M.S. Thesis Presentation by Brett J. Fennell
Wednesday, June 9, 1999

(Dr. Daniel Baldwin, advisor)

"Modeling Realignment in Next Generation Flip Chip Assembly"


Conventional Direct Chip Attach (DCA) processing involves 4 major steps: flux application, solder reflow, underfill distribution, and underfill cure.  The latter two steps are particularly time consuming.  In so much as these steps are completed in sequential order, the entire process time is the sum of the times associated with each individual step; thereby a major drawback of this procedure is the time factor.  To address this issue, a new flip chip process has been proposed in which, solder reflow and underfill cure occur simultaneously.  In this procedure, unlike conventional DCA, the underfill is dispensed onto the substrate prior to the placement of the chip.  During reflow, while the solder becomes molten creating interconnections between the chip and substrate, the underfill begins to gel and finally cure.  This new approach not only combines the processing times for reflow and underfill cure, but also eliminates the time required for underfill dispense and flow.  The net result is substantial increases in through put.

An attractive characteristic of conventional DCA is its capacity for chip self alignment during reflow.  This alignment occurs when the solder becomes molten and, being driven by surface tension, attempts to assume a low energy geometric shape; all the while in the presence of a thin film of flux.  Such alignment will always occur as long as the solder traces on the substrate, and solder bumps on the chip, are in contact with one another during reflow.  With the new proposed process, the solder traces and bumps would be in the presence of a liquidous medium during reflow.  In such an environment, there is no guarantee that the restoration forces due to surface tension in solder, could produce chip self alignment when being opposed by a damping force provided by the liquid underfill.  The goal of this thesis is the modeling solder self alignment mechanics in a liquidous medium, so as to predict those conditions which are more conducive to alignment, as well as those which are not.