Electroless over pad metallization for high temperature interconnections

Electrolessly plated over pad metallization (OPM) was evaluated for high temperature gold wire bonding applications. Bonding strength, measured by wire bond bump shear test, of 4N gold wire on electroless OPMs, such as electroless nickel/immersion gold (ENIG), electroless nickel/electroless palladium (ENEP), and electroless nickel/electroless palladium/immersion gold (ENEPIG), was carefully evaluated before and after wire bond was subjected to various environmental stresses. Failure modes, besides bonding strength, were particularly examined for overall bonding quality assessment and OPM screening. Authors´ intensive work found that ENEPIG OPM had the highest bonding strength and consistent failure mode over the other two OPM options: ENIG and ENEP. Even after 3000 hours of 175°C high temperature storage life test, there is no sign of bonding strength degradation. Processes of ENIG, ENEP and ENEP1G OPMs were compared, in conjunction with pros and cons of each OPM choice. Plating imperfections, found in almost every ENEP1G OPM cross section, are also discussed in details.