Pre-applied underfill Technique for Fine-pitch Cu Pillar 3D Die Stacking to Enable 2.5/3D Advanced Packaging

Abstract

Increased performance and smaller form factor have driven demand for next generation architectures for integrated circuit 3D (IC) chip packaging. Advanced chip packaging technologies, such as 2.5D and 3D, offer greater chip compatibility and lower power consumption. Given these advantages, the adoption of advanced packaging is inevitable. One key driver is the copper pillar interconnect technology which offers several benefits such as improved electromigration resistance, improved electrical and thermal conductivity, simplified under bump metallization (UBM), and higher input/output (I/O) density. The fine pitches that copper pillars are capable of helps the technology to supersede solder bump technology, which reaches its lowest pitch around 40 microns. Finer pitches allow for a higher I/O count, which enhances performance. One of the major innovations has been 2.5/3D integration, a technique of vertically stacking multiple dies. However, traditional encapsulation techniques become challenging because traditional capillary underfill methods become restrictive as enlarged surface area is required in order to provide underfill protection on multiple surfaces. Here, we discuss a novel approach of vertically stacking dies with an integrated underfill process that minimizes the required area for underfilling in fine-pitch IC chip packages by creating a technique to apply underfill during thermo-compression bonding. This process offers a reliable method for vertical stacking of multiple chips and eliminates post-processing need for underfill application. Additionally, the process provides precise temperature control to both the substrate and die by using a mechanism of dual heat application. This is used in combination with a controlled variable application of compressive force on the package. Through fine-tuning of the force and temperature applied to the IC components, the bonding process can be tailored to simultaneously provide the ideal underfill curing conditions while forming solder joints. In this work, we used dies with 80µm pitch Cu pillar flip chip copper pillars with tin/silver caps bonded to Electroless Nickel and Immersion Gold (ENIG) pads in conjunction with commercially available pre-applied underfill material. The interconnect strength was determined though destructive shear testing and cross-sectional analysis to evaluate for the bonding quality and strength. A scanning electron microscope was used to visually examine joint properties, and X-ray analysis was conducted to review for underfill voiding. Finally, completed assemblies were thermal-cycled to evaluate for the degradation of joint strength and voiding of underfill under induced stress.