Reliability of Au-Ti for High Temperature Termination on Ferrite LTCC Inductors

Abstract

While ferrite Low Temperature Co-fired Ceramic (LTCC) inductor and transformer developments have undergone thermal shock and high temperature aging that focused on the stability of their electrical characteristics (resistance, inductance), little attention has been paid to their termination reliability at high temperatures. Testing has been done on AgPt and AgPd terminations with Ag5Cd95 and Pb88Sn10Ag2 solders for 2000 and 25 hrs, respectively. However, Ag5Cd95 is unusable in commercial applications due to ROHS restrictions while Pb88Sn10Ag2 is undesirable because of the high lead content. Sn96 solder and wire bonding are common attachment methods that have not been vetted. Initial investigations show that high Sn solders may interfere with bonding between the AgPt and AgPd termination materials and the ferrite bulk of the part. An alternative terminal structure for using Sn96 solder is created by electroplating Au and Ni; however, electroplating to ferrite is challenging due to the masking involved. Also, the preferred materials for wire bonding are thick film, thin film or electroplated Au. To this end an alternative termination structure using Au sputter deposited onto sputter deposited Ti is being investigated. This structure was chosen for its potential to be a lower cost alternative to thick film Au and for its potential for simpler manufacturing than electroplating. Tests involved measuring bond strength and resistance after thermal ageing and thermal shock. Baseline solder joint pull tests show strength comparable to other termination methods. Some issues with solder wetting of the terminals have been noted.