Thermo-mechanical reliability and scalability of copper-pillar solder-cap interconnects for micro-LEDs

초록

Micro-LED display fabrication-for devices such as smartwatches and TVs-requires high-throughput masstransfer and bonding processes to assemble vast numbers of micro-LED chips from a temporary substrate, known as the interposer, onto the large-area backplane panel. Currently, a key strategy for reducing the manufacturing cost of these technologies is to significantly increase the interposer area to reduce panel-level manufacturing time, and this approach has emerged as a major issue. However, as chip sizes continue to shrink, the risk of mechanical damage during bonding rises, placing limits on the expansion of the interposer area. To overcome these limitations, this study performed thermo-compression bonding and thermal cycling simulations to examine the thermo-mechanical behavior of solder joints and to determine the maximum achievable transfer area under various bonding material conditions. The results reveal that copper pillars play a critical role in relieving stress and suppressing internal solder cracking. However, a clear trade-off exists between solder-bump height and diameter: reducing stress tends to aggravate internal cracking, whereas mitigating cracking leads to higher stress. Indium solder exhibited significantly improved reliability during both bonding and post-assembly operations compared with Sn-3.0Ag-0.5Cu, and its low-temperature bonding characteristics enabled a substantially larger maximum transferable area. The findings of this study provide actionable design guidelines for realizing largearea mass transfer and robust interconnection structures for next-generation micro-LED packaging.

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