Thermo-compression Bonding Assembly Technology
Authors: Chylak Bob 
Published: February 2024

With the slowing of Moore’s law heterogeneous integration and assembly became important drivers of the performance of semiconductor packaging. This “More than Moore” approach enables higher functional density and higher bandwidth packages. At the same time, heterogeneous integration drives an aggressive interconnect pitch roadmap, pushing the limits of Advanced Packaging technologies.

Flip chip with mass reflow has been the main assembly technology for fine pitch interconnect. However, the need for even finer pitch and higher packaging density was not compatible with the limitations imposed by the mass reflow process, therefore requiring new interconnect technologies. Local reflow flip chip via thermocompression bonding (TCB) overcomes limitations due to CTE-mismatch and warpage. Meanwhile, TCB based on solder micro-bumps became a well-established and cost effective fine-pitch interconnect process and Cu to Cu Hybrid Bonding is emerging as a technology for the future.

Normal TCB typically requires the application of flux and post-bond flux cleaning as any flux residue can result in post mold package reliability issues, even for ‘no-clean’ flux materials. Post-bond flux cleaning, however, becomes more and more difficult for increasing die size and with bump pitch and stand-off height shrinking even further. K&S has developed and demonstrated a flux-less TCB process for large and high-density interconnect die that utilizes in-situ Formic Acid (FA) vapor application for copper and solder oxide reduction. This process eliminates the need for flux and, therefore, there is no need for the tedious and time-consuming cleaning of the post bond flux residue. The FA process delivers nearly the same productivity as a conventional flux based TCB process. The FA delivery system can be integrated on our high-accuracy C2S and C2W TCB tools, leveraging the existing HVM TCB infrastructure and extending its application space from ~35µm towards ultra-fine pitch applications around 10µm.

Hybrid bonding created a lot of attention over the last years as this bump-less technology allows for long-term pitch scalability, possibly even down to 1µm. Hybrid bonding, however, requires a more complex process flow which relies on pristine die and target wafer surfaces as well as front-end level cleanliness conditions. Accordingly, the advantages and cost of Flux-less TCB as compared to Hybrid bonding should be explored in more detail. TCB can be viewed as an extension of flip chip. Using the existing assembly and metrology infrastructure as well as the existing die and substrate design layouts will be attractive to subcons. As a result, the overall cost and time to market should be much lower. An activity-based cost model has been developed along with SavanSys comparing flux-less TCB and Hybrid bonding. We present results on the process cost as well as the overall package cost as a function of die size and yield.

In summary, flux-less thermo-compression bonding should be considered for ultra-fine pitch applications. We have demonstrated pitch capability below 30µm and believe that a pitch of 10µm may be possible. Other applications that formic acid vapor TCB enables are large module bonding (50 X 50 mm, 70 X 70 mm) and Cu to Cu interconnects.