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Solder Joint Fatigue Analysis in PCBs: Causes, Prevention, and Testing Methods

Solder joints in electronic assemblies provide the critical electrical and mechanical interfaces between components and the printed circuit board (PCB). Nonetheless, due to repeated thermal and mechanical stress can cause solder joint fatigue, these interfaces can be subject to a gradual process of degradation. Solder joint fatigue is a basic failure mode due to cyclic heating and cooling. Solder joint fatigue can result in crack growth and high electrical resistance and can finally contribute to the failure of the overall device.

The article by FS PCBA provides an extensive review of the causes of solder joint fatigue, owing essentially to cyclic heating and cooling, and techniques used to prevent and characterize the reliability of solder joints.

What is Thermal Cycling?

Thermal cycling is the process of exposing materials or components to very high and low temperatures in a controlled cycle.  It is commonly used to evaluate how materials respond to thermal stress. Materials are heated and cooled within a defined temperature range.

A material, while heated, expands and contracts while cooled, relying on the thermal expansion coefficient. This process helps in finding the ability of the device and its reliability in response to changes in temperature.

What is Solder Joint Fatigue?

Solder joint fatigue is the gradual degradation of the solder connection because of repeated thermal and mechanical stress over time. It can be referred to as the cumulative damage that occurs in a soldered joint as a result of cyclic stresses, which in most cases are temperature-related.

While overload failure happens suddenly, solder joint fatigue failure occurs over time as a result of micro-cracks and deformations, which begin to form and propagate in the solder material with every cycle. The micro-cracks appear as a result of the solder joint material becoming fatigued, which may result in open circuits. The reliability of the entire electronics board is determined by the ability of the soldered joints to resist such a solder joint fatigue failure mode.

Why Does Thermal Cycling Cause Solder Joint Fatigue Failure?

Generally, solder joint fatigue is caused by thermal mismatch between components and the PCB. Several mechanisms contribute to this behaviour, as described below.

CTE Mismatch Between PCB and Components

CTE represents the rate of expansion and contraction of PCB with the change in temperature. Every PCB material has its own coefficient of expansion (CTE). For eg:

  • Copper: 18 ppm/°C
  • FR-4 PCB: 14-17 ppm/°C (in x and y-axis direction) and 55 -70 in (z-axis direction)
  • Silicon die: 3 ppm/°C

During the thermal cycling process, PCB materials expand more than the silicon chip. This happens due to their respective thermal expansion coefficients. And when the temperature decreases, the PCB shrinks again, which pulls the solder joints in the opposite direction. This repeated push and pull is what makes the solder joints fatigue.

Uneven Contraction and Bending

The temperature changes can result in an uneven contraction, creating a slight curvature on the PCB or on the individual components, applying a force of bending, in turn stressing the solder joints, especially if the joint is located at a corner of a component.

Viscoplastic Behaviour in Solder

The solders display viscoplastic behavior, where the material not only shows recoverable (elastic) and permanent (plastic and/or creep) deformation upon loading but, in each cycle, not all the deformation is recovered. The gradual accumulation of plastic deformation and creep results in the generation of micro-damage, such as grain growth and the formation of voids in the solder matrix. After the completion of thousands of cycles, the damages accumulate and form cracks, leading to fatigue of solder joints. 

How to Test Solder Joint Fatigue?

Thermal Cycling Testing

This is the primary test for reliability under thermo-mechanical fatigue.

  • Standards: Compliant to the JEDEC JESD22-A104 conditioning standard and the IPC-9701 performance testing
  • Process: Test PCBs are placed in a temperature chamber and tested with cyclic temperature variations, such as -40°C to 125°C. Dwell times need to be specified within these limits to ensure that the temperature stabilization is achieved. The procedure entails a cyclic test involving hundreds to thousands of cycles.
  • Monitoring: The resistance is monitored continuously (often daisy-chained) for intermittent opens or large increases in resistance, which would signal a crack. Cross-sectional analysis is done after the test to check for crack growth.

Vibration Fatigue Testing

It replicates operating conditions in autos, aircraft, and other industry applications.

  • Process: After the PCB is built, it is placed on a shaker table and tested with specific vibration levels (random vibration, sine sweep vibration). These levels include specific frequencies and Gs.
  • Monitoring: Evaluation involves the monitoring of resonance and electrical continuity failures induced by vibration stressing of solder connections.

How to Prevent Solder Joint Fatigues?

There are several ways to prevent solder joint fatigue, as outlined below.

Choose Highly Reliable Solder Alloys

Traditional SAC solders can be modified to make them more fatigue-resistant. Ductility and cracking can be furthered with the addition of additives such as bismuth (Bi) and indium (In) to these alloys. In high-reliability environments, low CTE solders or creep-resistant solders could be considered.

Optimize Component and PCB Layout

Component placement and pad design directly influence mechanical loading. Large, heavy, or power components should be routed away from the edges and corners, which are the most susceptible to bending. Optimize the pad shape to improve joint reliability. In large components, a staggered via pattern in the thermal pads helps to mitigate the stress associated with reflow.

Utilize Underfill/Reinforcement Materials

In the case of Ball Grid Array (BGA) and Chip Scale Package (CSP) components, capillary flow underfill epoxies can be used after soldering. These epoxies encase the solderballs, bond the device to the PCB substrate, and reroute stresses from specific solder joints. The impact of this process can lead to substantially improved fatigue life.

Control Heating and Cooling Rates

When assembling through the Reflow process and during functional operation, it is crucial to avoid thermal shock. By controlling and slowing down the rate of heating and/or cooling, the solder can have more time to relax internally, and the strain rates can be reduced. This method will prevent sudden thermal shocks.

Improve PCB Material Selection

If possible, it may be advantageous to use components with standard IC packages that have CTEs closer to those of the substrate material, thus reducing intrinsic CTE variation. Alternatively, for harsh environments or applications requiring improved performance, substrates with modified or balanced CTE (such as high Tg FR4 or Polyimide material) or MCPCBs may provide improved thermal conductivity.

Solder Joint Fatigue Analysis using FEA

Finite Element Analysis (FEA) is a powerful computational tool for improving a design proactively.

  • Process: A 3D detail model of a component, solder joints, and PCB is generated by engineers. Simulated thermal cycles or loads are applied through ANSYS or ABAQUS software.
  • Output: From the analysis, stress-strain concentration points in the solder joints are determined as well as temperature-related phenomena, such as creep. By applying fatigue life models (Coffin–Maunson model or Engelmaier model), it provides estimated cycles to failure.

Conclusion

Solder joint fatigue is an important aspect to be considered for the integrity and life of an electronic assembly subjected to cyclic thermal or mechanical stresses. Its optimization can only be achieved through a combination of proper material selection based on CTE, PCB design, the use of supporting materials like under-fill, and controlled manufacturing techniques.

Proper qualification testing in terms of cyclic temperature and vibration, in conjunction with thorough simulation analysis techniques such as FEA, is an important aspect in the validation of the design and subsequent life prediction of the product.

It is essential to ensure the reliability of your PCB assembly with respect to temperature and mechanical stress. For professional design and testing, contact us at FS PCBA. Contact us today and request a quote.

FAQs about Solder Joint Fatigue

Field failures often present as intermittent malfunctions, especially when the device is subjected to temperature changes or physical shock. Symptoms include unexplained resets, signal dropouts, or glitches that may temporarily resolve. This is because crack surfaces can make and break contact. Complete failure manifests as a permanent open circuit.

The transition to lead-free solders (like SAC alloys) generally resulted in joints that are stronger but less ductile than SnPb. This can make them more susceptible to brittle fracture under shock, but their performance under thermal fatigue cycling depends heavily on the specific alloy composition, microstructure, and service temperature range. Modern modified SAC alloys aim to address these challenges.

The required number of cycles is defined by the product’s target lifecycle and operating environment, often referenced in industry standards. For example, automotive electronics may require testing for 1000 to 3000 cycles between -40°C and +125°C, depending on the specific application’s severity level. IPC-9701 provides guidelines for tailoring test conditions.

While conformal coating protects against environmental factors like moisture and contamination, it provides negligible mechanical reinforcement against the strains caused by CTE mismatch. It is not a substitute for underfill in high-stress applications but is part of a comprehensive reliability strategy.

Components with large size or high thermal mass disparity to the PCB are most vulnerable. This includes large BGAs, QFNs, MLCCs (especially large-case sizes), and any through-hole connectors or transformers. The fatigue risk is highest at the corner solder joints farthest from the component’s neutral point.

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