Quantitative evaluation of vacuum conditions for void removal in flip-chip underfill encapsulation

Abstract

Void formation during underfill encapsulation remains a critical reliability challenge in flip-chip semiconductor packaging. This study quantitatively evaluates the effectiveness of vacuum-assisted degassing in eliminating entrapped voids within the underfill layer. The void-removal process was directly visualized during vacuum treatment, and image-based quantitative analysis was employed to assess degassing performance under well-controlled vacuum conditions. Experiments were conducted at three vacuum pressures (−0.03, −0.06, and −0.09 MPa) and holding times (1, 3, and 5 min), using the void-area fraction as the key performance metric. The results indicate that vacuum pressure is the primary factor governing void removal. A size-dependent behavior was observed, where larger voids responded more effectively to higher pressure differentials. To examine viscosity-related effects, three commercial underfill materials with distinct rheological properties were tested under identical vacuum conditions. Despite differences in viscosity, all materials achieved comparable final void-area fractions after degassing, suggesting that, under sufficiently strong vacuum and adequate holding time, material viscosity exerts a minimal influence on the overall void-removal efficiency. These findings provide quantitative insight for optimizing underfill processes by prioritizing vacuum conditions, thereby enhancing packaging reliability through improved void control.