Solder fatigue is the mechanical degradation of
solder
Solder (; North American English, NA: ) is a fusible alloy, fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces aft ...
due to
deformation under cyclic loading. This can often occur at
stress levels below the
yield stress
In materials science and engineering, the yield point is the point on a stress–strain curve that indicates the limit of elasticity (physics), elastic behavior and the beginning of plasticity (physics), plastic behavior. Below the yield point ...
of solder as a result of repeated temperature fluctuations, mechanical
vibration
Vibration () is a mechanical phenomenon whereby oscillations occur about an equilibrium point. Vibration may be deterministic if the oscillations can be characterised precisely (e.g. the periodic motion of a pendulum), or random if the os ...
s, or
mechanical loads. Techniques to evaluate solder fatigue behavior include
finite element analysis
Finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical models, mathematical modeling. Typical problem areas of interest include the traditional fields of structural ...
and semi-analytical
closed-form equations.
Overview
Solder is a
metal alloy
An alloy is a mixture of chemical elements of which in most cases at least one is a metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have properties ...
used to form electrical, thermal, and mechanical interconnections between the component and
printed circuit board
A printed circuit board (PCB), also called printed wiring board (PWB), is a Lamination, laminated sandwich structure of electrical conduction, conductive and Insulator (electricity), insulating layers, each with a pattern of traces, planes ...
(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 driven due to
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
cycling. Under thermal cycling, stresses are generated in the solder due to
coefficient of thermal expansion
Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions).
Substances usually contract with decreasing temp ...
(CTE) mismatches. This causes the solder joints to experience non-recoverable deformation via
creep and
plasticity
Plasticity may refer to:
Science
* Plasticity (physics), in engineering and physics, the propensity of a solid material to undergo permanent deformation under load
* Behavioral plasticity, change in an organism's behavior in response to exposur ...
that accumulates and leads to degradation and eventual
fracture
Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...
.
Historically, tin-lead solders were common alloys used in the
electronics industry
The electronics industry is the industry (economics), industry that produces electronic devices. It emerged in the 20th century and is today one of the largest global industries. Contemporary society uses a vast array of electronic devices that ar ...
. Although they are still used in select industries and applications,
lead-free solders have become significantly more popular due to
RoHS
The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Uni ...
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
Physics of failure is a technique under the practice of reliability design that leverages the knowledge and understanding of the processes and mechanisms that induce failure to predict reliability and improve product performance.
Other definition ...
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
Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymer ...
, 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
In thermodynamics, dissipation is the result of an irreversible process that affects a thermodynamic system. In a dissipative process, energy ( internal, bulk flow kinetic, or system potential) transforms from an initial form to a final form, ...
. 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
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
*Phase space, a mathematica ...
(s), grain size, and
intermetallics
An intermetallic (also called intermetallic compound, intermetallic alloy, ordered intermetallic alloy, long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Inte ...
. This affects susceptibility to
deformation mechanism
In geology and materials science, a deformation mechanism is a process occurring at a microscopic scale that is responsible for Deformation (physics), deformation: changes in a material's internal structure, shape and volume. The process involves p ...
s such as
dislocation
In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to sli ...
motion,
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
, and
grain boundary sliding. During thermal cycling, the solder's microstructure (grains/phases) will tend to coarsen as energy is dissipated from the joint. This eventually leads to crack initiation and
propagation
Propagation can refer to:
*Chain propagation in a chemical reaction mechanism
*Crack propagation, the growth of a crack during the fracture of materials
*Propaganda, non-objective information used to further an agenda
*Reproduction, and other forms ...
which can be described as accumulated fatigue damage.
The resulting bulk behavior of solder is described as
viscoplastic
Viscoplasticity is a theory in continuum mechanics that describes the rate-dependent inelastic behavior of solids. Rate-dependence in this context means that the deformation of the material depends on the rate at which loads are applied. The i ...
(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 Garofalo and Anand 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 law is employed to calculate accumulated damage.
Coffin–Manson model
The generalized Coffin–Manson model considers the elastic and plastic strain range by incorporating Basquin's equation and takes the form:
Here ''∆ε'' ⁄ 2 represents the elastic-plastic cyclic strain range, ''E'' represents elastic modulus, ''σ
m'' represents means stress, and ''N
f'' represents cycles to failure. The remaining variables, namely ''σ
f'',''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, in conjunction with the work of Wild, 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 (''N
f''). ''ε'
f'' and ''c'' are model constants where ''c'' is a function of mean temperature during thermal cycling (''T
s'') and thermal cycling frequency (''f'').
∆γ can be calculated as function of the distance from the neutral point (''L
D'') solder joint height (''h
s''), coefficient of thermal expansion (∆''α''), and change in temperature (Δ''T''). 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
Darveaux 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 ''N
0'' represents the number of cycles to crack initiation, ∆W represents inelastic work density, ''K
1'' and ''K
2'' are material model constants. In the second equation, da/dN represents the crack prorogation rate, ∆W represents inelastic work density, ''K
3'' and ''K
4'' are material model constants. In this case the crack propagation rate is approximated to be constant. ''N
f'' 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
Finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical models, mathematical modeling. Typical problem areas of interest include the traditional fields of structural ...
(FEA) simulation.
The Darveaux model has been found to be relatively accurate by several authors. 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.
Blattau model
The current solder joint fatigue model preferred by the majority of electronic
OEMs worldwide is th
Blattaumodel, which is available in the
Sherlock Automated Design Analysis Sherlock Automated Design Analysis is a software tool developed by DfR Solutions for analyzing, grading, and certifying the expected reliability of products at the circuit card assembly level. Based on the physics of failure, Sherlock predicts fail ...
software. The Blattau model is an evolution of the previous models discussed above. Blattau incorporates the use of strain energy proposed by Darveaux, while using closed-form equations based on classic mechanics to calculate the stress and strain being applied to the solder interconnect. An example of these stress/strain calculations for a simple leadless chip component is shown in the following equation:
Here α is the CTE, T is temperature, ''L
D'' 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
In materials science and solid mechanics, Poisson's ratio (symbol: ( nu)) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value ...
, and a is the edge length of the copper bond pad. The subscripts 1 refer to the component, 2 and b refer to the board, and s refer to the solder joint. The shear stress (∆τ) is then calculated by dividing this calculated force by the effective solder joint area. Strain energy is computed using the shear strain range and shear stress from the following relationship:
This approximates the
hysteresis
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
loop to be roughly equilateral in shape. Blattau uses this strain energy value in conjunction with models developed by Syed
[Syed, A., "Accumulated Creep Strain and Energy Density Based Thermal Fatigue Life Prediction Models for SnAgCu Solder Joints", ECTC 2004, pp. 737-746 - corrected.] to relate dissipated strain energy to cycles to failure.
Other fatigue models
The Norris–Landzberg model is a modified Coffin–Manson model.
Additional strain range and strain energy based models have been proposed by several others.
[
]
Vibration and cyclic mechanical fatigue
While not as prevalent as thermomechanical solder fatigue, vibration fatigue
Vibration fatigue is a mechanical engineering term describing material fatigue, caused by forced vibration of random nature. An excited structure responds according to its natural-dynamics modes, which results in a dynamic stress load in the mat ...
and cyclic mechanical fatigue are also known to cause solder failures. Vibration fatigue is typically considered to be high cycle fatigue (HCF) with damage driven by elastic deformation and sometimes plastic deformation. This can depend on the input excitation for both harmonic
In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the ''fundamental frequency'' of a periodic signal. The fundamental frequency is also called the ''1st har ...
and random vibration
In mechanical engineering
Mechanical engineering is the study of physical machines and mechanism (engineering), mechanisms that may involve force and movement. It is an engineering branch that combines engineering physics and engineering mathem ...
. Steinberg 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
In signal processing, the power spectrum S_(f) of a continuous time signal x(t) describes the distribution of power into frequency components f composing that signal. According to Fourier analysis, any physical signal can be decomposed into ...
or acceleration time history, the natural frequency of the circuit card, and the transmissibility. Blattau developed a modified Steinberg model that uses board level strains rather than displacement and has sensitivity to individual package types.
Additionally, low-temperature isothermal mechanical cycling is typically modeled with a combination of LCF and HCF strain range or strain energy models. The solder alloy, assembly geometry and materials, boundary conditions, and loading conditions will affect whether fatigue damage is dominated by elastic (HCF) or plastic (LCF) damage. At lower temperatures and faster strain rates the creep can approximated to be minimal and any inelastic damage will be dominated by plasticity. Several strain range and strain energy models have been employed in this type of a case, such as the Generalized Coffin–Manson model. In this case, much work has been done to characterize the model constants of various damage models for different alloys.
See also
* Cold solder joint
* Creep (deformation)
In materials science, creep (sometimes called cold flow) is the tendency of a solid material to undergo slow deformation while subject to persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress ...
* Fatigue (material)
In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of ...
* Plasticity (physics)
In physics and materials science, plasticity (also known as plastic deformation) is the ability of a solid material to undergo permanent Deformation (engineering), deformation, a non-reversible change of shape in response to applied forces. For ...
* Poor metal
* Potting (electronics)
In electronics, potting is the process of filling a complete electronic assembly with a solid or gelatinous compound. This is done to exclude water, moisture, or corrosive agents, to increase resistance to shocks and vibrations, or to prevent gas ...
* Vibration fatigue
Vibration fatigue is a mechanical engineering term describing material fatigue, caused by forced vibration of random nature. An excited structure responds according to its natural-dynamics modes, which results in a dynamic stress load in the mat ...
References
Further reading
*
* {{Cite web , title=Unknown Title
External links
Solder joint fatigue calculators
Soldering defects
Fracture mechanics