Controlling Underfill Lateral Flow to Improve Component Density in Heterogeneously Integrated Packaging Systems

This paper investigates the control of lateral underfill flow through the use of laser ablation demarcations. It is shown that such demarcations can be successfully created on various substrate surfaces, such as solder masks or polyimides, by adapting industry standard laser ablation processes used in the microelectronic packaging portfolio. In particular, both IR-based laser sources used for substrate identification marking and UV-based laser sources used for laser grooving and dicing of semiconductor devices are studied. Laser demarcations of various patterns, sizes and proximities are investigated to assess their underfill and resin bleed spread control capability. For IR laser demarcations, lower energy ablations on solder mask surfaces produced regions of material build-up that were not capable of impeding the underfill flow. Higher energy ablations produced grooves that controlled underfill spread but only grooves that penetrated the entire thickness of the solder mask could be created, thus exposing the underlying copper circuitry and introducing a reliability concern. A UV laser source produced narrow grooves at different energy levels. Investigation of various patterns subjected to underfill spread testing demonstrated that a first 30µm wide groove of about 5 µm in depth successfully pinned the underfill. Subsequent flip chip assembly experiments validated these results with UV laser grooves located as close as 0.5mm from chip edge. Cross-sections at the groove regions support the pinning phenomenon at the first edge of the groove where the apparent contact angle has increased in accordance with the geometrical Gibbs criterion. Despite effective underfill control, the demarcations were not able to impede progression of resin bleed. However, given its negligible thickness, the bleed material was readily removed by an ArH2 RIE plasma treatment. Finally, preliminary experimentation using UV laser demarcations on polyimide and Si3N4 surfaces showed similar underfill control, suggesting that this approach can be extended to 2.5D and 3D integration applications.