A study of copper thick film conductor adhesion mechanisms on ceramic substrates

In this study, copper thick film adhesion mechanisms and its failure modes on different ceramic substrates (glass ceramic, $\rm Al\sb2O\sb3$ and AlN) were investigated as a function of aging time at 150$\sp\circ$C in the presence of Pb/Sn solders. The copper thick film inks were formulated from copper powder, glass frit and CuO, an ink additive. The inks were then screen printed onto the substrates and fired at 900$\sp\circ$C in N$\sb2$ ambient containing less than 10 ppm of O$\sb2$. A statistical multiple linear regression analysis was successfully employed in the development of the glass frit and the selection of the ink additives. Adhesion strength of the fired films was measured by a modified Dupont peel test procedure. Various surfaces and interfaces of the thick film samples were analyzed using optical microscopy, electronprobe microanalysis (EPMA), and high resolution scanning electron microscopy (SEM) techniques. X-ray diffraction and thermal analyses (TMA and TGA) techniques were used to analyze the ink additives.Results indicated that the bonding between the thick film and the substrate was predominantly controlled by the relative amounts of glass frit and CuO added to the ink. With 1.00-2.00 wt% added glass frit, direct bonding of the copper thick film to the substrate occurred through the formation of a ternary (CaO-CuO-Cu$\sb2$O) eutectic liquid in the glass ceramic substrate systems, and the formation of this eutectic liquid and a spinel compound, $\rm CuAl\sb2O\sb4$ in the $\rm Al\sb2O\sb3$ substrate systems. At higher frit contents (2.50-5.00 wt%) bonding occurred through an interfacial glassy layer. Both bonding mechanisms resulted in high unaged pull strengths (6-8 Lbs/80 mil x 80 mil square). Thick films did not adhere to the substrates for glass frit concentrations $$2.50 wt%) and the bond strength was significantly reduced in relatively short aging times ($<$100 hours) at 150$\sp\circ$C with failures occurring primarily at the thick film/glass interface, due to Sn diffusion to the interface. In contrast, the aged adhesion strengths for the direct bonded copper films exceeded well over 2 Lbs/80 mil x 80 mil square, even after 1000 hours of aging at 150$\sp\circ$C. The aged failures in these samples occurred predominantly along the Cu/Sn intermetallic and solder interface. With AlN substrates no direct bonding was achieved even for low concentrations of the glass frit.U of I OnlyETDs are only available to UIUC Users without author permissio