Characterization of Coated Silver Wire Bond Interface Using TEM

Abstract

Coated silver (Ag) wire is designed and developed to replace gold (Au) wire of soft nature. Ion-milling the reliability tested coated-Ag bonded device revealed intact Ag ball bonds to Al-0.5wt%Cu pad exhibiting good compatibility with green epoxy mold compound. The paper primarily reports the characterization of bond interfaces of the reliability tested coated-Ag ball bonds. Where, thin samples parted from the bond interface using focus-ion beam (FIB) and analyzed using selected area electron diffraction (SAED) phase analysis, attached to transmission electron microscope (TEM). During thermal cycling ((TC) -40 to 125°C, 2000cycles)) and biased highly accelerated stress test (bHAST - 130°C, 85%RH, +3.3V, 192h) bond interfaces revealed predominant Ag2Al phase formation along with growth of AlAu4 phase. Moreover, thermal ageing (high temperature storage ((HTS), 175°C, 3000h)) revealed predominant Ag3Al phase growth along with AuAl/Au2Al phases and pure Au/Ag regions. Relatively, Ag diffuses rapidly compared to Au and Al at the interface, thus consuming entire Al pad. Aging at high temperature for longer periods transform Ag3Al to Au2Al phases, perhaps stable than other gold/silver-aluminide phases. This indicates that the phases formed at the bond interface Ag3Al, Ag2Al, AuAl, Au4Al, Au2Al are harmless and do not contribute to the reliability failures.