Ti/NiCu/Ag and Al/NiCu/Ag thin film deposited on positive temperature coefficient (PTC) thermistor by direct current magnetron sputtering

Metallization of positive temperature coefficient (PTC) thermistors deposited by direct current magnetron sputtering has been investigated. Two kinds of multilayer thin films, namely Ti/NiCu/Ag and Al/NiCu/Ag, are proposed as electrode materials. Conventional NiCr/Ag thin film serves as reference electrode in the experiment. Experimental results show that the newly proposed electrodes exhibit good performance including low ohmic contact, firm adhesion, fine solderability and stability. The surface resistances at room temperature of Ti/NiCu/Ag and Al/NiCu/Ag PTC range from 4.3 to 6.2 Ω, and they change little in moisture condition. The average tensile strength is 6.3 Mpa, which provides firm adhesion between electrode and substrate. At solderability test, the new electrodes can withstand the soakage of lead-free solder at 350 °C for 5 s and the properties remain stable after high temperature treatment.