Fluxless Bonding Process Using Thermo-Compression Micro-Scrub for 61 µm Pitch SnAg Solder 3-D Interconnections

In this research study, we evaluated the use of a micro-scrub bonding process as a potential method to eliminate the use of flux, and its associated cleaning, during flip chip thermo-compression bonding. Additionally, this process requires no plasma pre-treatment to eliminate tin oxide and contaminants from the micro-bump surface. This evaluation used a test vehicle with 20-μm/15-μm thick Cu/SnAg micro-bumps and Ni/Au pads at 61 μm pitch. The top die has more than 170,000 micro-bumps and both the top die and bottom die were larger than 600 mm2. The top die and bottom die were bonded with parameters developed for a micro-scrub bonding process. After each die makes contact with precise alignment, micro-scrub is applied under a specific force, temperature, and atmosphere during the bonding process. The mechanical micro-scrub action breaks up the native tin oxide film and improves the wettability of the solder. X-ray images confirmed there was no solder bump bridging across the full die area. Neither voids nor cracks were observed at the solder joints by SEM cross sectional analysis. CSAM results indicated there were no voids in the underfill.