Influence of metallic diffusion on the adhesion of screen printed silver films

It is known that the peel strength of a soldered silver thick film will decrease with time of exposure to elevated temperature. The cause of this phenomenon was investigated. Cross sections of thick silver films on dielectric bodies and of the silver film dielectric interface were examined using the electron microprobe. The silver film is bonded to a glass layer, which in turn is bonded to the dielectric body. The bond failure before thermal aging most often occurs at the glass-dielectric interface. After thermal aging, the failure occurs at the silver-glass interface and the magnitude of the peel strength is reduced compared to the former value. In the latter case, the tin from the tin-lead solder was found to have replaced the silver at the silver-glass interface. No lead was found at the interface. The sources of metallic diffusion are considered to be lattice diffusion of the silver into the tin component of the solder, and surface diffusion of the tin over the silver.