Bonding of Non-Planarized Fine-Pitch (10 μm) Cu Pillar Through Ag Cap

Fine-pitch copper (Cu) pillar interconnects have been widely adopted in flip-chip packaging to enable high-density integration. However, achieving reliable bonding with non-planarized Cu pillars is challenging which typically requires harsh bonding conditions to ensure adequate bonding quality. In this study, we successfully achieved non-planarized Cu pillar bonding under mild bonding conditions by utilizing a silver (Ag) cap as the bonding intermediate layer. Non-planarized Cu pillar chips with a pitch size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~\mu $ </tex-math></inline-formula>m were bonded using the Ag cap at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$200~^{\circ }$ </tex-math></inline-formula>C for 3 minutes under a low applied pressure of 5 MPa, and the shear strength can reach as high as 144.1 MPa. The bonding structures were examined using SEM and TEM, revealing a flawless bonding interface achieved by Ag interdiffusion. Furthermore, we demonstrated the applicability of the Ag cap in the packaging of a commercialized fine-pitch Cu pillar test element group (TEG) chip. The electrical resistance of the Ag-capped TEG sample was comparable to that of a solder-based structure, and remained stable after 1000 cycles of severe thermal cycling and 168 hours of highly accelerated stress test.