Fused or sintered Cu nanoparticle structures are potential alternatives to solder for ultra-fine pitch flip chip assembly and to sintered Ag for heat sink attach in high temperature microelectronics. Meaningful testing and interpretation of test results in terms of what to expect under realistic use conditions do, however, require a mechanistic picture of degradation and damage mechanisms. As far as fatigue goes, such a picture is starting to emerge. The porosity of sintered nano-particle structures significantly affects their behavior in cycling. The very different sensitivities to parameters, compared to solder, means new protocols will be required for the assessment of reliability. The present study focused on fatigue in both isothermal and thermal cycling. During the latter, all damage occurs at the low temperature extreme, so life is particularly sensitive to the minimum temperature and any dwell there. Variations in the maximum temperature up to 125 °C did not affect, but a maximum temperature of 200 °C led to much faster damage. Depending on particle size and sintering conditions deformation and damage properties may also degrade rapidly over time. Our picture allows for recommendations as to more relevant test protocols for vibration, thermal cycling, and combinations of these, including effects of aging, as well as for generalization of test results and comparisons in terms of anticipated behavior under realistic long-term use conditions. Also, the fatigue life seems to vary with the ultimate strength, meaning that simple strength testing becomes a convenient reference in materials and process optimization.

Solder joints in electronic assemblies experience damage due to cyclic thermomechanical loading that eventually leads to fatigue fracture and electrical failure. While solder joints in smaller, die-sized area-array packages largely experience shear fatigue due to thermal expansion mismatch between the component and the substrate, larger area-array packages experience a combination of cyclic shear and axial tensile/compressive loads due to flexure of the substrate. Additionally, on larger processor packages, the attachment of heatsinks further exacerbates the imposed axial loads, as does package warpage. With the increase in size of packages due to 2.5D heterogeneous integration, the above additional axial loads can be significant. Thus, there exists a critical need to understand the impact on fatigue life of solder joints with superposed compressive/tensile loads on the cyclic shear loads. In this paper, we describe a carefully constructed multi-axial microprecision mechanical tester as well as fatigue test results on Sn3.0Ag0.5Cu (SAC305) solder joints subjected to controlled cyclic shear and constant compressive/tensile loads. The tester design allows one to apply cyclic shear loads up to 200 N while maintaining a constant axial load of up to 38 N in tension or compression. The tester is capable of maintaining the axial load to within a tolerance of ±0.5 N during the entirety of fatigue experiment. Carefully constructed test specimens of Sn3.0Ag0.5Cu solder joints were isothermally fatigued under systematically increased compressive and tensile loads imposed on the test specimen subject to repeated loading (R = 0) under lap-shear. In general, the imposition of the superposed compressive load increases the fatigue life of the solder joint compared to application of pure cyclic shear, while the imposition of the superposed tensile load decreases the fatigue life. At larger compressive loads, friction between fractured surfaces is responsible for significant energy dissipation during the cyclic load–unload cycles.

Thermal fatigue failure of microelectronic chip often initiates from the interface between solder and substrate, and the service life of the chip is largely dependent on the singular stress–strain at this interface. To provide a reasonable life evaluation method, three thermal fatigue evaluation models, including strain-based and stress–strain based, have been established in terms of the interfacial singular fields. Thermal fatigue lives of different chips under different thermal cycles are obtained by thermal fatigue tests, and the stress and strain intensity factors and singular orders at the solder/substrate interface are computed at the same conditions, to determine the material constants in the established models. The thermal fatigue lives predicted are in acceptable agreement with the experimental results. What is more, the application of these thermal fatigue models demonstrates a fact that the thermal fatigue of the microelectronic chips can be evaluated uniformly no matter what the shapes, dimensions of the chip, and the thermomechanical properties of the solders are, as long as the relevant stress–strain intensity factors and singular orders are obtained.

In displacement controlled mechanical fatigue of ball grid solder interconnect arrays, decrease in maximum load monitors total increase in crack area in an array while electrical resistance monitors only the area of large cracks that lead to electrical failure. Small cracks with good electrical contact between the crack surfaces have only minor effect on the resistance of the array. In this mechanical fatigue research of ball grid arrays, the fatigue damage was continually followed by simultaneously measuring maximum load and electrical resistance. Experimental details, results, and analysis of the results are given including a Paris relation fit to the data.

The fatigue life of a material varies with the strain rate if it has time-dependent deformation. An interesting phenomenon related to the effect of the strain rate on the fatigue life can be observed when a cyclic tension-compression loading of which strain rate in the tensile region is different from that in the compressive region is employed for the fatigue test. Different fatigue lives due to different strain rates in the tensile and compression regions originate from the difference of development behaviors of creep strain generated in the cyclic loading. This paper investigates the effects of creep strain on the difference of fatigue life due to the different strain rate in the tensile and compression regions. The creep strain of the lead-free solder Sn–3.0Ag–0.5Cu subjected to a cyclic loading was investigated using stepped ramp wave loading. The experimental results reveal that the creep strain develops differently in the tensile and compression regions. A new parameter is proposed for estimating fatigue life when the strain rate varies in the loading direction.

Isothermal three-point and four-point cyclic bend fatigue test methods have been developed for Sn–Ag–Cu solder joints. Reported bend tests from the literature were conducted at room temperature (25°C) and there is lack of data for lead-free solder joints. In this study, very-thin quad flat no-lead (VQFN) assembly with Sn–Ag–Cu lead-free solder was tested under three-point and four-point cyclic bending loads at both room temperature (25°C) and high temperature (125°C). The correlation between three-point and four-point bend tests was developed. Two different board surface finishes of electroless Ni and immersion gold (ENIG) and organic solderability preservatives (OSP) were investigated. Bending fatigue resistance of VQFN with OSP finish is slightly better than ENIG finish case. The acceleration factor of failure at high temperature (125°C) is higher than that at room temperature (25°C). Finite element analysis modeling and simulation were performed for different test conditions to investigate the solder joint stress-strain behavior. Volume-averaged energy density was used as a fatigue damage parameter and energy-based bending fatigue models were developed for VQFN with Sn–Ag–Cu solder joint under cyclic bending load at both 25°C and 125°C.

No-flow underfill materials reduce assembly processing steps and can potentially be used in fine-pitch flip chip on organic board assemblies. Such no-flow underfills, when filled with nano-scale fillers, can significantly enhance the solder bump reliability, if the underfills do not prematurely delaminate or crack. Therefore, it is necessary to understand the risk of underfill delamination during assembly and during further thermal excursions. In this paper, the interface between silicon nitride ( SiN ) passivation and a nano-filled underfill (NFU) material is characterized under monotonic as well as thermo-mechanical fatigue loading, and fracture parameters have been obtained from such experimental characterization. The passivation-underfill interfacial delamination propagation under monotonic loading has been studied through a fixtureless residual stress induced decohesion (RSID) test. The propagation of interfacial delamination under thermo-mechanical fatigue loading has been studied using sandwiched assemblies and a model for delamination propagation has been developed. The characterization results obtained from this work can be used to assess the delamination propagation in flip-chip assemblies. Though the methods presented in this paper have been applied to nano-filled, no-flow underfill materials, their application is not limited to such materials or material interfaces.

To give a proper and accurate estimation of the fatigue life of ball grid array (BGA) solder joints, a mechanical fatigue test method under mixed-mode loading is proposed. Experiments were conducted with 63 Sn ∕ 37 Pb and Sn ∕ 3.5 Ag ∕ 0.75 Cu solder joints in room temperature. The mechanical low cycle fatigue tests were performed under several loading angles. The loading angle is controlled by several grips which have specific surface angle to the loading direction. Constant displacement controlled tests are performed using a micro-mechanical test apparatus. It was found that the normal deformation significantly affects the fatigue life of the solder joint. Throughout the whole test conditions at room temperature, Sn ∕ 3.5 Ag ∕ 0.75 Cu solder alloy had longer fatigue life than 63 Sn ∕ 37 Pb alloy. Failure patterns of the fatigue tests were observed and discussed. A morrow energy model was examined and found to be a proper low cycle fatigue model for solder joints under mixed mode loading condition.

Fatigue tests were performed using single lap-joint specimens to obtain near-threshold fatigue crack growth data of solder joint under mode-II load. Attention was focused on the effect of high temperature aging and microstructures separately from the intermetallics. As a result, it was shown that the long cast time yielded the intermetallics and microstructures of the solder invariable regardless of aging condition. The granular micro-structure of the air-cooled specimens was shown to be inferior to the laminated micro-structure of the furnace-cooled specimens. Also, transition of fatigue crack behavior with Δ J and the procedure of fatigue crack propagation from the pre-crack tip were discussed.

Thermal reliability of the solder sealing ring of Agilent Technologies’ bubble-actuated photonic cross-connect switches has been investigated in this paper. Emphasis is placed on the determination of the thermal-fatigue life of the solder sealing ring under shipping/storing/handling conditions. The solder ring is assumed to obey the Garofalo-Arrhenius creep constitutive law. The nonlinear responses such as the deflections, stresses, creep strains, and creep strain energy density of the 3-D photonic package have been determined with a commercial finite element code. In addition, isothermal fatigue tests have been performed to obtain the relationship between the number of cycle-to-failure and the strain energy density. Thus, by combining the finite element results and the isothermal fatigue test results, the average thermal-fatigue life of the solder sealing ring is readily determined and is found to be more than adequate for shipping/storing/handling the photonic switches.

This study investigates the effect of quasi-static bending loads (strain rate=0.05/s) on the durability of 0.5 mm pitch Chip Scale Package (CSP) interconnects when assembled on FR4 substrates. The substrates have rows of CSPs and are subjected to three-point bending loads. Overstress curvature limits are experimentally determined and used to identify limits for zero-to-max cyclic bending loads. The test configuration is simulated using finite element modeling (FEM) and the total strain accumulated in the solder joints is estimated. Using the FEM model, a calibration curve is constructed to relate the cyclic curvature range in the substrate to the cyclic strain range in the critical solder joint. Bending moments along the substrate are estimated from the forces applied at the center of the board during the fatigue test. Strains measured on the substrate surface and the bending displacements measured at the center are used to estimate curvatures at different locations along the substrate. Using the calibration curve, the total strains in the solder joint are obtained for the applied loading. A strain-range fatigue damage model proposed by Coffin and Manson, is used to predict the cycles to failure for the applied loading. Predicted durability is compared to experimental measurements. Concave substrate curvature is found to be more damaging than convex curvature, for interconnect fatigue. Finite element simulations are repeated for life-cycle loading to predict acceleration factors. Using the acceleration factors, the product durability is estimated for life-cycle environments.

To investigate the effect of stencil thickness and reflow ambient atmosphere on the reliability of ceramic ball grid array (CBGA) assemblies, three levels of stencil thickness, 0.10, 0.15, and 0.20 mm, were used to print solder paste on printed circuit board (PCB). After the CBGA modules were placed on PCBs, the specimens were divided into two groups, and reflowed in nitrogen and compressed air separately. Properties of the six groups of assemblies, such as shear strength, bending fatigue life, thermal shock cycles, and vibration fatigue life, were tested to find out the optimum assembling process. The results show that assemblies prepared with a stencil 0.15 mm thick yield maximized performance. And the nitrogen ambient atmosphere demonstrates a remarkable effect on improving the fatigue life. Theoretical models are given to qualitatively explain the relationship between the solder joint volume and performance. This work provides a guideline on how to determine the soldering process parameters of CBGA assemblies.

We have developed a new technique that uses a noncontact fiber optic displacement sensor to investigate the crack growth along polymer interfaces under thermal fatigue conditions. This technique has been used to test the underfill/passivation interface of a direct chip attach (DCA) assembly, the thermal fatigue driven delamination of which is a major cause for failure of DCA assemblies. The sample is prepared as a multilayered cantilever beam by capillary flow of the underfill over a polyimide coated metallic beam. During thermal cycling the crack growth along the interface from the free end changes the displacement of this end of the beam and we measure this displacement at the lowest temperature in each thermal cycle. The change in beam displacement is converted into crack growth knowing the geometry of the specimen. The crack growth rate depends on the maximum difference in the strain energy release rate of the crack in each cycle and the mechanical phase angle. This paper outlines the theoretical basis of the technique and provides initial results obtained for a variety of underfills dispensed over a commercial (PMDA/ODA) polyimide. The technique was validated by comparing the crack growth measured by displacement changes with direct optical microscopy measurements of the crack length.

Thermal fatigue resistance was investigated for insertion mount solder joints, manufactured with 58Bi-42Sn (wt. %), 43Sn-43Pb-14Bi, 52In-48Sn and 40In-40Sn-20Pb low-temperature alloys. Accelerated thermal cycling was used in conjunction with metallographic analysis to determine the fatigue resistance and to elucidate the failure mode for each solder composition. Additionally, the behavior of each solder alloy was compared to that of 63Sn-37Pb solder, tested under identical conditions. A two-phase microstructure with lamellar morphology was observed in 58Bi-42Sn, whereas a globular morphology was prevalent in the remaining alloys. The Bi-containing solders demonstrate fatigue resistance comparable to 63Sn-37Pb, but greater than the In-bearing alloys. These differences and microstructure changes prior to and during thermal fatigue testing are discussed.

A solder joint specimen has been designed to determine the stress/strain hysteresis response and fracture behavior of 90 percent wtPb/10 percent wtSn solder alloy. The specimen consists of an Al 2 O 3 beam and an Al 2024-T4 beam bonded together at the ends with solder. The specimen is subjected to thermal cycling to failure between 40°C to 140°C with a 10°C/min ramp rate and 10-minute hold times. Stress/strain hysteresis loops were experimentally determined as a function of thermal cycles. A method based on the stress relaxation data at hold times has been developed to determine the steady state creep parameters of the solder. A constitutive equation for the solder alloy based on elastic and creep deformation has been formulated and implemented in a finite element code, ABAQUS. Good agreement was obtained between the finite element model and the experimental results. In the thermal fatigue test, crack length versus number of thermal cycles was measured for two different shear strain ranges, and the fracture surface was examined with SEM. The SEM results show a combined transgranular and intergranular fracture. In addition, a significant amount of secondary cracks and voids were generated during thermal fatigue which led to material weakening. A thermal fatigue model based on the C* integral, the measured stress history, and creep properties was employed to model the fracture behavior.

Room temperature, displacement-controlled tensile test and displacement-controlled tension/compression fatigue tests were performed on as-cast specimens of eutectic Pb-Sn solder with lamellar microstructure. A range of strain rates were used to identify how failure varies with strain rate. Under tensile loading at high strain rate (6.7 × 10 −2 /s) failure was observed after about 20 percent strain. There was evidence of shear localization, and failure was transgranular with a dimpled fracture surface. The low strain rate (6.7 × 10 −6 /s ) tensile specimen failed by cavitation. This cavitation coincided with sliding at boundaries between colonies of the eutectic (grain boundaries); there was little evidence of sliding in the high strain rate specimen. Two behaviors were observed in the fatigue tests. At low frequency and high strain range (2 × 10 −3 /s, 4 percent, N f ≈ 300−1000) numerous intergranular cracks initiated at the surface, eventually linking up and propagating along grain boundaries to the interior. What appeared to be fatigue striations were found on the crack face. At high frequency and low strains (>0.1/s, <1 percent) transgranular failure dominated.

Isothermal and thermomechanical fatigue of 63Sn/37Pb solder is studied under total strain-controlled tests. A standard definition of failure is proposed to allow inter-laboratory comparison. Based on the suggested failure criterion, load drop per cycle, the Young’s modulus and the ratio of the maximum tensile to maximum compressive stresses remain constant, and the fatigue response of the solder is stable before failure, although cyclic softening was observed from the beginning. Experimental results of isothermal fatigue tests for a total strain range from 0.3 to 3 percent show that the log-log plot of the number of cycles to failure versus the plastic strain range has a kink at the point where the elastic strain is approximately equal to the plastic strain. In this paper, it is shown how the isothermal fatigue life of near-eutectic solder at lower strain ranges can be predicted by using the experimental data of fatigue tests at high strain ranges and early stage information of a fatigue test at the strain range in question. A thermomechanical fatigue life prediction is also given based on a dislocation pile-up model. Comparison with experimental results shows a good agreement.

Modules attached to circuit cards by peripheral J- and gullwing leads were studied for their behavior under flexure. Three aspects of mechanical behavior were focused upon: the stiffness of the system, the forces arising in the leads, and the fatigue strength of the latter. The effective stiffness of a module-reinforced circuit card was measured experimentally in several configurations (load on card and load-on-module, double-sided and stacked). The leaded attachments were in two parallel rows. Analytical modeling of these tests were performed considering the leads as a continuous elastic foundation connecting the module and the card; test results were corroborated. Experiments were also conducted to establish the elastic and elastoplastic range of lead stiffness in three perpendicular directions: in two shearing planes and axially. The latter was the stiffest and most significant direction, motivating much of the present analysis. For lead force, the analytical procedure yielded values which were confirmed by finite element computation methods described previously by Engel (1990). Fatigue tests were performed on both J- and gullwing leads. Solder joints failed in the former, while lead failures occurred in the latter.