Atmospheric corrosion of Ag/Cu plated FeNi based leadframe used in MOSFET package

Environmental conditions, such as humidity, temperature, corrosive gas, play a significant effect on the reliability and lifetime of electronic packages. Presence of small amount of hydrogen sulphide (H2S), even at ppb level, could bring severe problems in electronics. The most susceptible materials to H2S should be silver (Ag) and copper (Cu), as H2S will corrode these metals and form sulfides, especially with the presence of humidity. In this paper, a type of molded MOSFET package (Metal Oxide Semiconductor Field Effect Transistor) encountered electricity opening in the field after about one year's service. Failure analysis located the failure area was the leadframe, and subsequent cross-section showed the leadframe was severely corroded from the package edge to inside. This is an Ag and Cu plated FeNi based leadframe (5 mu m Ag/5 mu m Cu/NiFe based). Physical failure analysis and surface analysis were employed to characterize the leadframe failure mechanism. The corrosion mechanism of Ag/Cu plated FeNi based leadframe was studied in details. It was found that the Ag plating layer which close to the device edge was almost corroded, whilst the Cu layer underneath it was slightly corroded. The NiFe base was intact. However, it is interesting to found that there was no silver sulphide (AgSx) corrosion compound detected in the Ag layer. On the other hand, there was copper sulphide (CuSx) detected. It is suggested that this was a sulphur originated corrosion of leadframe. The MOSFET molded device served close to the coal-generation power plants, where the environmental gas contained high level sulphur (like H2S). The gas penetrated into the molded device through the leadframe and molded compound interface. S attacked the leadframe top layers Ag and Cu. Because Ag ions were inclined to migrate, the Ag ions migrated via the lead surface to the external leads, and accumulated at the lead at the package edge. On the other hand, Cu ions transported away from its original sites and eventually deposited on the cathodic areas as Cu salts, i.e. CuSx which was detected by EDX and AES.

Automated summary

This paper emphasizes the significant impact of environmental conditions on the reliability and lifetime of electronic packages, especially in the presence of corrosive gases like hydrogen sulphide (H2S). The study reveals the corrosion mechanism of Ag/Cu plated FeNi based leadframes in high sulphur environments and the migration of Ag and Cu ions leading to failure.