two mixed solder joints: partial mixing and full mixing between Sn-Pb and Sn-Ag-Cu solders were reached with the peak reflowing temperature of 190 and 250 °C, respectively. During isothermal aging, the intermetallic compound (IMC) layer increased with aging time and its growth was diffusion controlled. There was also no obvious affect from the solder composition on IMC growth. After electromigration with the current density of 2.0 × 10³ A/cm², Sn-37Pb solder joints showed the shortest lifetime with the cracks observed at the cathode for the stressing time < 250 h. In Sn-3.0Ag-0.5Cu Pb-free solder joints, current stressing promoted the growth of IMC layer at the interfaces, but the growing rate of IMC at the anode interface was far faster than that at the cathode interface. Therefore, there existed an obvious polarity effect on IMC growth in Sn-Ag-Cu Pb-free solder joints. After Sn-37Pb was mixed with Sn-3.0Ag-0.5Cu Pb-free solder, whether the partial mixing or the full mixing between Sn-Pb and Sn-Ag-Cu can obviously depress both the crack formation at the cathode side and the IMC growth at the anode.     

Mechanical reliability of Sn-rich Au–Sn/Ni flip chip solder joints fabricated by sequential electroplating method

In this study, we evaluated the mechanical reliability of Sn-rich, Au–Sn/Ni flip chip solder bumps by using a sequential electroplating method with Sn and Au. After reflowing, the average diameter of the solder bump was approximately 80 μm and only a (Ni,Au)₃Sn₄ intermetallic compound (IMC) layer was formed at the interface. Due to the preferential consumption of Sn atoms within the solder matrix during aging, the solder matrix was transformed sequentially in the following order: β-Sn and η-phase, η-phase, and η-phase and ε-phase. In the bump shear test, the shear force was not significantly changed despite aging at 150 °C for 1000 h and most of the fractures occurred at the interfaces. The interfacial fracture was significantly related to the formation of brittle IMCs at the interface. The Sn-rich, Au–Sn/Ni flip chip joint was mechanically much weaker than the Au-rich, Au–Sn/Ni flip chip joint. The study results demonstrated that the combination of Sn-rich, Au–Sn solder and Ni under bump metallization (UBM) is not a viable option for the replacement of the conventional, Au-rich, Au–20Sn solder.     

Effect of nano Al2O3 particles doping on electromigration and mechanical properties of Sn–58Bi solder joints

In the present study, the effect of Al₂O₃ nanoparticles on performances of Sn–58Bi solder were investigated in aspects of electro-migratio, shear strength and microhardness. The experimental results show that the Al₂O₃ nanoparticles significantly improved microstructure and mechanical performances of solder joints. With the addition of 0.5 wt% Al₂O₃, the intermetallic compounds (IMC) reduced from 2.5 μm to 1.27 μm after 288 aging hours at 85 °C. Furthermore, after electromigration test under a current density of 5 × 10³ A/cm² at 85 °C, Bi-rich layers formed at the anode side of both Al₂O₃ doped and plain solder. Moreover, the addition of Al₂O₃ nanoparticles reduced the mean thickness of Bi-rich layer. In addition, the growth rate of the IMC layer of Al₂O₃ doped solder decreased by 8% compared with the plain solder. Besides, the Al₂O₃ doped solder exhibited better performance than plain solder in microhardness after different aging times. While, the addition of Al₂O₃ significantly impeded the degradation of the shear strength of solder joint after aging for 48 and 288 h. Furthermore, it was worth noting that the fracture surface of doped solder showed a typical rough and ductile structure. However, plain solder exhibited a relatively smooth and fragile surface.     

Effect of elevated temperature on PCB responses and solder interconnect reliability under vibration loading

In this paper, vibration tests are conducted to investigate the influence of temperature on PCB responses. A set of combined tests of temperature and vibration is designed to evaluate solder interconnect reliability at 25 °C, 65 °C and 105 °C. Results indicate that temperature significantly affects PCB responses, which leads to remarkable differences in vibration loading intensity. The PCB eigenfrequency shifts from 290 Hz to 276 Hz with an increase of test temperature from 25 °C to 105 °C, during which the peak strain amplitude is almost the same.Vibration reliability of solder interconnects is greatly improved with temperature rise from 25 °C to 105 °C. Mean time to failure (MTTF) of solder joint at 65 °C and 105 °C is increased by 70% and 174% respectively compared to that of solder joint at 25 °C. Temperature dominates crack propagation path of solder joint during vibration test. Crack propagation path is changed from the area between intermetallic compound (IMC) layer and Cu pad to the bulk solder with temperature increase.     

A Modified Electromigration Test Structure for Flip Chip Interconnections

Due to decreasing dimensions, electromigration becomes a major concern for flip chip joint reliability. A novel test structure for in-situ monitoring of electromigration in a flip chip connection is proposed. With this structure, small resistance changes at the cathodic and anodic sides can be mounted separately. This allows studying the asymmetric behavior of solder joints under electromigration conditions. The design of the new test structure is described and compared to the traditional measurement method. As a first test case, a Cu–Sn–Cu flip chip joint, stressed with a current of 6000Amp/cm$^2$at 150$^circ$C, is studied. First tests clearly indicate diverging resistance values when comparing the cathodic and anodic side of the flip chip bumps. Microstructural analysis shows extensive Cu-migration from cathode to anode.     

High-lead flip chip bump cracking on the thin organic substrate in a module package

Flip chip bump cracking was observed after Si die attach reflow on the organic substrate of a module package. High-lead bump and eutectic SnPb cladding were used on Si die and the substrate sides, respectively. The reflow peak temperature was 260 °C for compatibility with passive components attach using lead-free solder. Flip chip bump cracking occurred at high-lead solder close to the die side. The cracking was eliminated by lowering the reflow peak temperature down to 225 °C. Main cause of the cracking at 260 °C reflow was attributed to the extensive Sn diffusion into high lead bump. This decreased the melting point of the high-lead solder around the die side, which in turn worsened the adhesion between solder and die due to the coexistence of solid and liquid. Diffusion length estimation showed both of the liquid- and solid-state diffusions of Sn. Crack gap in the solder bump was consistent with thermal expansion mismatch between Si die and organic substrate. The bump cracking was mitigated by use of 225 °C reflow, limiting Sn diffusion and providing a good integrity of high lead bumps on die side.     

Power cycle reliability of Cu nanoparticle joints with mismatched coefficients of thermal expansion

The power cycle reliability of Cu nanoparticle joints between Al₂O₃ heater chips and different heat sinks (Cu-40 wt.%Mo, Al-45 wt.%SiC and pure Cu) was studied to explore the effect of varying the mismatch in the coefficient of thermal expansion (CTE) between the heater chip and the heat sink from 4.9 to 10.3 ppm/K. These joints were prepared under a hydrogen atmosphere by thermal treatment at 250, 300 and 350 °C using a pressure of 1 MPa, and all remained intact after 3000 cycles of 65/200 °C and 65/250 °C when the CTE mismatch was less than 7.3 ppm/K, despite vertical cracks forming in the sintered Cu. When the CTE mismatch was 10.3 ppm/K, the Cu nanoparticle joint created at 300 °C endured the power cycle tests, but the joint created at 250 °C broke by lateral cracks in the sintered Cu after 1000 cycles of 65/200 °C. The Cu nanoparticle joint created at 350 °C also broke by vertical cracks in the heater chip after 1000 cycles of 65/250 °C, suggesting that although sintered Cu can be strengthened to tolerate the stress by increasing the joint temperature, this eventually causes the weak and brittle chip to fracture through accumulated stress. The calculation results of stresses on the heater chip showed that the stress can be higher than the strength of Al₂O₃ when the CTE mismatch is 10.3 ppm/K and the Young's modulus of the sintered Cu is higher than 20 GPa, suggesting that the heater chip can be broken.     

Studies on various chip-on-film (COF) packages using ultra fine pitch two-metal layer flexible printed circuits (two-metal layer FPCs)

Various fine pitch chip-on-film (COF) packages assembled by (1) anisotropic conductive film (ACF), (2) nonconductive film (NCF), and (3) AuSn metallurgical bonding methods using fine pitch flexible printed circuits (FPCs) with two-metal layers were investigated in terms of electrical characteristics, flip chip joint properties, peel adhesion strength, heat dissipation capability, and reliability. Two-metal layer FPCs and display driver IC (DDI) chips with 35 μm, 25 μm, and 20 μm pitch were prepared. All the COF packages using two-metal layer FPCs assembled by three bonding methods showed stable flip chip joint shapes, stable bump contact resistances below 5 mΩ, good adhesion strength of more than 600 gf/cm, and enhanced heat dissipation capability compared to a conventional COF package using one-metal layer FPCs. A high temperature/humidity test (85 °C/85% RH, 1000 h) and thermal cycling test (T/C test, ?40 °C to + 125 °C, 1000 cycles) were conducted to verify the reliability of the various COF packages using two-metal layer FPCs. All the COF packages showed excellent high temperature/humidity and T/C reliability, however, electrically shorted joints were observed during reliability tests only at the ACF joints with 20 μm pitch. Therefore, for less than 20 μm pitch COF packages, NCF adhesive bonding and AuSn metallurgical bonding methods are recommended, while all the ACF and NCF adhesives bonding and AuSn metallurgical bonding methods can be applied for over 25 μm pitch COF applications. Furthermore, we were also able to demonstrate double-side COF using two-metal layer FPCs.     

Electromigration Behavior in Sn-37Pb and Sn-3.0Ag-0.5Cu Flip-Chip Solder Joints under High Current Density

The electromigration of conventional Sn-37Pb and Pb-free Sn-3.0Ag-0.5Cu (in wt.%) solder bumps was investigated with a high current density of 2.5 × 10⁴ A/cm² at 423 K using flip-chip specimens comprised of an upper Si chip and a lower bismaleimide triazine (BT) substrate. Electromigration failure of the Sn-37Pb and Sn-3.0Ag-0.5Cu solder bumps occurred with complete consumption of electroless Ni immersion Au (ENIG) underbump metallization (UBM) and void formation at the cathode side of the solder bump. Finite element analysis and computational simulations indicated high current crowding of electrons in the patterned Cu on the Si chip side, whereas the solder bumps and Cu line of the BT substrate had a relatively low density of flowing electrons. These findings were confirmed by the experimental results. The electromigration reliability of the Sn-3.0Ag-0.5Cu solder joint was superior to that of Sn-37Pb.     

Discrete phase method particle simulation of ultra-fine package assembly with SAC305-TiO2 nano-reinforced lead free solder at different weighted percentages

This paper presents a 3D numerical simulation of nano-reinforced lead (Pb)-free solder at the ultra-fine joint component for 01005 capacitor with dimension of 0.2 × 0.2 × 0.4 mm³. The nano-reinforced particles introduced in the Sn-3.0Ag-0.5Cu (SAC305) solder is titanium oxide (TiO₂) nanoparticles with approximate diameter of ≈ 20 nm at different weight percentages of 0.01, 0.05 and 0.15 wt% respectively. The 3D model developed is based on the reflow thermal profile of nano-reinforced Pb-free solder in the wetting zone temperature of 217 °C–239 °C. A two way interactions utilizing both volume of fluid method (VOF) and discrete phase method (DPM) are introduced in the current study. The study effectively shows the distribution of the nanoparticles as it is being doped in the molten solder after undergoing soldering process. Based on the findings, it was shown that good agreement can be seen between experimental data obtained using High Resolution Transmission Electron Microscope (HRTEM) system as compared to multiphase DPM based simulation. At weight percentage of SAC305 + 0.05% TiO₂ nanoparticles, the nanoparticles are well distributed. The fillet height of nano-reinforced solder also meets the minimum requirement for 01005 capacitor. Additionally, as the weight percentage of the doped nanoparticles increases, the time required for the formation of wetted solder also increases. In terms of the velocity and pressure distribution of the nano-reinforced lead (Pb)-free solder, higher weight percentage of doped nanoparticles have higher velocity distribution and lower pressure distributions.     

Creep lifetime prediction of solder joint for heat sink assembly

The heat sink assembly in a server station is anticipated to creep to fail at the solder joint under a constant load and temperature condition. To predict the lifetime of solder joint in the system, accelerated creep-rupture tests are conducted. Three loads of 4, 6, and 8 kg and temperatures of 35, 55, and 65 °C are selected for the tests. Larson–Miller model is adopted for the lifetime prediction, which requires tested lifetime data and stress analyses for the solder joint. An FE model for the stress analyses is developed and validated experimentally. Analyzed Larson–Miller constants show different tendency in the 8 kg load cases. Extensive failure analyses on the failed solder joints reveal the transition of failure mechanism at 8 kg load cases from the intergranular to the transgranular creep. Using only the validated test data of 4 and 6 kg load cases, creep lifetime prediction model for the solder joint in the heat sink assembly is developed and applied for a field condition.     

Effect of type of thermo-mechanical excursion on growth of interfacial intermetallic compounds in Cu/Sn-Ag-Cu solder joints

This study reports the effect of different types of thermo-mechanical excursion (TME) on growth of intermetallic compound (IMC) layer formed at the interface of Sn-3.0%Ag-0.5%Cu solder and Cu substrate. 1 mm thick solder joints were prepared by reflowing at 270 °C for either 60 or 90 s. Solder joints were then exposed to one of the following TME: (i) isothermal aging at 60 °C for 48, 96 and 144 h, (ii) thermal cycling between − 25 and 125 °C for 100, 200 and 400 cycles, and (iii) thermo-mechanical cycling between − 25 and 125 °C for 100, 200 and 400 cycles, wherein a shear strain of 10% per cycle was imposed on the joint. Finite element analysis (FEA) was performed to ascertain the effects of imposed shear strain and volumetric expansion due to the formation of IMC on the stress field in the solder joint. Irrespective of the type of TME, the thickness of the IMC layer increased with time. However, IMC thickness increased relatively more rapidly under thermo-mechanical cycling condition, indicating strain enhanced coarsening of the interfacial IMC layer. FEA showed that high stresses were generated in the IMC layer and near solder-IMC interface due to the formation of IMC layer as well as imposed external strain, which might then not only enhance the IMC growth kinetics, but also affect the morphology of the IMC layer.     

Electromigration in the Flip Chip Solder Joint of Sn-8Zn-3Bi on Copper Pads

Electromigration in Sn-8Zn-3Bi flip chip solder bumps on Cu pads has been studied at 120°C with an average current density of 4 × 10³ A/cm² and 4.5 × 10⁴ A/cm². Due to the polarity effect, the thickness of the intermetallic compound Cu-Zn (γ-phase) formed at the anode is much greater than that at the cathode. The solder joint fails after 117 h of stressing at 4.5 × 10⁴ A/cm², and void formation at the cathode can clearly be seen after polishing. However, it is the melting at the edge of the bump that causes the solder joint to fail. A simulation of the current density distribution indicates that the current density is not distributed uniformly, and current crowding occurs inside the bump. The results indicate that the increase of current density associated with Joule heating has affected melting and enhanced damage in the solder joint during electromigration.     

Investigation on thermal fatigue of SnAgCu,Sn100C,and SnPbAg solder joints in varying temperature environments

Thermal cycling tests have been performed for a range of electronic components intended for avionic applications, assembled with SAC305, SN100C and SnPbAg solder alloys. Two temperature profiles have been used, the first ranging between −20 °C and +80 °C (TC1), and the second between −55 °C and +125 °C (TC2). High level of detail is provided for the solder alloy composition and the component package dimensions, and statistical analysis, partially supported by FE modeling, is reported. The test results confirm the feasibility of SAC305 as a replacement for SnPbAg under relatively benign thermomechanical loads. Furthermore, the test results serve as a starting point for estimation of damage accumulation in a critical solder joint in field conditions, with increased accuracy by avoiding data reduction. A computationally efficient method that was earlier introduced by the authors and tested on relatively mild temperature environments has been significantly improved to become applicable on extended temperature range, and it has been applied to a PBGA256 component with SAC305 solder in TC1 conditions. The method, which utilizes interpolated response surfaces generated by finite element modeling, extends the range of techniques that can be employed in the design phase to predict thermal fatigue of solder joints under field temperature conditions.     

Junction temperature and reliability of high-power flip-chip light emitting diodes

In this work, infrared micro-imaging, emission microscope measurements are performed on the chip surface of flip-chip light emitting diodes (FCLEDs). The temperature deviation on the chip surface increases from 19 to 146 °C when the injection current changes from 20 to 2000 mA. When the structure of FCLED is optimized, the temperature deviation becomes smaller. And the thermal resistance is achieved to as low as 10.4 °C/W. The finite element method calculation based on the model of steady-state current field and temperature field is carried out to investigate the effects of current spreading on thermal performance of FCLED.     

Experimental analysis of Sn-3.0Ag-0.5Cu solder joint board-level drop/vibration impact failure models after thermal/isothermal cycling

Sn-3.0Ag-0.5Cu board-level lead-free solder joint drop (1000g, 1 ms)/vibration (15g, 25–35 Hz) reliability after thermal (− 40–125 °C, 1000 cycle)/isothermal (150 °C, 500 h) cycling was reported in this study. The failure performance of solder joint and testing life were analyzed under design six testing conditions (1. Single drop impact, 2. Order thermal cycling and drop impact, 3. Order isothermal cycling and drop impact, 4. Single vibration 5. Order thermal cycling and vibration 6. Order isothermal cycling and vibration). The results revealed that the pre-cracks initiation during thermal cycling do not affect the solder joint drop impact reliability, but decrease the vibration reliability. The formation of voids weaken both drop and vibration reliability of solder joint. After thermal cycling, the crack initiated from β-Sn near IMC layer, and continued propagation through the same path when under second in order vibration impact. But propagation path turn to IMC layer when under second in order drop impact. The drop life increases from 41 times to 49 times, and vibration life decrease from 77 min to 45 min. After isothermal cycling, the formation of voids let the cracks occurred at IMC layer under second in order no matter drop impact or vibration. The drop and vibration life is 19 times and 62 min respectively.     

High cycle fatigue behaviour and generalized fatigue model development of lead-free solder alloy based on local stress approach

This paper gives an insight into high cycle fatigue (HCF) behaviour of a Pb-free solder alloy in the region between 10⁴ up to 10⁹ fatigue cycles using fatigue specimen. By means of a local stress approach, the method can be translated into solder joint fatigue evaluation in an application. The effect of temperatures (35 °C, 80 °C, 125 °C) on the fatigue property of Pb-free solder alloy is considered in this work to understand the possible fracture mechanisms and micro structural changes in a solder alloy at elevated temperature. Experiments are performed for different interaction factors under mean stresses (R = 0, − 1, − 3), stress concentration (notched, un-notched) and surface roughness. SN (stress-life) diagrams presented in this work will compare the fatigue performance of Sn_3.8Ag_0.7Cu solder alloy for different conditions. Furthermore, mathematical fatigue model based on FKM guideline (in German “Fachkuratorium Maschinenbau) is extracted out of the experiments under all these external effects. The models can be exported later for lifetime evaluation purposes on applications. The paper thereby proposes the use of FKM guideline in the field of microelectronics.     

Young's modulus of a sintered Cu joint and its influence on thermal stress

Al₂O₃ chips and pure Cu plates were joined by Cu nanoparticles at 250 °C and 350 °C, and the Young's moduli of the sintered Cu were evaluated by nanoindentation tests. The average Young's moduli were 52.7 ± 19.8 GPa and 76.5 ± 29.7 GPa at 250 °C and 350 °C, respectively, indicating that the sintered structures were strengthened at higher temperatures. The calculation results indicated that the joint at 350 °C has a high Young's modulus, but make the stress higher than the chip strength, resulting in breakage of the chip during 65/250 °C power cycling.     

Abnormal Failure Behavior of Sn-3.5Ag Solder Bumps Under Excessive Electric Current Stressing Conditions

Abnormal failure behavior of flip chip Sn-3.5Ag solder bumps with a Cu underbump metallurgy under excessive electric current stressing conditions is investigated with regard to electromigration lifetime characteristics and damage evolution morphologies. Abnormal behavior such as abrupt changes in the slope of the resistance versus stressing time curve correlate well with the changes in mean time to failure and the standard deviation with respect to␣the resistance increase ratio, which seems to be strongly related to highly␣accelerated electromigration test conditions of 120°C to 160°C and 3 × 10⁴ A/cm² to 4.6 × 10⁴ A/cm². This is closely related to changes in the damage evolution mechanism with time, even though the activation energy for electrical failure is primarily controlled by Cu diffusion through Cu-Sn intermetallic compound layers.     

A study on thermal cycling (T/C) reliability of anisotropic conductive film (ACF) flip chip assembly for thin chip-on-board (COB) packages

In this work, thermal cycling (T/C) reliability of anisotropic conductive film (ACF) flip chip assemblies having various chip and substrate thicknesses for thin chip-on-board (COB) packages were investigated. In order to analyze T/C reliability, shear strains of six flip chip assemblies were calculated using Suhir’s model. In addition, correlation of shear strain with die warpage was attempted.The thicknesses of the chips used were 180 μm and 480 μm. The thicknesses of the substrates were 120, 550, and 980 μm. Thus, six combinations of flip chip assemblies were prepared for the T/C reliability test. During the T/C reliability test, the 180 μm thick chip assemblies showed more stable contact resistance changes than the 480 μm thick chip assemblies did for all three substrates. The 550 μm thick substrate assemblies, which had the lowest CTE among three substrates, showed the best T/C reliability performance for a given chip thickness.In order to investigate what the T/C reliability performance results from, die warpages of six assemblies were measured using Twyman–Green interferometry. In addition, shear strains of the flip chip assemblies were calculated using measured material properties of ACF and substrates through Suhir’s 2-D model. T/C reliability of the flip chip assemblies was independent of die warpages; it was, however, in proportion to calculated shear strain. The result was closely related with material properties of the substrates. The T/C reliability of the ACF flip chip assemblies was concluded to be dominatingly dependent on the induced shear strains of ACF layers.