40μm Ag-AgIn flip-chip interconnect process at 180°C

40μm silver-silver indium (Ag-AgIn) flip-chip interconnect process at 180°C by transient liquid phase (TLP) bonding process is reported. Array of 50×50 flip-chip joints with 100μm pitch and 40μm joint diameter was fabricated on silicon (Si) and bonded to copper (Cu) substrate at 180°C for 5 minutes with a static pressure of 0.4-0.7MPa (60-100psi). The corresponding load for each joint is 0.05gm. In the bonding, no flux was used. Cross section SEM images show that Ag columns are well bonded to the Cu substrate without visible voids or cracks. EDX data indicate that the resulting column structure is Ag/(Ag)/Ag₂In/(Ag). Each joint has height approximately 50μm. In this structure, Ag₂In is a dominating intermetallic compound (IMC) in the Ag-In system with melting temperature of 660°C. (Ag) is a solid solution phase of Ag with In composition up to 20 at. %. It has a solidus temperature range of 695 to 962°C depending on In composition. In long-term operation, (Ag)/Ag₂In/(Ag) is expected to gradually convert to a single (Ag) phase which is more reliable. Thus, the flip-chip joints will get better in use. The process temperature of this new interconnect method is 80°C below typical reflow temperature of tin-based lead-free solders. The free shear strain of this 50μm Ag-AgIn flipchip interconnect between Si and Cu is around 0.14. This strain value is relatively small for ductile material such as Ag.