Publication: Effects of pitch size, cu pillar diameter, and height during reflow soldering using thermal fluid-structure interaction analysis
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Date
2023-07-01
Authors
Lee Jing Rou
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Abstract
The copper (Cu) pillar with solder cap interconnection has been introduced as an alternative for the solder bump interconnection in electronic packaging. This is because of their ability to tackle limitations such as the collapsing nature and larger pitch size of the solder bump. However, the thermal and mechanical performances of Cu pillar bumps are a concern for achieving reliable electronic products. Hence, this study aims to develop a one-way fluid-structure interaction (FSI) thermal coupling method to model the reflow soldering process of the Cu pillar bumps, and study their thermal and mechanical behaviours. The experimental work was used as a benchmark for developing the numerical model, then the simulated reflow temperature was compared with the experiment result, which was in good agreement. When comparing the reflow temperature profiles, the Cu pillar bumps were found to have comparable thermal performance to solder bumps, showing that Cu pillar bumps are an alternative option. Furthermore, the fluid and thermal analysis were conducted in ANSYS FLUENT, whereas the structural analysis was performed in ANSYS STATIC STRUCTURAL. The thermal loads obtained from the FLUENT were applied on the STATIC STRUCTURAL using one-way FSI thermal coupling method. The simulated flow field also showed that radiation is the dominant heat transfer mode in the oven. The heat transfer was affected by the airflow circulation and leaded to uneven temperature distribution, causing temperature deviation between each bump. Besides, an in-depth study using the simulation approach revealed that the thermal and mechanical performances of the Cu pillar bumps were dependent on several parameters, which were pitch size, Cu pillar diameter and Cu pillar height. From the parametric studies, the simulation result revealed that the overall reflow temperature, the temperature difference between the coldest and hottest bumps, deformation, and stress and strain distribution, influenced the Cu pillar bump reliability. Based on the results of these parametric studies, the suggested configuration for the Cu pillar bump was proposed, with 0.40 mm pitch size, 0.20 mm diameter and 0.09 mm height. Thus, this study provides a comprehensive guide for monitoring the temperature distributions on Cu pillar bumps and its capability to resist deformation and stress, which are crucial criteria for achieving high-quality bonding and reliable electronic products.