A new family of flip chip underfills (JM8800 series) have been developed based on Moisture Resistance Cyanate Ester (MRCE) chemistry. These 100% convertible underfills demonstrate superior moisture resistance, excellent adhesion, and robust workability over those based on epoxy/anhydride chemistry. For organic substrates, JM8805 is capable of underfilling down to 1 mil gap from 60/spl deg/C to about 100/spl deg/C with exceptionally low void; and cures in 10 minutes at 165/spl deg/C, or 30 minutes at 150/spl deg/C. For ceramic substrates, JM8801 can underfill 3 mil gaps from 60/spl deg/C to about 120/spl deg/C, and be cured in 15 minutes at 210/spl deg/C, or 90 minutes at 165/spl deg/C. Unlike epoxy/anhydride underfills which are moisture sensitive even at ambient conditions and can be hydrolyzed rather easily leading to low molecular weight, low adhesion, low glass transition temperature (Tg), and high coefficient of thermal expansion (CTE) product, MRCE underfills are stable even after stage time (before curing) of up to 24 hours at 30/spl deg/C/60% relative humidity with little effect. These MRCE underfills have demonstrated to pass JEDEC level 1 reliability on ceramic substrate, and passed JEDEC level 2 reliability on organic substrate. Furthermore, the low dielectric constant (3.4) and high volume resistivity (E16 ohm-cm) of MRCE underfills make them uniquely suitable for high frequency devices.