A Study About Facile Interconnect Formations Involving SB2-JET Solder Ball Stacking and Colonnade Patterning in Hybrid Package Architectures

Abstract

The current study aims at presenting a cost-effective, facile and novel interconnection approach that employs a unique solder ball jetting process (SB2) for stack formations. Conventional methodologies for bridging boards, chips or filling VIAs can be substituted by a solder ball stacking process. In the SB2 process, a bond-head singulates solder spheres which are then liquefied by laser energy and expelled onto a target using nitrogen pressure. Multiple solder droplets could be dispensed at the same location before subsequent translocation of the bond-head. Continued jetting outlined herein could be used to form single or multiple columns (colonnade), or even fill VIA structures. The challenge is to select a parametric window which prevents disintegration of solder stacks at the interface of boards or chips due to parasitic air inclusions or other factors [1]. The process window for three different stacks of solder spheres with varying diameters and aspect ratios (> > 5) are presented and discussed in this work. This work highlights possible combinations of solder stack configurations regarding the size and number of solder balls per stack and the resulting geometrical characteristics. The metallurgical properties of the solder interface were inspected by cross sectional analysis and X-ray. The mechanical strength and corresponding fracture modes were analyzed with a shear test unit and an optical microscope. The vertical offsets associated with column heights were examined with a 3D profilometer. An FEM simulation was performed, confirming and supplementing the experimental findings. Test vehicles pertaining to board-to-board or chip to chip connections using solder ball stacking are demonstrated. As an evaluation material, Si Chips with AlNiAu metallic pads were populated by SAC_305 (Sn 96.5%, Ag 3.0%, Cu 0.5%) stacked solder ball columns. The chip-to-chip assembly was accomplished additionally using a laser-assisted bonding process (LAB). To demonstrate the wide range of applicability and opportunities for column design, low melting-point solder spheres were placed on the corresponding chip to form a solder interface with the pre-processed columns/stacks during the flip-chip process. The ensuing colonnade hybridswere used to support conventional bonding in standard reflow ovens. Finally, future prospects of intended reliability and stability studies are elucidated as well as the opportunity to use this process technique for filling VIAs.