Hermetic Sealing of Kovar Hybrid Packages by Laser Welding

Abstract

Laser welding of Kovar (54% Fe, 16% Co, 29% Ni, <1% Mn, Cr) has been evaluated in order to develop a reliable welding method for hermetic encapsulation of hybrid packages. In particular, the design of the welding joint and the impact of Au/Ni plating on the joint quality have been examined. It has been found that for the specific combination of case and lid used, the sealings are hermetic (leak rate <2 × 10−9 mbar l/s) if the following requirements are fulfilled: the contact area of the case against the lid is ground; the case and the lid are unplated or Ni electroplated (2–5 µm); when using a Nd‐YAG laser the pulse time is relatively long (furthermore the pulse power shall be low in order to avoid splatter of molten metal into the case); the welding is performed in a pure N2 gas flushed covered chamber; the joint is formed in such a manner that the molten metal ‘runs over’ the edge of the case. In addition, it was found that Au plating increases the probability of gross point leaks. When there is an increasing percentage of Au in the joint, the number and size of the leaks also tend to increase. Finally, material analysis including SEM and EDX is reported together with details of weld schedules, equipment and a comparison with the reviewed literature.