On Thermomechanical Durability Analysis Combined With Computational Fluid Dynamics Thermal Analysis

Manufacturers of aerospace, defense, and high performance (ADHP) equipment are currently facing multiple challenges related to the reliability of electronic systems. The continuing reduction in size of electronic components combined with increasing clock frequencies and greater functionality, results in increased power density. As an effect, controlling the temperature of electronic components is central in electronic product development in order to maintain and potentially improve the reliability of the equipment. Simultaneously, the transition to lead-free electronic equipment will most probably propagate also to the ADHP industry. Compared to well-proven tin-lead solder, the knowledge about field operation reliability of lead-free solders is still limited, as well as the availability of damage evaluation models validated for field temperature conditions. Hence, the need to fill in several knowledge gaps related to reliability and reliability prediction of lead-free solder alloys is emphasized. Having perceived increasing problems experienced in the reliability of fielded equipment, the ADHP industry has suggested inclusion of physics-of-failure (PoF) in reliability prediction of electronics as one potential measure to improve the reliability of the electronic systems.This thesis aims to contribute to the development of reliable ADHP systems, with the main focus on electronic equipment for the aerospace industry. In order to accomplish this, the thesis provides design guidelines for power distribution on a double-sided printed circuit board assembly (PBA) as a measure to improve the thermal performance without increasing the weight of the system, and a novel, computationally efficient method for PoF-based evaluation of damage accumulation in solder joints in harsh, non-cyclic field operation temperature environments.Thermal fatigue failure mechanisms and state‑of‑the‑art thermal design and design tools are presented, with focus on the requirements that may arise from avionic use, such as low weight, high reliability, and ability to sustain functional during high vibration levels and high g-forces. Paper I, II, and III describes an in-depth investigation that has been performed utilizing advanced thermal modeling of power distribution on a double-sided PBA as a measure to improve the thermal performance of electronic modules.Paper IV contributes to increasing the accuracy of thermal fatigue life prediction in solder joints, by employing existing analytical models for predicting thermal fatigue life, but enhancing the prediction result by incorporating advanced thermal analysis in the procedure.Papers V and VI suggest and elaborate on a computational method that utilizes surrogate stress and strain modeling of a solder joint, to quickly evaluate the damage accumulated in a critical solder joint from non-cyclic, non-simplified field operation temperature profiles, with accuracy comparable to finite element modeling. The method has been tested on a ball grid array package with SnAgCu solder joints. This package is included in an extensive set of accelerated tests that helps to qualify certain packages and solder alloys for avionic use. The tests include -20°C to +80°C and -55°C to +125°C thermal cycling of a statistically sound population of a number of selected packages, assembled with SnAgCu, Sn100C, and SnPbAg solder alloys. Statistical analysis of the results confirms that the SnAgCu-alloy may outperform SnPbAg solder at moderate thermal loads on the solder joints.In Papers VII and VIII, the timeframe is extended to a future, in which validated life prediction models will be available, and the suggested method is expected to increase the accuracy of embedded prognostics of remaining useful thermal fatigue life of a critical solder joint.The key contribution of the thesis is the added value of the proposed computational method utilized in the design phase for electronic equipment. Due to its ability for time-efficient operation on uncompressed temperature data, the method gives contribution to the accuracy, and thereby also to the credibility, of reliability prediction of electronic packages in the design phase. This especially relates to applications where thermal fatigue is a dominant contributor to the damage of solder joints.