Laser assisted transfer of solder material from a solid-state solder layer for mask-less formation of micro solder depots on Cu-pillars and ENIG pad structures

The current work introduces a process for the transfer of solder-material from a thin solder layer on a carrier substrate to contact pads of semiconductor devices. The process principle is based on laser assisted bonding (LAB) and de-bonding (LAdB) methods described by Fettke et al [1] where the solder material of a semiconductor device is heated by a laser to form or to separate an interconnection. As a precursor, a glass-carrier was plated with a TiW seed layer and a SnAg solder film. Then, a chip was positioned on the plated solder film and exposed with a short IR laser pulse. This chip was later used for the assembly. Through a dynamic vertical axis stroke, a specific amount of solder material was transferred from the glass carrier onto the pads of the chip. The current work illustrates this process principle along with the thermal and temporal characteristics of the transfer. Furthermore, this study correlates solder thickness on the carrier substrate, laser energy and resulting solder depot volume. Moreover, the process capability of the solder transfer step was investigated and characterized with respect to different I/O configurations. The contact interface of the Si test vehicles had one of the two finishes: ENIG (electroless nickel immersion gold) or Cu-pillar. The SnAg layer was used as a solder material for the transfer trials. The ensuing morphology and metallurgy of the solder depots were analyzed using an optical microscope and Scanning Electron Microscopy (SEM). The intermetallic compound (IMC) and the grain structures were characterized by Focused Ion Beam (FIB) -SEM and Energy Dispersive X-ray Spectroscopy (EDX). The mechanical strength of the formed solder caps was measured using a shear tester and the topography was analyzed using a surface profilometer. The nature of the transferred solder material in terms of forming a stable and reliable interconnection was investigated by bonding a series of flip-chip test vehicles in a chip-on-chip configuration. The solder joint quality was characterized by cross-sectional grinding and polishing, SEM and microscopic inspection. Finally, potential applications and future prospects of intended reliability and stability will be outlined.