US Patent 7656042 Stratified underfill in an IC package

A method includes joining an integrated circuit die having at least one low-k dielectric layer to a package substrate or printed circuit board using a plurality of solder bumps located between the die and the package substrate or printed circuit board. The low-k dielectric layer has a dielectric constant of about 3.0 or less. The solder bumps have a lead concentration of about 5% or less. A stratified underfill is formed between the die and the package substrate or printed circuit board.