Authored By:
Raed Al Athamneh, Mohammed Abueed, Dania Bani Hani, Sa’d Hamasha
Department of Industrial and Systems Engineering
Auburn University, 3301 Shelby Center, Auburn, AL 36849
Summary
Lead-free solder joints are used to provide electrical and mechanical connections between the printed circuit board and electronic components. The robustness of every single solder joint is vital for the reliability because any failure in a solder joint may ruin the overall function of an electronic device. Mechanical and thermal cyclic stresses are the most common factors that lead to failures in solder joints. Aging is another factor that changes the mechanical properties. The most critical applications of electronic assemblies are found in harsh environments where solder joints are under cyclic stress at elevated temperatures for a long time.
This study aims to assess the reliability of the most common solder material under different cyclic stress levels and aging times at an elevated temperature. Individual SAC305 (96.5 % tin, 3% silver, and 0.5% copper) solder joints are cycled in an accelerated shear fatigue experiment using Instron 5948 Micromechanical Tester. The stress amplitude levels are 16 MPa, 20 MPa, and 24 MPa and the aging times are 0, 2 hrs, 10 hrs 100 hrs and 1000 hrs where the aging temperature is held constant at 100ᵒC. The number of the experimental combinations tested is fifteen. Seven solder joints for each combination were used. Two-parameter Weibull distribution was developed for each combination to assess the reliability.
The plastic strain and the inelastic work per cycle were calculated from the hysteresis loops. The results showed that increasing the stress amplitude leads to less reliability and larger inelastic work per cycle and plastic strain. The results also showed that increasing the aging time leads to less reliability and larger inelastic work per cycle and plastic strain. Power equations were used to fit the correlations between the characteristic fatigue life, stress amplitude, aging time, inelastic work per cycle, and plastic strain. A general model was developed as well to predict the reliability based on the stress amplitude level and aging time.
Conclusions
This paper studied the reliability of lead-free solder joints SAC305 with OSP surface finish under different aging conditions and cyclic shear stress amplitudes. Isothermal accelerated fatigue shear test was utilized to demonstrate the reliability of the solder joint. The aging temperature was 100ºC and four different aging times were implemented (2 hrs, 10 hrs, 100 hrs, and 1000 hrs) under three different cyclic shear stress amplitudes (16 MPa, 20 MPa, and 24 MPa). Two-parameter Weibull distribution was utilized to assess the reliability of the solder joint at each service condition.
The results show a reduction in the fatigue life when the cyclic shear stress amplitude has higher value and an increase in the fatigue life when the aging time has a lower value. The relationship between the stress amplitude and the characteristic life was identified. The fitted power equations that relate them were determined for each aging time and the fitted power equations that relate aging time and characteristic life were built for each stress amplitude. Prediction reliability models to predict the characteristic life were constructed as a function of the stress amplitude and aging time. The model is obtained by calculating the average of the shape parameters and using the relationships between the fatigue life, stress amplitude and aging times.
The earlier failures that are associated with SAC305 solder joint at different stress amplitudes sand aging times were studied as well. B10 was used to represent the earlier failure. The relationships between the B10 and stress amplitude at different aging times and its fitted prediction equations were formulated. In order to predict the inelastic work per cycle and the plastic strain, the steady state region was defined. To determine the inelastic work and plastic strain, the hysteresis loops at different aging times and stress amplitudes were demonstrated. The relationships between the aging time, the inelastic work per cycle and plastic strain were obtained, and the power equation was utilized to form a set of fitted prediction equations. As a result from this analysis, a model form from a set of fitted equations was originated as a function of aging time and stress amplitude to predict the fatigue life, inelastic work, and plastic strain.
Initially Published in the SMTA Proceedings
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