Development of compliant Cu pillar for flip chip package

A compliant flip chip bump design compromises of a polymer core inside a Cu pillar, a polymer encapsulated Cu pillar and the process flow to make such bumps for fine pitch flip chip package is proposed in this study. Both polymer core and polymer encapsulated Cu pillar structures are able to provide the compliance to the Cu pillar structure and reduce stress on low K dielectric layers. A structural analysis showed the polymer encapsulated bump and polymer core bump reduce the low K dielectric stress by 35% and 20%, respectively, compared to conventional Cu pillar structure. Also both compliant bump structures (polymer encapsulated and polymer core bump) increase solder joint thermal cycle life time by 55% and 30%, respectively.