Underfill selection methodology for fine pitch Cu/low-k FCBGA packages

A systematic underfill selection approach has been presented to characterize and identify suitable underfill encapsulants for large size flip chip ball grid array (FCBGA) packages. In the selection scheme, a total of six evaluation factors such as fracture toughness, coefficient of moisture expansion, flowability, delamination performance and filler settlement were considered. Driving stresses for package failure were also included as a factor of consideration, which clearly depends on the package size and geometry. Based on the approach adopted, underfill material that is suitable for 35×35mm2 packages with 15mm die size and 45×45mm2 packages with 21mm die size was selected. Target value for underfill properties has also been revised.