Influence of fabrication parameters on bond strength of adhesively bonded flip-chip interconnects

Abstract

The push for miniaturization in microelectronics, coupled with conversion towards Pb-free electronics has led to increasing interest in adhesive bonding of flip-chip dies directly to printed wiring boards (PWBs). However, the interconnect strength and durability have not been adequately studied, which hampers proper reliability assessment. This paper focuses on the integrity of joints made with non-conducting adhesives (NCAs). The durability of such joints comes partly from the residual compressive stresses generated by the curing-induced shrinkage of the adhesive and partly from direct metal-to-metal bonding between the mating bond pads. The focus in this paper is specifically on the nature of the second mechanism, viz. metal-to-metal bonding and its dependence on the bonding parameters. To further explore this issue, detailed experiments are conducted on specially fabricated test specimens that consist of a pair of gold-bumped flip-chip dies that are bonded to each other without any adhesive between them. An experimental matrix is designed, where the bonding pressure, bonding temperature, and bonding time are systematically varied in order to understand the metal-to-metal bonding mechanism(s). The bond strength is found to increase with increase in bonding pressure, bonding temperature, and bonding time. The results suggest that bonding occurs by a sequence of plastic flattening at the Au–Au interfacial asperities followed by a time-dependent bonding mechanism. Since the temperature is too low for classical diffusion bonding between Au, a possible explanation is that the growth of bonding strength with bonding time/temperature could be a ‘cold-welding’ phenomenon, partially due to creep-assisted growth of the area of atomistically flat contact regions. This hypothesis is supported by the fact that the experimental data agree well with: (i) theoretical diffusion models, commonly used in the literature for solid-state diffusion bonding studies; and (ii) computational creep models of time-dependent deformation and flattening at the contact asperities.

Keywords:

• Interconnect
• microelectronics
• welding
• durability
• pull
• strength
• flip-chip
• diffusion

Acknowledgments

The work reported here was sponsored by the members of the Electronic Products and Systems Consortium at the Center for Advanced Life Cycle Engineering at the University of Maryland. Test specimens were provided by Philips Research in The Netherlands.