Development of a Rapid Fatigue Life Testing Method for Reliability Assessment of Flip-Chip Solder Interconnects

The underlying physics of failure are critical in assessing the long term reliability of power packages in their intended field applications, yet traditional reliability determination methods are largely inadequate when considering thermomechanical failures. With current reliability determination methods, long test durations, high costs, and a conglomerate of concurrent reliability degrading threat factors make effective understanding of device reliability difficult and expensive. In this work, an alternative reliability testing apparatus and associated protocol was developed to address these concerns; targeting rapid testing times with minimal cost while preserving fatigue life prediction accuracy. Two test stands were fabricated to evaluate device reliability at high frequency (60 cycles/minute) with the first being a single-directional unit capable of exerting large forces (up to 20 N) on solder interconnects in one direction. The second test stand was developed to allow for bi-directional application of stress and the integration of an oven to enable testing at elevated steady-state temperatures. Given the high frequency of testing, elevated temperatures are used to emulate the effects of creep on solder fatigue lifetime. Utilizing the mechanical force of springs to apply shear loads to solder interconnects within the devices, the reliability of a given device to withstand repeated cycling was studied using resistance monitoring techniques to detect the number of cycles-to-failure (CTF). Resistance monitoring was performed using specially designed and fabricated, device analogous test vehicles assembled with the ability to monitor circuit resistance in situ. When a resistance rise of 30 % was recorded, the device was said to have failed. A mathematical method for quantifying the plastic work density (amount of damage) sustained by the solder interconnects prior to failure was developed relying on the relationship between Hooke’s Law for springs and damage deflection to accurately assess the mechanical strength of tested devices

COMPARISON OF INTERCONNECT FAILURES OF ELECTRONIC COMPONENTS MOUNTED ON FR-4 BOARDS WITH SN37PB AND SN3.0AG0.5CU SOLDERS UNDER RAPID LOADING CONDITIONS.

Electronic circuit boards can experience rapid loading through shock or vibration events during their lives; these events can happen in transportation, manufacture, or in field conditions. Due to the lead-free migration, it is necessary to evaluate how this rapid loading affects the durability of a leading lead free solder alternative (Sn3.0Ag0.5Cu) assemblies as compared with traditional eutectic lead based solder Sn37Pb assemblies. A literature review showed that there is little agreement on the fatigue behavior of Sn37Pb solder assemblies and Sn3.0Ag0.5Cu solder assemblies subjected to rapid loading. To evaluate the failure behavior of Sn37Pb and Sn3.0Ag0.5Cu solder assemblies under rapid loading conditions, leadless chip resistors (LCR), ball grid arrays (BGA), small outline integrated circuits (SOIC), and small outline transistors (SOT) were subjected to four point bend tests via a servo-hydraulic testing machine at printed wiring board (PWB) strain rates greater than 0.1/s. The PWB strain was the metric used to evaluate the failures. The PBGAs and LCRs were examined with both Sn37Pb and Sn3.0Ag0.5Cu solders. There was no significant difference found in the resulting test data for the behavior of the two solder assembly types in the high cycle fatigue regime. PBGA assemblies with both solders were also evaluated at a higher strain rate, approximately 1/s, using drop testing. There was no discernable difference found between the assemblies as well as no difference in the failure rate of the PBGAs at this higher strain rate. The PWB strain was converted to an equivalent solder stress index using finite element analysis. This equivalent stress index value was used to compare the results from the LCR and BGA testing for Sn37Pb and Sn3.0Ag0.5Cu. Independently generated BGA data that differed with respect to many testing variables was adjusted and incorporated to this comparison. The resulting plot did not show any significant differences between the behaviors of the two solder assemblies under rapid loading outside of the ultra low cycle fatigue regime, where the assemblies with Sn37Pb solder outperformed the assemblies with SnAgCu solder

MODELING RATE DEPENDENT DURABILITY OF LOW-Ag SAC INTERCONNECTS FOR AREA ARRAY PACKAGES UNDER TORSION LOADS

The thesis discusses modeling rate-dependent durability of solder interconnects under mechanical torsion loading for surface mount area array components. The study discusses an approach to incorporate strain-rate dependency in durability estimation for solder interconnects. The components under study are two configurations of BGAs (ball grid array) assembled with select lead-free solders. A torsion test setup is used to apply displacement controlled loads on the test board. Accelerated test load profile is experimentally determined. Torsion test is carried out for all the components under investigation to failure. Strain-rate dependent (Johnson-Cook model) and strain-rate independent, elastic-plastic properties are used to model the solders in finite element simulation. Damage model from literature is used to estimate the durability for SAC305 solder to validate the approach. Test data is used to extract damage model constants for SAC105 solder and extract mechanical fatigue durability curve

MICROSTRUCTURAL CHARACTERIZATION AND THERMAL CYCLING RELIABILITY OF SOLDERS UNDER ISOTHERMAL AGING AND ELECTRICAL CURRENT

Solder joints on printed circuit boards provide electrical and mechanical connections between electronic devices and metallized patterns on boards. These solder joints are often the cause of failure in electronic packages. Solders age under storage and operational life conditions, which can include temperature, mechanical loads, and electrical current. Aging occurring at a constant temperature is called isothermal aging. Isothermal aging leads to coarsening of the bulk microstructure and increased interfacial intermetallic compounds at the solder-pad interface. The coarsening of the solder bulk degrades the creep properties of solders, whereas the voiding and brittleness of interfacial intermetallic compounds leads to mechanical weakness of the solder joint. Industry guidelines on solder interconnect reliability test methods recommend preconditioning the solder assemblies by isothermal aging before conducting reliability tests. The guidelines assume that isothermal aging simulates a "reasonable use period," but do not relate the isothermal aging levels with specific use conditions. Studies on the effect of isothermal aging on the thermal cycling reliability of tin-lead and tin-silver-copper solders are limited in scope, and results have been contradictory. The effect of electrical current on solder joints has been has mostly focused on current densities above 104A/cm2 with high ambient temperature (≥100oC), where electromigration, thermomigration, and Joule heating are the dominant failure mechanisms. The effect of current density below 104A/cm2 on temperature cycling fatigue of solders has not been established. This research provides the relation between isothermal aging and the thermal cycling reliability of select Sn-based solders. The Sn-based solders with 3%, 1%, and 0% silver content that have replaced tin-lead are studied and compared against tin-lead solder. The activation energy and growth exponents of the Arrhenius model for the intermetallic growth in the solders are provided. An aging metric to quantify the aging of solder joints, in terms of phase size in the solder bulk and interfacial intermetallic compound thickness at the solder-pad interface, is established. Based on the findings of thermal cycling tests on aged solder assemblies, recommendations are made for isothermal aging of solders before thermal cycling tests. Additionally, the effect of active electrical current at 103 A/cm2 on thermal cycling reliability is reported

The durability of solder joints under thermo-mechanical loading; application to Sn-37Pb and Sn-3.8Ag-0.7Cu lead-free replacement alloy

Solder joints in electronic packages provide mechanical, electrical and thermal connections. Hence, their reliability is also a major concern to the electronic packaging industry. Ball Grid Arrays (BGAs) are a very common type of surface mount technology for electronic packaging. This work primarily addresses the thermo-mechanical durability of BGAs and is applied to the exemplar alloys; traditional leaded solder and a popular lead-free solder. Isothermal mechanical fatigue tests were carried out on 4-ball test specimens of the lead-free (Sn-3.8Ag-0.7Cu) and leaded (Sn-37Pb) solder under load control at room temperature, 35°C and 75°C. As well as this, a set of combined thermal and mechanical cycling tests were carried out, again under load control with the thermal cycles either at a different frequency from the mechanical cycles (not-in-phase) or at the same frequency (both in phase and out-of-phase). The microstructural evaluation of both alloys was investigated by carrying out a series of simulated ageing tests, coupled with detailed metallurgical analysis and hardness testing. The results were treated to produce stress-life, cyclic behaviour and creep curves for each of the test conditions. Careful calibration allowed the effects of substrate and grips to be accounted for and so a set of strain-life curves to be produced. These results were compared with other results from the literature taking into account the observations on microstructure made in the ageing tests. It is generally concluded that the TMF performance is better for the Sn-Ag-Cu alloy than for the Sn-Pb alloy, when expressed as stress-life curves. There is also a significant effect on temperature and phase for each of the alloys, the Sn-Ag-Cu being less susceptible to these effects. When expressed as strain life, the effects of temperature, phase and alloy type are much diminished. Many of these conclusions coincided with only parts of the literature and reasons for the remaining differences are advanced

Numerical analysis and thermal fatigue life prediction of solder layer in a SiC-IGBT power module

Limited by the mechanical properties of materials, silicon (Si) carbide insulated gate bipolar transistor (IGBT) can no longer meet the requirements of high power and high frequency electronic devices. Silicon carbide (SiC) IGBT, represented by SiC MOSFET, combines the excellent performance of SiC materials and IGBT devices, and becomes an ideal device for high-frequency and high-temperature electronic devices. Even so, the thermal fatigue failure of SiC IGBT, which directly determines its application and promotion, is a problem worthy of attention. In this study, the thermal fatigue behavior of SiC-IGBT under cyclic temperature cycles was investigated by finite element method. The finite element thermomechanical model was established, and stress-strain distribution and creep characteristics of the SnAgCu solder layer were obtained. The thermal fatigue life of the solder was predicted by the creep, shear strain and energy model respectively, and the failure position and factor of failure were discussed

Creep-Fatigue Behaviours of Sn-Ag-Cu Solder Joints in Microelectronics Applications

Electronic manufacturing is one of the dynamic industries in the world in terms of leading in technological advancements. At the heart of electronic assembly lies the 'soldering technology' and the 'solder joints' between electronic components and substrate. During the operation of electronic products, solder joints experience harsh environmental conditions in terms of cyclic change of temperature and vibration and exposure to moisture and chemicals. Due to the cyclic application of loads and higher operational temperature, solder joints fail primarily through creep and fatigue failures. This paper presents the creep-fatigue behaviours of solder joints in a ball grid array (BGA), soldered on a printed circuit board (PCB). Using finite element (FE) simulation, the solder joints were subjected to thermal cycling and isothermal ageing. Accelerated thermal cycling (ATC) was carried out using a temperate range from 40℃ to 150℃, and isothermal ageing was done at -40,25,75 and 150℃ temperatures for 45 days (64,800 mins). The solders studied are lead-based eutectic Sn63Pb37 and lead-free SAC305, SAC387, SAC396 and SAC405. The results were analysed using the failure criterion of equivalent stress, strain rate, deformation rate, and the solders' strain energy density. The SAC405 and SAC396 are found to possess the least stress magnitude, strain rate, deformation rate, and strain energy density damage than the lead-based eutectic Sn63Pb37 solder; they have the highest fatigue lives based on the damage mechanisms. This research provides a technique for determining the preventive maintenance time of BGA components in mission-critical systems. Furthermore, it proposes developing a new life prediction model based on a combination of the damage parameters for improved prediction.N/

Modelling of the reliability of flip chip lead-free solder joints at high-temperature excursions

At high-temperature operations of electronic control devices, Tin-Silver-Copper (SnAgCu) alloy solder joints used to assemble the component of the devices are functioning at homologous temperature above 0.8. In such ambient temperatures, solder alloys have limited mechanical strength and will be sensitive to strain rate. The sensitivity of solder properties to creep/visco-plastic deformation increases the rate of accumulation of plastic damage in the alloy and decreases the number of cycles to failure (Nf) of the joints. Most untimely rupture of solder joints in high-temperature electronics (HTE) system usually culminates in colossal loss of resources and lives. Typical incidences are reported in recent automotive and aircraft crashes as well as the collapse of oil-well logging equipment. To increase the mean time to failure (MTTF) of solder joints in HTE, an in-depth understanding and accurate prediction of the response of solder joints to thermally induced plastic strain damage is crucial. This study concerns the prediction of the reliability of lead-free solder joints in a flip chip (FC) model FC48D6.3C457 which is mounted on a substrate and the assembly subjected to high-temperature excursions. The research investigates the effect of the high-temperature operations on reliability of the joints. In addition, the investigation examines the impact of control factors (component stand-off height (CSH), inter-metallic compound (IMC) thickness, number of thermal cycle and solder volume) on Nf of the joints. A model developed in the course of this investigation was employed to create the assembly solder joints architecture. The development of the model and creation of the bump profile involve a combination of both analytical and construction methods. The assembled package on a printed circuit board (PCB) was subjected to accelerated temperature cycle (ATC) employing IEC standard 60749-25 in parts. The cycled temperature range is between -38 oC and 157 oC. Deformation behaviour of SnAgCu alloy solder in the joints is captured using Anand’s visco-plasticity model and the response of other materials in the assembly were simulated with appropriate model. The results demonstrate that the reliability of solder joints operating at elevated temperatures is dependent on CSH, thickness of IMC and solder volume. It also shows that incorporating the IMC layer in the geometric models significantly improves the level of accuracy of fatigue life prediction to ± 22.5% (from the ± 25% which is currently generally accepted). The findings also illustrate that the magnitude of the predicted damage and fatigue life are functions of the number of ATC employed. The extensive set of results from the modelling study has demonstrated the need for incorporating the IMC layer in the geometric model to ensure greater accuracy in the prediction of solder joint service life. The technique developed for incorporating the IMC layer will be of value to R&D engineers and scientists engaged in high-temperature applications in the automotive, aerospace and oil-well logging sectors. The results have been disseminated through peer reviewed journals and also presentations at international conferences

Thermomechanical behavior of monolithic Sn-Ag-Cu solder and copper fiber reinforced solders

Solder joints provide both electrical and mechanical interconnections between a silicon chip and the packaging substrate in an electronic application. The thermomechanical cycling in the solder causes numerous reliability challenges, mostly because of the mismatch of the coefficient of thermal expansion between the silicon chip and the substrate. The actual transition to lead-free solders and the trend towards hotter-running, miniaturized and higher current density chips aggravate this situation. Therefore, improved solder joints, with higher resistance to creep and low cycle fatigue, are necessary for future generations of microelectronics. This study focuses on a thermomechanical behavior comparison between monolithic Sn-Ag-Cu, copper fiber and copper ribbon cylindrical reinforced solders. The composite solders were found to reduce the inelastic strain range of the joint relative to monolithic solder, but at the expense of increased stress range.http://archive.org/details/thermomechanical109452062Approved for public release; distribution is unlimited

Influence of the microstructure on the creep behaviour of Tin-Silver-Copper solder

A common failure mode of electronic printed circuit boards (PCB’s) is the appearance of cold solder joints between the component and PCB, during product life. This phenomenon is related to solder joint fatigue and is attributed mainly to the mismatch of the coefficients of thermal expansion (CTE) of component-solder-PCB assembly. With today’s solder joint thickness decreasing and increasing working temperatures, among others, the stresses and strains due to temperature changes are growing, leading to limited fatigue life of the products. As fatigue life decreases with increasing plastic strain, creep occurrence should have significant impact, especially during thermal cycles and, thus, should be studied. Through the cooling phase, on the production of PCB assembly’s by the reflow technology, the hoven atmosphere temperature is adjusted in order to control the cooling rate. Narrow criteria is used so as to control the inter-metallic compounds (IMC) thickness, PCB assembly distortion and defects due to thermal shock. The cooling rate also affects solder microstructure, which has direct impact on creep behaviour and, thus, on the soldered joint reliability. In this paper, a dynamic mechanical analyser (DMA) is used to study the influence of the solder cooling rate on its creep behaviour. SAC405 samples with two distinct cooling rates were produced: inside a hoven cooling and by water quenching. Creep tests were made on three-point-bending clamp configuration, isothermally at 25 °C, 50 °C and 75 °C and under three separate levels of stress, 3, 5 and 9 MPa. The results show that creep behaviour has a noticeable cooling rate dependence. It was also noticed that creep propensity is exacerbated by the temperature at which stresses are applied, especially for the slower cooling rates. Creep mechanisms were related to the solder microstructural constituents, namely by the amount of phases ant their morphology.The authors would like to express his acknowledgments for the support given by the Portugal Incentive System for Research and Technological Development. Project in co-promotion This research is sponsored by the Portugal Incentive System for Research and Technological Development. This work is supported by: European Structural and Investment Funds in the FEDER component, through the Operational Competitiveness and Internationalization Programme (COMPETE 2020) [Project nº 002814; Funding Reference: POCI-01-0247-FEDER-002814]. This work was financed by FCT, under the Strategic Project UID/SEM/04077/2013; PEst2015-2020 with the reference UID/CEC/00319/2013 and UID/FIS/04650/2013