Effects of Viscoelastic-Anisotropic Properties of Prepreg on Diagonal Warpage of Multilayer PCB Substrates

As artificial intelligence (AI) continues to advance, printed circuit board substrates (PCBs) are becoming increasingly important as substrates for stacking various chips. Multilayer PCBs, in particular, are crucial for enhancing performance and reliability through high-density circuit design, improved electrical performance, and miniaturization. These substrates must possess excellent thermal stability to ensure that the stacked chips maintain high performance and reliability even at elevated temperatures. However, high temperatures during production or operation can cause diagonal warpage in multilayer PCBs, potentially leading to reliability issues and malfunction of stacked chips. Therefore, understanding the mechanism behind diagonal warpage in multilayer PCBs subjected to thermal loads is essential for improving package reliability. In this study, the effects of prepreg (PPG) properties on the diagonal warpage of multilayer PCBs were investigated. The viscoelastic behavior of 170 μm thick PPG was quantitatively measured through stress relaxation tests and characterized using the Williams-Landel-Ferry (WLF) model. Additionally, the flexural moduli and coefficient of thermal expansion (CTE1, CTE2) values in the 0°, 45°, 90°, and 135° directions were precisely evaluated using the three-point bending (TPB) test and 3D digital image correlation (3D-DIC) method. Finite element analysis (FEA) simulations demonstrated that the diagonal warpage of PPG is primarily attributed to its viscoelastic-anisotropic properties. By understanding the effects of these properties on warpage behavior, this study provides insights for predicting and enhancing the reliability of multilayer PCBs.