The beneficial effect of underfilling on the reliability of flip-chip joints

Recently, several low cost alternatives for flip-chip interconnection technology have been investigated. The scope of this paper is a reliability study of flip-chip assemblies on various substrates (mainly FR4). The deterioration (characterised by the electrical resistance) of isotropically conductive adhesive and of solder (63Sn37Pb, 96.5Sn3.5Ag) bumps caused by both thermal cycling and thermal shocks are described. These investigations confirm that in low cost flip-chip technology, there is practically no reliable bump interconnection without using underfill material.