Techniques to evaluate solder fatigue behavior include finite element analysis and semi-analytical closed-form equations.
[1] 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 thermomechanically[2] driven due to temperature cycling.
This causes the solder joints to experience non-recoverable deformation via creep and plasticity that accumulates and leads to degradation and eventual fracture.
Although they are still used in select industries and applications, lead-free solders have become significantly more popular due to RoHS regulatory requirements.
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.
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.
Global and local mismatches of coefficient of thermal expansion (CTE) between the component, component leads, PCB substrate, and system level effects[4] drive stresses in the interconnects (i.e. solder joints).
The deformation characteristics of various solder alloys can be described at the microscale due to the differences in composition and resulting microstructure.
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 coarsen[5] as energy is dissipated from the joint.
This eventually leads to crack initiation and propagation which can be described as accumulated fatigue damage.
[6] The resulting bulk behavior of solder is described as viscoplastic (i.e. rate dependent inelastic deformation) with sensitivity to elevated temperatures.
Several constitutive models have been developed to capture the creep characteristics of lead and lead-free solders.
These model parameters are often incorporated as inputs in FEA simulations to properly characterize the solder response to loading.
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.
These model constants are fit from experimental testing and simulation for different solder alloys.
The generalized Coffin–Manson[11][12][13][14] model considers the elastic and plastic strain range by incorporating Basquin's equation[15] and takes the form:
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.
∆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).
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 [citation needed] that Engelmaier and other models that are based on strain range do not provide a sufficient degree of accuracy.
Darveaux[18][19] 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.
Model constants can be fit for different solder alloys using a combination of experimental testing and finite element analysis (FEA) simulation.
Here α is the CTE, T is temperature, LD is the distance to the neutral point, E is elastic modulus, A is the area, h is the thickness, G is shear modulus, ν is Poisson's ratio, and a is the edge length of the copper bond pad.
Steinberg[29] developed a vibration model to predict time to failure based on the calculated board displacement.
This model takes into account the input vibration profile such as the power spectral density or acceleration time history, the natural frequency of the circuit card, and the transmissibility.
Blattau developed a modified Steinberg model[30] that uses board level strains rather than displacement and has sensitivity to individual package types.
At lower temperatures and faster strain rates the creep can approximated to be minimal and any inelastic damage will be dominated by plasticity.