High throughput thermo-compression bonding with pre-applied underfill for 3D memory applications

As demand for smaller and faster electronic products increases rapidly, 3D packaging with higher electrical performance and density are desirable [1]. Flip chip interconnection technologies using copper pillars have been widely used in many microelectronic applications for high performance systems as well as consumer electronics in recent years [2]. The technology offers improved electrical and thermal performance due to shorter electrical paths between die and substrate. Memory applications remain as a key growth segment for 3D packaging due to the drive for reduced package height and increased numbers of stacked die. Traditionally, capillary underfill is applied after the flip chip interconnections are formed. The underfill resin flows into the gap between the die and substrate by a capillary force [3]. However, with shrinking interconnection gaps and the limitations in underfill flow and also inhomogeneity in the resin system, capillary underfill proves to be difficult as it is a slow process and can result in voids between copper pillars in fine-pitch packages [4]. In order to solve these problems, thermo-compression bonding (TCB) processes using preapplied non-conductive film (NCF) were developed and are commonly used in fine pitch devices [5-7]. Due to the high entry equipment investment capital required, additional breakthroughs for a high throughput TCB process is required to enable widespread industrial adoption. Currently, 3D memory stacking using a TCB process is performed layer by layer involving several temperature ramping steps in order to activate, flow, and cure the NCF before reaching the solder melting temperature and completing the solder reflow. Moreover, since NCF is sensitive to high temperatures, there is a cooling step at the end of the bonding process in every layer to cool down the bond head before picking up the next die. Collective bonding, together with K&S APAMA® flip chip bonder patent pending Contactless Transfer feature, is presented in this paper to demonstrate a high throughput TC-NCF process. Collective Bonding is done by tacking all the layers in a memory stack at a lower temperature. After that, a TCB process with higher temperature ramping is performed only at the last layer. For conventional TC-NCF bonding, sub 100°C temperatures during transfer of the die from picker to bond head has to be used, which results in lower throughput. A contactless transfer process allows higher transfer and alignment temperatures to be used, enabling higher UPH as no additional time is required for cooling down of the bond head. In addition, a higher transfer and alignment temperature was shown to reduce NCF voiding issues by removing the volatiles and moisture in the NCF material prior to die placement. Collective Bonding process factors affecting the bondability of interconnect, such as solder wetting and solder thickness control were studied. NCF underfill performance such as voids, fillet control, and film outgassing were also investigated. Lastly, an empirical UPH model will be presented to illustrate potential throughput improvement.