Mechanism of Adhesive Film Popcorn in Electronic Packaging

Popcorn generally happens in lead frame based package, where the interface between molding compound and die (or die paddle) is sometimes weak during pre-conditioning reflow. With the increasing use of adhesive film in substrate based package, adhesive film popcorn is frequently encountered in the film between silicon die and plastic substrate. The feature of adhesive film popcorn is a combination of two seemingly contradictory phenomena: voiding and cracking, very different from that of past lead frame package popcorn, in which the main failure mode is interface delamination. Void growth generally suggests very good plasticity while cracking is a brittle indicator. A popcorn mechanism behind the phenomena is introduced in this paper. First the adhesive popcorn failure modes are introduced. To analyze the failure, the material behavior of adhesive film and the stress source which may cause failure are qualitatively analyzed. Based on the material behavior and stress condition, the void growth and cracking criteria are presented. An innovative experiment is designed to validate the criteria and mechanical stress analysis through finite element method is found consistent to the popcorn failure modes. It finally comes to the conclusion that initial film defect size and the largest strain the film can sustain plays an important role in the adhesive film popcorn failure mechanism. The understanding here can be used in new adhesive film development and selection to avoid package popcorn and delamination.