Low-Temperature Sealing of Titanium for Hermetic Implant Packages

Implantable medical electronics are almost exclusively packaged in titanium shells to ensure reliability and longevity. Additional silicone rubber encapsulation improves structural biocompatibility and limits the diffusion of ion from the body to package materials. These capsules must be hermetically sealed and provide electrical feedthroughs that are insulated against each other by connecting ceramic parts. In this study, a novel RoHS-compliant low-temperature soldering process is presented, allowing the required assembly. Tungsten/titanium as adhesion material and platinum as wetting layer were deposited by physical vapor deposition as thin films and were structured by a rapid prototyping laser process. Different devices were soldered with a tin-silver alloy below 300 °C resulting in low thermal stress. The platinum acts as an excellent solder pad. A 50-nm tungsten/titanium adhesion promoter efficiently strengthens the bond to the titanium substrate. The results show that the platinum layer must be thicker than 300 nm to guarantee high bonding strength. Analyses of cross sections of the joint show occurrence of diffusion of platinum and silver during the soldering process. At soldering times above 30 s, a large intermetallic layer forms at the soldering contact, which is prone to crack formation. Likewise, intermixing of tungsten and platinum were observed but without detrimental effects. Limitations in solder times are therefore advisable, for example, by introduction of automatic processes. Soaking tests were performed with sealed capsules in water at 60 °C for one year without measurable increase of humidity inside the package. In the used accelerated aging model, this would correspond to a hermetic sealing of about five years at 37 °C.