Solder fatigue is the mechanical degradation of solder due to deformation under cyclic loading. This can often occur at stress levels below the yield stress of solder as a result of repeated temperature fluctuations, mechanical vibrations, or mechanical loads. Techniques to evaluate solder fatigue behavior include finite element analysis and semi-analytical closed-form equations.Serebreni, M., Blattau, N., Sharon, G., Hillman, C., Mccluskey, P. "Semi-analytical fatigue life model for reliability assessment of solder joints in qfn packages under thermal cycling". SMTA ICSR, 2017. Toronto, ON, https://www.researchgate.net/publication/317569529_SEMI-ANALYTICAL_FATIGUE_LIFE_MODEL_FOR_RELIABILITY_ASSESSMENT_OF_SOLDER_JOINTS_IN_QFN_PACKAGES_UNDER_THERMAL_CYCLING
Overview Solder is a metal alloy used to form electrical, thermal, and mechanical interconnections between the component and printed circuit board (PCB) substrate in an electronic assembly. Although other forms of cyclic loading are known to cause solder fatigue, it has been estimated that the largest portion of electronic failures are thermomechanicallyG. Sharon, "Temperature Cycling and Electronics", https://www.dfrsolutions.com/hubfs/Resources/services/Temperature-Cycling-and-Fatigue-in-Electronics-White-Paper.pdf driven due to temperature cycling.Wunderle, B.; B. Michel, "Progress in Reliability Research in Micro and Nano Region", Microelectronics and Reliability, V46, Issue 9-11, 2006. Under thermal cycling, stresses are generated in the solder due to coefficient of thermal expansion (CTE) mismatches. This causes the solder joints to experience non-recoverable deformation via creep and plasticity that accumulates and leads to degradation and eventual fracture.
Historically, tin-lead solders were common alloys used in the electronics industry. Although they are still used in select industries and applications, lead-free solders have become significantly more popular due to RoHS regulatory requirements. This new trend increased the need to understand the behavior of lead-free solders.
Much work has been done to characterize the creep-fatigue behavior of various solder alloys and develop predictive life damage models using a Physics of Failure approach. These models are often used when trying to assess solder joint reliability. The fatigue life of a solder joint depends on several factors including: the alloy type and resulting microstructure, the joint geometry, the component material properties, the PCB substrate material properties, the loading conditions, and the boundary conditions of the assembly.
Thermomechanical solder fatigue During a product's operational lifetime it undergoes temperature fluctuations from application specific temperature excursions and self-heating due to component power dissipation. Global and local mismatches of coefficient of thermal expansion (CTE) between the component, component leads, PCB substrate, and system level effects drive stresses in the interconnects (i.e. solder joints). Repeated temperature cycling eventually leads to thermomechanical fatigue.
The deformation characteristics of various solder alloys can be described at the microscale due to the differences in composition and resulting microstructure. Compositional differences lead to variations in phase(s), grain size, and intermetallics. This affects susceptibility to deformation mechanisms such as dislocation motion, diffusion, and grain boundary sliding. During thermal cycling, the solder's microstructure (grains/phases) will tend to coarsenCrina Rauta, Abhijit Dasgupta, Craig Hillman, "Solder Phase Coarsening, Fundamentals, Preparation, Measurement and Prediction", https://www.dfrsolutions.com/hubfs/Resources/services/Solder-Phase-Coarsening-Fundamentals-Preparation-Measurement-and-Prediction.pdf?t=1514473946162 as energy is dissipated from the joint. This eventually leads to crack initiation and propagation which can be described as accumulated fatigue damage.
The resulting bulk behavior of solder is described as viscoplastic (i.e. rate dependent inelastic deformation) with sensitivity to elevated temperatures. Most solders experience temperature exposures near their melting temperature (high homologous temperature) throughout their operational lifetime which makes them susceptible to significant creep. Several constitutive models have been developed to capture the creep characteristics of lead and lead-free solders. Creep behavior can be described in three stages: primary, secondary, and tertiary creep. When modeling solder, secondary creep, also called steady state creep (constant strain rate), is often the region of interest for describing solder behavior in electronics. Some models also incorporate primary creep. Two of the most popular models are hyperbolic sine models developed by GarofaloGarofalo, F., 1965, "Fundamentals of Creep and Creep-Rupture in Metals", Macmillan, New York. and AnandAnand, L., 1985, "Constitutive Equations for Hot Working of Metals", J. Plasticity, 1(3), pp. 213–231Brown, S. B.; Kim, K. H.; Anand, L., 1989, "An Internal Variable Constitutive Model for Hot Working of Metals," Int. J. Plasticity, 5(2), pp. 95–130 to characterize the steady state creep of solder. These model parameters are often incorporated as inputs in FEA simulations to properly characterize the solder response to loading.
Fatigue models Solder damage models take a physics-of-failure based approach by relating a physical parameter that is a critical measure of the damage mechanism process (i.e. inelastic strain range or dissipated strain energy density) to cycles to failure. The relationship between the physical parameter and cycles to failure typically takes on a power law or modified power law relationship with material dependent model constants. These model constants are fit from experimental testing and simulation for different solder alloys. For complex loading schemes, Miner's linear superposition damage lawM. A. Miner, "Cumulative damage in fatigue", Journal of applied mechanics, vol. 12, pp. 159-164, 1945 is employed to calculate accumulated damage.
Coffin–Manson model The generalized Coffin–MansonL. F. Coffin, "The Problem of Thermal Stress Fatigue in Austenitic Steels", Special Technical Publication 165, ASTM, 1954, p. 31L. F. Coffin, "A study of the Effects of Cyclic Thermal Stresses on a Ductile Metal", Trans. ASME, 76, 931–950 (August 1954).S. S. Manson, "Behavior of materials under conditions of thermal stress", Proceedings of the Heat Transfer Symposium, University of Michigan Engineering Research Institute, Ann Arbor, Mich, pp. 9-75, 1953Dowling, N. E., "Mechanical Behavior of Materials", 2nd Edition, Upper Saddle River, New Jersey, 1999. model considers the elastic and plastic strain range by incorporating Basquin's equationBasquin, O. H. (1910). "The exponential law of endurance test". Proceedings of the American Society for Testing and Materials. 10: 625–630. and takes the form:
Here ∆ε ⁄ 2 represents the elastic-plastic cyclic strain range, E represents elastic modulus, σm represents means stress, and Nf represents cycles to failure. The remaining variables, namely σf,ε'f,b, and c are fatigue coefficients and exponents representing material model constants. The generalized Coffin–Manson model accounts for the effects of high cycle fatigue (HCF) primarily due to elastic deformation and low cycle fatigue (LCF) primarily due to plastic deformation.
Engelmaier model In the 1980s Engelmaier proposed a model,Engelmaier, W., "Fatigue Life of Leadless Chip Carrier Solder Joints During Power Cycling", Components, Hybrids, and Manufacturing Technology, IEEE Transactions on, vol.6, no.3, pp. 232-237, September 1983 in conjunction with the work of Wild,Wild, R. N., "Some Fatigue Properties of Solders and Solder Joints", IBM Tech. Rep. 73Z000421, January 1973. that accounted for some of the limitations of the Coffin–Manson model, such as the effects of the frequency and temperature. His model takes a similar power law form:
Engelmaier relates the total shear strain (∆γ) to cycles to failure (Nf). ε'f and c are model constants where c is a function of mean temperature during thermal cycling (Ts) and thermal cycling frequency (f).
∆γ can be calculated as function of the distance from the neutral point (LD) solder joint height (hs), coefficient of thermal expansion (∆α), and change in temperature (ΔT). In this case C is empirical model constant.
This model was initially proposed for leadless devices with tin-lead solder. The model has since been modified by Engelmaier and others to account for other phenomena such as leaded components, thermal cycling dwell times, and lead-free solders. While initially a substantial improvement over other techniques to predict solder fatigue, such as testing and simple acceleration transforms, it is now generally acknowledged that Engelmaier and other models that are based on strain range do not provide a sufficient degree of accuracy.
Darveaux model DarveauxDarveaux, R., 1997, "Solder Joint Fatigue Life Model", in Design & Reliability of Solder and Solder Interconnections, Proceedings of the TMS, The Minerals, Metals & Materials Society, Orlando, Florida, February 1997.Darveaux, R. (2000) Effect of simulation methodology on solder joint crack growth correlation. Electronic Components and Technology Conference, 2000 IEEE, pp 158–169 proposed a model relating the quantity of volume weighted average inelastic work density, the number of cycles to crack initiation, and the crack propagation rate to the characteristic cycles to failure.
In the first equation N0 represents the number of cycles to crack initiation, ∆W represents inelastic work density, K1 and K2 are material model constants. In the second equation, da/dN represents the crack prorogation rate, ∆W represents inelastic work density, K3 and K4 are material model constants. In this case the crack propagation rate is approximated to be constant. Nf represents the characteristic cycles to failure and a represents the characteristic crack length. Model constants can be fit for different solder alloys using a combination of experimental testing and finite element analysis (FEA) simulation.
The Darveaux model has been found to be relatively accurate by several authors.Ye, Yuming, et al. "Assessment on reliability of BGA package double-sided assembled". High Density Packaging and Microsystem Integration, 2007. HDP'07. International Symposium on. IEEE, 2007Meifunas, M., et al. "Measurement and prediction of reliability for double-sided area array assemblies". Electronic Components and Technology Conference, 2003. Proceedings. 53rd. IEEE, 2003 However, due to the expertise, complexity, and simulation resources required, its use has been primarily limited to component manufacturers evaluating component packaging. The model has not received acceptance in regards to modeling solder fatigue across an entire printed circuit assembly and has been found to be inaccurate in predicting system-level effects (triaxiality) on solder fatigue.https://www.dfrsolutions.com/hubfs/Developing%20Damage%20Models%20for%20Solder%20Joints%20Exposed%20to%20Complex%20Stress%20States.pdf, Hillman, C., "Developing Damage Models for Solder Joints Exposed to Complex Stress States: Influence of Potting, Coating, BGA Mirroring, and Housing on Solder Joint Fatigue", Proceedings of the EMPC, Warsaw, Poland, September, 2017
Blattau model