Definition

Underfill is an epoxy material dispensed under BGA, CSP, and flip chip packages after soldering, then cured to form a solid bond between the component and PCB. The underfill mechanically couples the component to the board, distributing thermal expansion stress across the entire interface rather than concentrating it at solder joints. This dramatically improves reliability under thermal cycling - critical for automotive, aerospace, and portable electronics that experience repeated temperature swings. For PCB designers, underfill-compatible designs need adequate clearance around BGAs for dispensing (typically 0.5-1mm), and via-in-pad should use filled/capped vias to prevent underfill wicking into holes.