A Novel Chip-join Assembly Using Heterogeneous Sn-Ag Bumps

In this article, a flip-chip assembly using a heterogeneous solder bump structure is studied. The metallurgical and shape evolution, from bump fabrication through final assembly, is presented, characterized, and discussed. Aiming for lower residual stress during flip-chip assembly, a sequentially plated bump is fabricated to ensure a ductile low Ag content solder proximate to the fragile chip back end of line (BEOL) layers while providing a separate solder portion with a high Ag content. Such a heterogeneous structure is enabled by the integration of a Ni cap barrier within the sequential electroplating process that serves to prevent Ag diffusion between the two distinct solder portions. The heterogeneous structure is shown to remain intact through the chip join operation, thereby proposing improved BEOL integrity during joint solidification. Further, the cap barrier induces a unique, pillar-like solder structure that provides the opportunity to optimize fine pitch assembly processes without the use of a stiff Cu-pillar layer. Finally, the cap barrier is shown to successfully break down using an additional reflow after underfill reinforcement, thus homogenizing the structure into a high-Ag content solder to optimize electromigration resistance.