Effects of Epoxy Molding Compound on Electrical Resistance Degradation of Pd-Coated Cu Wire Bonds in the 175 °C to 225 °C Range

We present electrical resistance measurements of PCC wires bonded to Al pads and aged at temperatures of 175 °C, 200 °C, and 225 °C. Wires were arranged to enable four-wire measurements of pairs of wires in series, and monitored throughout the aging process. The wires used were 25 μm diameter PCC and bare Cu. Bonded ball diameters were 61 μm and 75 μm, and the mean ball shear strength was between 120 MPa and 130 MPa for all of the bond groups. We tested samples without encapsulation or with one of two different encapsulants and Al pad thicknesses of 800 nm and 3000 nm in various combinations. We also tested the effect of plasma cleaning of substrates before bonding compared to the combination of solvent and plasma cleaning of substrates and test ovens. Lifetimes were longer for unencapsulated bonds. At 225 °C, for bonds on 3000 nm thick pads, the median time until 10 % resistance increase ranged from 253 h with the worse encapsulant and limited cleaning to 986 h with no encapsulant and the full cleaning procedure. The activation energy for failures on 3000 nm thick pads, calculated from the time to reach 10 % resistance change for each temperature, was 0.37 ± 0.05 eV. We present cross-sections of bonds after aging for failure analysis.