To keep up with the demands for smaller antennas with increased performance and decreased costs, most next generation architectures mandate higher IC (integrated circuit) chip integration. Compared to conventional packaging configurations, advanced chip packaging technologies, such as 2.5D and 3D, offer greater chip compatibility and lower power consumption. Given these advantages, the adoption of advanced packaging is inevitable. Within advanced packaging, the copper pillar interconnect is a key enabling technology, and the next logical step. This technology offers several benefits, including improved electromigration resistance, improved electrical and thermal conductivity, simplified underbump metallization (UBM), and higher I/O (input/output) density. The fine pitches that copper pillars allow helps the technology to supersede solder bump technology, which reaches its lowest pitch around 40 microns. Finer pitches allow for a higher I/O count, which increases performance.

In this work, assembly of ultra-thin MMIC (monolithic microwave integrated circuit) GaN (Gallium Nitride) fine-pitch copper pillar flip chip assemblies on high density interposers was successfully demonstrated. Using 150μm pitch copper pillar flip chip, the assembly processes for both organic PCB (printed circuit board) and silicon interposers were evaluated, with both an ENIG (Electroless Nickel Immersion Gold) and eutectic tin-lead solder pad finish evaluated. For the 2D/2.5D/3D assembly process development, a standard in-house pick and place tool was used, followed by mass solder reflow, finished with an underfill for reliability test. The interconnect robustness was determined by die pull strengths, a flux stamping investigation, and cross-sections. Complete reliability and qualification test data on GaN copper pillar flip chip 2D assembly was completed, including 700 temperature cycles and UHAST (unbiased highly accelerated temperature/humidity stress test).

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