Solder Flux Evolution for Heterogeneous Integration

Abstract

Advanced packaging techniques continue to evolve within the semiconductor industry. Within advanced packaging, heterogeneous integration has become a solution for those wishing to integrate distinct and complex dies into smaller packages. Many application trends are emerging to meet the challenges of heterogeneous integration which are causing the assembly process to become more intricate, with these trends requiring soldering material technology, especially that of solder flux, to be up to date and cutting-edge. In this paper, three assembly processes that necessitate advanced solder flux will be discussed: flip-chip assembly, BGA ball-attach assembly, and assemblies which have advanced to necessitate fluxless component attach. First, flip-chip assembly methods will be discussed and how different assembly technologies, such as standard mass reflow, thermocompression bonding (TCB), and laser assisted bonding (LAB), require differently formulated fluxes. This paper will then discuss the ball grid array assembly process, first outlining common process steps which negatively affect the solderability of ball-grid array pads, such as flip-chip reflow and molding compound applications, then discuss advancements in ball-attach fluxes which help eliminate these extra process steps, or ‘prefluxing’ steps. Finally, this paper will then discuss assembly trends which have led to fluxless component attach, and will discuss materials designed for fluxless processes, such as formic acid reflow, and how these fluxless processes tie into flip-chip assembly.