18.0 INTRODUCTION

IBM first introduced flip chip (FC) technology in 1964 in the solid logic technology (SLT) hybrid modules in the System 360 mainframe. ¹ The technology was developed by IBM to replace wire bonding as a means of interconnecting semiconductor dies to thick-film metallization on alumina. The dies were three-terminal transistors with Au/Ni-plated Cu balls embedded in a Pb/Sn solder bump on the three input-output (I/O) pads of the transistor. A Cr/Cu/Au interface layer was deposited between the Al transistor bond pads and the solder bump. The device was assembled to the hybrid substrate by inverting (flipping) the dies and reflow soldering the copper balls to corresponding metal pads on the substrate. The copper balls maintained a constant standoff between the dies and the substrate after reflow.

As the I/O count of the dies increased, the copper spheres were replaced with high-lead tin-lead alloy solder bumps. ² The solder balls collapse somewhat during the reflow soldering process, balancing the weight of the chip and the surface tension forces of the molten solder. This phenomenon gave rise to the IBM terminology controlled collapse chip connection (C4). The flow of the solder (collapse of the chip-substrate spacing) was controlled during reflow by controlling the solder volume and the wettable metal exposed on the dies and the substrate.

The advantages of the FC assembly process for high-volume manufacturing were recognized by the automotive industry. FC-on-ceramic (FCOC) was adopted by the automotive industry in the...