Board Level Underfill – Moisture Related Voids

The importance of board-level underfill has increased significantly in modern electronics. One of the primary issues with underfill is the formation of voids, with moisture-related ones being one of the top three causes. This presents a significant challenge in surface mount technology production, significantly affecting the quality and reliability of the assembly. This work investigates the impact of several factors on void formation, including printed circuit board (PCB) exposure to moisture, drilled holes in the PCB, and using an underfilled syringe nearing the end of its floor life. The findings identify the most common diameter of moisture-related voids in board-level underfill and highlight the influence of individual parameters on their formation. Studies have revealed that the formation of voids is influenced not only by moisture exposure in materials but also by the design of the board and the drilled technological non-plated through hole (NPTH) vias within it. Similarly, using an underfill syringe at the third quarter of its guaranteed floor life leads to an increase in void formation. Using scanning acoustic microscopy (SAM) in this work, 38% more voids were detected than with cross-section optical evaluation.