The underfill process for flip chip packaging presents unique challenges, particularly with designs that include large die, tight gaps, and high-density ball placement. Capillary underfill is traditionally used in these flip chip packages to ensure the highest reliability. The process of capillary underfill is quite simple. The fluid is driven into the gap between the die and substrate via capillary forces. However, this simple dispensing process meets many challenges, particularly with advanced chip designs including die size, gap, bump density, and differing die passivation materials. Specific challenges include wicking speed, fillet height and balance, and underfill adhesion to various materials. All of these challenges are related to the properties of the substrate, underfill fluid, and flip chip device.
Plasma-enhanced surface preparation prior to the underfill process has proven to increase underfill wicking speed, increase fillet height and uniformity, minimize voiding, and improve underfill adhesion. The mechanisms for these improvements include surface energy and surface chemical composition changes.
The figures below display the effect of plasma for underfill fillet height and uniformity.
Adhesion enhancement between the underfill and the polyimide die passivation is illustrated in the figure below. The adhesion strength between the underfill and the die passivation is increased by over a factor of two with plasma.
Die Attach |
Wire Bonding | Underfill | Encapsulation & Mold