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Effect of Encapsulation Materials on Tensile Stress
Effect of Encapsulation Materials on Tensile Stress
In this paper FEA modeling was performed to observe stress-strain behavior of solder joints during thermal expansion for various potting materials.
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Authored By:
Maxim Serebreni, Dr. Nathan Blattau, Dr. Gilad Sharon, Dr. Craig Hillman - DfR Solutions

Summary
Electronic assemblies use a large variety of polymer materials with different mechanical and thermal properties to provide protection in harsh usage environment. However, variability in the mechanical properties such as the coefficient of thermal expansion and elastic modulus effects the material selection process by introducing uncertainty to the long term impacts on the reliability of the electronics.

Typically, the main reliability issue is solder joint fatigue which accounts for a large amount of failures in electronic components. Therefore, it is necessary to understand the effect of polymer encapsulations (coatings, pottings and underfills) on the solder joints when predicting reliability. It has been shown that there is a large reduction in fatigue life when tensile stresses exist in the solder due to the thermal expansion of the polymer encapsulation. The inclusion of tensile stress subjects solder joints to cyclic multiaxial stress state which is found to be more damaging than a conventional cyclic shear loading.

Isolating the tensile stress component is necessary in order to understand its influence on a reduced fatigue life of microelectronic solder joints. Therefore, a unique specimen was constructed in order to subject Pb-free solder joints to the fluctuating tensile stress conditions. This paper presents the construction and validation of a thermo-mechanical tensile fatigue specimen. The thermal cycling range was matched with potting expansion properties in order to vary the magnitude of tensile stress imposed on solder joints. Solder joint geometries were designed with a scale factor that is relevant to BGAs and QFN solder joints while maintaining a simplified stress state.

FEA modeling was performed to observe the stress-strain behavior of solder joints during thermal expansion for various potting material properties. The magnitude of axial stress in solder joints is shown to be dependent on both the coefficient of thermal expansion and modulus along with the peak temperature of thermal cycles. Results from thermal cycling of the specimen assist in correlating the magnitude of tensile stress experienced by solder joints due to the thermal expansion of potting material with various expansion properties and provides new insight into low cycle fatigue life of solder joints in electronic packages with encapsulations.

Conclusions
The cyclic fatigue life of encapsulated solder joints is reduced significantly due to specific combination of mechanical properties. The CTE and modulus of the encapsulant generates axial stresses and strain during thermal expansion. Fatigue performance of solder joints under predominantly axial loading is necessary for incorporating the contribution of axial loads to TMF life models. Therefore; a novel test coupon is constructed for testing solder joints under axial cyclic loading driven by thermal expansion of potting materials. The specimen has shown to successfully place solder joints with BGA geometries under purely axial loading during thermal cycling.

The test coupon enables simplified control of fatigue strain range with temperature by selection encapsulants with specific CTE and modulus. Displacement compatibility equation with temperature dependent modulus and CTE properties shown to provide a stress level that represents failure mode from low cycle fatigue to overstress failure. However; the analytical equation fails to provide a correlation of the fatigue life regimes due to the displacement control nature of the specimen.

Finite element analysis was performed to capture stresses and strains observed by solder joints during thermal cycles of the test coupon. Potting materials with high CTE and low modulus were found to cause higher axial strains in solder joints compared to potting materials with low CTE and high modulus. Results from the test coupon could provide insight to the contribution of axial loading to fatigue life of solder joints and combined with shear based fatigue models to better predict reliability of solder interconnects in a variety of electronic packages under axial and mixed mode stress state.

Initially Published in the IPC Proceedings

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