Fine pitch Copper Pillar interconnection with C4 (Mass Reflow) Processing

Flip chip interconnection design and process capability has been extended with the use of copper pillar, specially for finer pitches, beyond what was possible with area array and standard C4 reflow and solder bumps. Many trends in device packaging are fueling this trend, specially the needs for thin packaging, increased function integration, challenging thermal, mechanical and physical specifications. In terms of processing, devices using Cu Pillar are most commonly assembled using thermo-compression bonding due to sizable challenges in extending the conventional mass reflow solution, typical for solder bumps. TC is specially recommended for handling very thin die in sparse or peripheral bump layouts, which are a predominant share of all devices converting to Cu pillar, where a bonding head is used to both hold the die flat and in true alignment with the substrate while supplying the thermal energy necessary to complete the interconnection. This process most often also requires the use of a non-conductive paste/film for stress absorption in recognition to the fact that underfill materials have great limitations in filling the underdie cavity. In this paper, we explore avenues to extend the current processing envelope for Cu pillar bumped devices using C4 reflow. Benefits of this approach are shown as decreased cost of ownership for materials, lower fixed costs due to using mostly depreciated equipment. Yet, challenges in defining new design rules, reformulating materials and extending process flows are among some of the barriers for further adoption of this solution in production