Underfill characterization for low-k dielectric / Cu interconnect IC flip-chip package reliability

Due to low dielectric constant inter-metal-dielectric (low-k IMD) materials possessing weaker mechanical properties and higher coefficient of thermal expansion (CTE) compared with IC silicon substrates, the integrity of advanced IC low-k IMD layer/silicon substrate structures, in a packaged form, becomes of great concern. Flip-chip underfill characterization was conducted to investigate the key factors of underfill material properties that can yield good low-k IC package integrity after reliability tests. A simplified stress-coupling-index concept was proposed and used to select five underfills for evaluation, based on their different property characteristics. 27×27 mm² FCBGA packages with heatspreader were built using 8×10 mm² low-k test die, and tested by temperature cycling. The test results showed that an FC package with low Tg underfill yielded good performance for low-k package integrity.