Study on the influence of viscoelastic degree of curing of underfill on the warpage of fan-out packaging based on the generalized Maxwell model

• Main Authors: Zhou, Junhao, Yang, Daoguo, He, Fangxian, Feng, Shenhuai, Dong, Hengzhi
• Format: Conference Proceeding
• Language: English
• Subjects: Computational modeling; Curing; Degree of curing; Fan-out packaging; Filling; Packaging; Substrates; Temperature measurement; The generalized Maxwell model; Thermodynamics; Underfill; Viscoelasticity; Warpage
• ISSN: 2836-9734
• DOI: 10.1109/ICEPT63120.2024.10667662

At present, fan-out packaging has become the mainstream chip packaging technology and is considered one of the latest and most promising technologies. However, after the filler is filled and cured into the mold, the problem of chip warpage due to stress is unavoidable. Therefore, accurately predicting the viscoelastic behavior of the underfill during the curing process can help predict warpage and reasonably improve the packaging structure to reduce the warpage effect. In this paper, the influence of the curing degree of the bottom filling layer on the warpage of the chip during the curing process of liquid bottom filling was investigated. The study began with the experimental characterization of temperature-dependent material properties of the underfill system, through dynamic mechanical analysis (DMA) and thermodynamic analysis. In addition, the heat of reaction and degree of cure during curing are measured by differential scanning calorimetry (DSC) testing. In order to save computing power, reduce the difficulty of simulation calculation, and the model has a high degree of symmetry, 1/4 of the simulation model was taken for research. The model consists of a substrate, a bottom filling, a micro-bump, an RDL layer, a chip and an EMC layer. Due to the focus on the viscoelastic mechanical behavior of the bottom filler, the RDL layer is simplified to two layers of PI polyamide sandwiched between a copper layer. The temperature load given to the entire model was 25°C to 110°C for 20 minutes, then back to room temperature. The simulation results show that significant changes in warpage occur after the underfill reaches 90% cure degree. Specifically, starting from the cooling stage, the warpage decreases substantially and reaches zero at a certain temperature point. Finally, reverse warpage forms at room temperature, changing from a "smile" to a "frown.".