Solder Joint Specimens Research Articles

Main and interaction effects of manufacturing variables on microstructure and fracture of solder-copper connections

Finding the optimized set of manufacturing parameters to produce strong solder-copper connections requires investigating the main and interaction effects of processing variables on the joint strength and microstructure. In this study, solder joint specimens were prepared at different levels of cooling rate, time above liquidus (TAL), and soldering temperature. Mode I fracture experiments were designed and performed at a strain rate of 0.5 s−1. The fracture load remained constant from the cooling rate of 0.1 to 1.4 °C/s and then decreased by almost 34% with further increase in the cooling rate to 34 °C/s. Increasing TAL from 60 to 120 s reduced the fracture load by almost 27%, while it was almost unchanged from TAL of 120 to 240 s. The influence of soldering temperature on fracture load was negligible. Microstructural examination showed the cooling rate and TAL both affected intermetallic compound (IMC) thickness and this caused the interaction between them.

Geometry Influence on Fracture Behavior of Lap-Shear Solder Joints

Single lap-shear (SLS) specimens of 63Sn37Pb solder joints were prepared with three different adherend thicknesses at three varying joint lengths. The fracture force was measured at a shear strain rate of 0.01 s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> for different geometries. The elastic–plastic fracture mechanics (EPFM) theory was used to find the energy dissipated in each case using a finite element model (FEM), and the fracture energy was obtained by cohesive zone modeling (CZM). Both 2-D and 3-D models were used to explain the variations in fracture energy by the level of constraint on the joint. Also, the plastic zone area and stress distribution along the solder layer were calculated at the moment of fracture. A phase angle definition was proposed and compared in different solder lengths to show its influence on fracture energy. It was concluded that the fracture energy was influenced by the local phase angle and plastic zone area at the moment of fracture. The effect of adherend thickness on fracture load was significant especially for long joints (i.e., lengths of 6.35 and 12.7 mm). The observations on the crack path showed that the crack was initiated near one of the adherends and tended to propagate within the intermetallic compound (IMC) layer. Also, the planar fracture surface was observed in all the specimens except for the short solder joints (i.e., with the length of 2.54 mm).

The Effect of Multi-Walled Carbon Nanotubes Reinforcement and Multiple Reflow Cycles on Shear Strength of SAC305 Lead-Free Solder Alloy

Abstract In this study, the effect of multi-walled carbon nanotubes (MWCNT) reinforcement on joint shear strength and microstructural development of tin-3.0silver-0.5copper (SAC305)/copper solder joint subjected to multiple reflow cycles was investigated. The MWCNT-reinforced SAC305 solder systems (SAC305-x MWCNT; x = 0.01, 0.05, 0.1, and 0.5 wt.%) were developed by a mechanical dispersion method. The microstructural, mechanical, and melting properties of SAC305 composite solders were evaluated as a function of different wt.% of MWCNT addition. The melting behavior of composite solders was analyzed using differential scanning calorimetry. The morphology and intermetallic compound growth at the solder joint interface were studied using scanning electron microscopy. The copper/solder/copper micro-lap-shear solder joint specimens reflowed for multiple reflow cycles were systematically characterized to evaluate the joint shear strength. The results showed that the reinforcement in the range of 0.01–0.05 wt.% of MWCNT resulted in the improvement of joint shear strength and better wettability compared to plain SAC305 solder alloy. Amongst all compositions analyzed, SAC305-0.05MWCNT nanocomposite suppressed the intermetallic compound layer growth effectively leading to improvement in the joint shear strength under multiple reflow cycles.

Effect of Multiple Reflow Cycles and Al2O3 Nanoparticles Reinforcement on Performance of SAC305 Lead-Free Solder Alloy

The effect of Al2O3 nanoparticles reinforcement on melting behavior, microstructure evolution at the interface and joint shear strength of 96.5Sn3Ag0.5Cu (SAC305) lead-free solder alloy subjected to multiple reflow cycles was investigated. The reinforced SAC305 solder alloy compositions were prepared by adding Al2O3 nanoparticles in different weight fractions (0.05, 0.1, 0.3 and 0.5 wt.%) through mechanical dispersion. Cu/solder/Cu micro-lap-shear solder joint specimens were used to assess the shear strength of the solder joint. Differential scanning calorimetry was used to investigate the melting behavior of SAC305 solder nanocomposites. The solder joint interfacial microstructure was studied using scanning electron microscopy. The results showed that the increase in melting temperature (TL) and melting temperature range of the SAC305 solder alloy by addition of Al2O3 nanoparticles were not significant. In comparison with unreinforced SAC305 solder alloy, the reinforcement of 0.05-0.5 wt.% of Al2O3 nanoparticles improved the solder wettability. The addition of nanoparticles in minor quantity effectively suppressed the Cu6Sn5 IMC growth, improved the solder joint shear strength and ductility under multiple reflow cycles. However, the improvement in solder properties was less pronounced on increasing the nanoparticle content above 0.1 wt.% of the solder alloy.

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.

A Novel Method for Impact Testing of Small Specimens - Numerical Simulation and Experiments

A novel method for dynamic testing of small specimens, based on impact loading, is proposed. This is motivated by the desire to elicit the dynamic mechanical response of single solder joints measuring 0.5 mm smaller in size. Impact loading is generated through impact of a tubular striker against a loading plate which is initially in contact with the specimen. The mechanical response of the specimen is derived from noting the stress waves generated in slender bars attached to the loading plate and specimen. Through this approach, dynamic tension or compression can be conveniently applied using the same test apparatus, and exact alignment of the slender bars attached is not critical. In addition, wave dispersion or attenuation in the bars (e.g., when polymer bars are used) has much less influence on the accuracy of results obtained. The equations governing this novel approach are derived and validated by finite element modeling. A prototype of a miniature impact tester was consequently fabricated, and tests on small aluminum specimens showed results comparable to that employing traditional split Hopkinson bar systems. The test arrangement was utilized for dynamic testing of lead-free single solder joint specimens, and initial results obtained are presented.

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.

Brittle Versus Ductile Failure of a Lead-Free Single Solder Joint Specimen Under Intermediate Strain Rate

The failure mode of Sn-4.0Ag-0.5Cu solder joints under the intermediate strain rates is explored. Tensile and shear tests were performed on miniature solder joints at two different displacement rates, i.e., 5 and 50 mm/s. For the uniaxial tensile test, brittle failure occurred at the intermetallic compound (IMC)/Cu pad interface on the board side under both loading rates. However, for shear loading at 5 mm/s, ductile failure along the solder matrix was detected. With the increase of the shear loading rate up to 50 mm/s, the failure mode is converted into the (ductile+brittle) mixed mode. Numerical simulation results based on the extracted elastic-plastic solder properties indicate that the maximum stress concentration is located at the interfaces of the board side, which indicates the weak position of the crack initiation. The plastic deformation of the solder alloy is significantly suppressed at 50 mm/s loading rate. The amplitude of the solder plastic deformation under shear loading is much greater than that for the tensile test. Furthermore, most plastic deformation under shear loading is in the solder alloy adjacent to solder/IMC interface of the board side and no visible plastic deformation is detected in the middle of solder ball, which results in cohesive failure at the rate of 5 mm/s and the (ductile+brittle) mixed failure mode at the rate of 50 mm/s.

Endochronic Fatigue Life Prediction of SN/3.5AG/0.75CU BGA Solder Joints Under Oblique Displacement Cyclic Tests

ABSTRACTIn this paper, cyclic damage behavior of cyclically load drop curves and their fatigue initiation life of Sn/3.5 Ag/0.75Cu BGA solder joint specimens under oblique displacement cyclic tests were investigated by the theory of damage — coupled endochronic viscoplasticity.By linearly unloading with damage elastic modulus and the linearly damage-free behavior of grip system, the damage loops of force-Φ angle oblique displacement of BGA solder joint specimen were converted successfully into damage loops of the representative solder ball under cyclically proportional straining, which can be predicted by the endochronic constitutive equations. These results established the relationship of the BGA oblique displacement amplitudes da(Φ) and the effective inelastic strain amplitudes of solder ball: da (Φ)= . Based on the phenomena of cyclic damage and its fatigue life, a Φ dependent degree of damage in the evolution equation of damage under proportional strain path was proposed to depend positively on and N cycles. Using this parameter in the damage per cycle computed by the endochronic theory, a Φ modified cycles N(Φ)/β(Φ) can be defined and then derive the Φ modified Lee-Coffin-Manson (Φ-LCM) equation for the fatigue initiation life of solder ball:Finally, a Φ modified Lee's BGA (Φ LBGA) equation for BGA solder joint specimens can be derived:This equation can predict quite well the life data of Sn/3.5Ag/0.75Cu solder joint specimens under Φ ϵ [0π/2]. As a consequence, a vehicle to study the fatigue initiation life of BGA solder joint specimens is constructed completely by the workable methodology and the theory discussed in the paper.

Cyclic Stress-Strain Behavior of BGA (Sn/3.5Ag/0.75Cu) Solder Joint under Cyclically Oblique Displacement Tests and Endochronic Viscoplastic Predictions

ABSTRACTIn this paper, a methodology with workable procedures was proposed and successfully transferred the hysteresis loops of BGA solder joint specimen held by loading-directionally aligned grip system under oblique displacement controlled cyclic tests, into hysteresis loops of “representative” solder ball itself under proportional straining and constant strain rate cyclic tests. Under the elastic unloading during cyclic test, value of elastic modulus of bulk solder specimens and the linear behavior of grip system were used. Following the study of Endochronic cyclic viscoplasticity, the kernel function ρ(Z) = ρ0 exp (−KZ)/Zα was found to be independent of the oblique angle (Φ) of straining paths. However, the steady cyclic behavior of material function f(ξ) ≡ f0 in the definition of intrinsic time Z contained two functions: (1) is the effective inelastic strain amplitude, and (2) f(Φ) Φ(rad) between Φ = 0 (uniaxial) and Φ = π/2 (shear). With (ρ0, α, K) = (4MPa, 0.84, 46) and f0 = [0.24(π/2−Φ)2 − 0.018(π2−Φ) + 1.2] , the endochronic theory predicted experimental hysteresis loops of BGA Sn/3.5Ag/0.75Cu solder joint specimens and Φ = 0°, 27°, 45°, 63°, 90° quite well.Results of ρ(Z) under f0 = 1 were found under independent study based on cyclic tests of bulk specimens under constant uniaxially strain amplitude and various constant strain rates. The values of (ρ0, α, K) = (7.3MPa, 0.84, 30) with f0 = 1.0 were all independent of strain rates. Comparisons of results of both types of specimens revealed that the values of (α, K) were almost the same but (1) ρ0 was smaller for micro-size solder ball, (2) is a material function of Sn/3.5Ag/0.75Cu itself (3) f(Φ) is not a material function, rather it is a special feature to reflect the effects of detail design of solder joint specimen and its connecting methods to the substrates

Electrical conductivity changes of bulk tin and Sn-3.0Ag-0.5Cu in bulk and in joints during isothermal aging

The changes of electrical conductivity (resistance) between Sn-3.0Ag-0.5Cu solder joints and printed circuit board (PCB) assembly during aging at 125°C were investigated by the four-point probe technique. The microstructural characterizations of interfacial layers between the solder matrix and the substrate were examined by optical microscopy and scanning electronic microscopy. Different types of specimens were designed to consider several factors. The experimental results indicate that electrical conductivities (resistances) and residual shear strengths of the solder joint specimens significantly decrease after 1000 h during isothermal aging. Microcracks generate in the solder matrix at the first 250 h. Besides, the evolutions of microstructural characterizations at the interface and the matrix of solder joints were noted in this research.

Influence of Asymmetrical Waveform on Low-Cycle Fatigue Life of Micro Solder Joint

The effects of waveform symmetry on the low-cycle fatigue life of the Sn-3.0Ag-0.5Cu alloy have been investigated, using micro solder joint specimens with approximately the same volume of solder as is used in actual products. Focusing on crack initiation life, fatigue tests on Sn-Ag-Cu micro solder joints using asymmetrical triangular waveforms revealed no significant reduction in fatigue life. A slight reduction in fatigue life at low strain ranges caused by an increase in the fatigue ductility exponent, which is the result of a weakening microstructure due to loads applied at high temperature for long testing time, was observed. This was due to the fact that grain boundary damage, which has been reported in large-size specimens subjected to asymmetrical triangular waveforms, does not occur in Sn-Ag-Cu micro size solder joints with only a small number of crystal grain boundaries.

Mapping the failure envelope of board-level solder joints

Single solder interconnects were subjected to a series of combined tension–shear and compression–shear tests to determine their failure load. The failure envelope of these interconnects was obtained by plotting the normal component against the shear component of the failure load. The interconnect failure force map was found to be elliptical like the failure envelopes of many materials. The failure map can be described by a simple mathematical expression to give a simple force-based criterion for combine loading of solder joints. Post mortem analyses were conducted on the solder joint specimens to identify the failure mechanisms associated with various segments of the failure map. Computational simulations of actual board tests show that the failure map obtained for joint tests provides good predictions of board-level interconnect failures and hence suggest that such failure maps are useful in the design and analysis of board assemblies subjected to mechanical loads. The industry could adopt the methodology to obtain failure envelopes for solder joints of different alloys, bump size and reflow profiles which they could later use to aid in board-level and system-level designs of their products for mechanical reliability.

Simulation of dynamic fracture along solder–pad interfaces using a cohesive zone model

In this paper, dynamic fracture of a single solder joint specimen is numerically simulated using the finite element method. The solder–IMC and IMC–Cu pad interfaces are modeled as cohesive zones. The simulated results show that under pure tensile loading, damage typically starts at the edge of the solder–IMC interface, then moves to IMC–Cu pad interface. Eventual failure is typically a brittle interfacial failure of the IMC–Cu interface.

Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.

Microstructure and Creep Deformation of Sn-Ag-Cu-Bi/Cu Solder Joints

Sn-Ag-Cu solder is one of the candidate alternatives to Sn-Pb-based solders. In order to improve its performance, different materials have been added to Sn-Ag-Cu-based solders. Several studies on Sn-Ag-Cu-based solders with Bi additions have shown Sn-Ag-Cu-Bi to be a class of solders with good wetting behavior and good performance that show great promise for use in the electronics assembly and packaging industry. To investigate the mechanical reliability of the Sn-Ag-Cu-Bi solders further, single-lap shear creep characteristics have been studied in this work. Dog-bone-type solder joint specimens were formed using five types of solder alloys, Sn-3.0Ag-0.5Cu and Sn-3.0 Ag-0.5Cu-xBi (x = 1 wt.% to 4 wt.%) with Cu substrates, and creep tests were performed at temperatures of 120°C and 150°C under stresses of 5 MPa to 10 MPa. Results indicate that the rupture times for Sn-3.0Ag-0.5Cu-xBi solder joints up to 4 wt.% of Bi are longer than the rupture time for Sn-3.0Ag-0.5Cu. Stress exponents ranged from 3 to 7 for temperatures of 150°C and 120°C with stresses under 10 MPa. Microstructural analyses using scanning electron microscopy (SEM) were performed and related to the creep behavior of the solder joints.

Incremental recrystallization/grain growth driven by elastic strain energy release in a thermomechanically fatigued lead-free solder joint

A single shear lap solder joint specimen with lead-free eutectic Sn–Ag solder on copper substrates was exposed to 1500 thermomechanical fatigue (TMF) cycles between −15° (3.5 h) and 150 °C (20 min) to generate intrinsic thermal strains. The 0.15-mm-thick solder joint had a cross-sectional area of 1 mm 2. Orientation imaging microscopy was used to examine changes in the tin microstructure as a function of the number of TMF cycles. The tin phase consisted of embedded minority orientations within an initial dominant orientation, which varied by about 25° from one end of the joint to the other, by means of lattice curvature and low angle boundaries. Between about 150 and 750 cycles, an incremental recrystallization/grain growth process occurred where a minority solidification twin orientation grew and consumed the dominant initial orientation. An elastic FEM analysis incorporating anisotropic thermal expansion and elastic constants indicated that the final orientation had an internal stress state that was 20–100% smaller than the initial orientation, when subjected to a temperature change. The release of elastic strain energy provided the thermodynamic driving force for recrystallization/grain growth.

In-situ electromigration study on Sn−Ag−Cu solder joint by digital image speckle analysis

The phenomenon of electromigration in Pb-free Sn-Ag-Cu solder joint specimen subject to high current density was characterized. Electromigration measurements in the direction of the electron flux, via Cu metal lines, ENIG metallization, and Sn-Ag-Cu solder joint is documented. The consequence of this migration phenomenon causes void formation and the significant accumulation of voiding is a serious reliability concern. Digital image speckle analysis (DISA) was used to measure the in-situ micro-deformation of a cross sectioned solder joints, which is subject to electromigration with a current density of 5*10/sup 3/A/cm/sup 2/ under a constant temperature of 150/spl deg/C. The deformation and strain field of the solder joint were analyzed by a digital image correlation software. After 120hrs thermal-current test, higher strain near large voids was detected. The IMC growth behavior with/without current are compared. It was noted that IMC layer growth on anode interface is faster than cathode interface, and both are faster than isothermal aging.

Grain boundary sliding on near-7°, 14°, and 22° special boundaries during thermomechanical cycling in surface-mount lead-free solder joint specimens

To investigate the effect of external loads arising from differential thermal expansion between a substrate and a surface-mount component during thermomechanical cycling, specimens with a nickel surface-mount component on a copper substrate were prepared. Specimens consisted of two 100 μm thick 1 mm 2 solder joints about 9 mm apart, with two designs. In one specimen (denoted “ dual-shear”), the as-fabricated joints were not stressed due to differential contraction during solidification and cool down. In the other specimen (denoted “ component”), a continuous copper substrate between the joints caused the nickel component to be put in compression during cool down, which imposed shear on the joints. To impose differential thermal shear strains, the “dual-shear” specimen was clamped to a copper block to cause a significant reversal in sign of the shear imposed on the solder joint during cycling. In the “component” specimen configuration, the existing compressive strain in the component increased with cooling, but the stress was reduced with heating. Both joints were polished on the side, and then subjected to thermomechanical cycling for 3.5 h at −15 °C and 20 min at 150 °C. The effect of the two strain histories on microstructural evolution and heterogeneous straining was compared using scanning electron microscopy and orientation imaging microscopy. Each joint was made up of one to three very large grains containing low-angle boundaries and embedded minority orientations that evolved with cycling. The “component” specimen exhibited less heterogeneous strain than the “dual-shear” specimen that had reversed shear stresses. Since the dominant tin orientation was different in each joint, the resolved shear stress on various slip systems differed. In both specimens, the joint with the largest resolved shear parallel to the c-axis showed the greatest amount of heterogeneous strain arising primarily from sliding on [1 1 0] tilt boundaries misoriented by 7°, 14°, or about 20°. These boundary misorientations are close to misorientations with a coincident site lattice geometry.

Dislocation Activity and Slip Analysis Contributing to Grain Boundary Sliding and Damage during Thermomechanical Fatigue in Dual Shear Lead-Free Solder Joint Specimens

To investigate the effect of external loads arising from differential thermal expansion between a substrate and a surface mount component, specimens with a simulated surface mount component (nickel) on a copper substrate having a 1 mm2 joint area and solder thickness of about 100 µm were prepared to induce extrinsic shear in joints undergoing thermomechanical fatigue (TMF) cycling. The specimens were fabricated stress free and later clamped to a copper block to cause a significant reversal in sign of the shear imposed on the solder joint during TMF cycling for 20 minutes at 150°C and 3.5 hr at -15°C. The evolution of surface damage and microstructure was examined using SEM and Orientation Imaging Microscopy (OIM). The joints were almost single crystals. However, the orientations of the tin in each joint is different, leading to different resolved stresses on a given slip system. The joint with the largest resolved shear aligned with the crystal caxis showed the most damage. Low angle tilt boundaries developed, and sliding was observed on boundaries near 7 and 14° that have a coincident site lattice. Schmid factor analysis was carried out in regions that showed ledges or grain boundary sliding. Slip on (110) planes correlated well with some of the ledges.