On the Influence of Nickel Deposition Techniques on Solder Joint Phase Formation

In this paper, we present the differences in intermetallic phase formation between electroplated and physical vapor deposited (PVD) nickel coating layers with soft solder. We investigate the adhesion layer between a power electronic module baseplate and its system solder. The baseplates of these modules consists of a metal matrix composite material, which needs to be coated to become solderable. Today, it is state of the art technology to coat the baseplate with electroplated nickel to form an adhesion layer to the system solder. During the publicly funded project ReffiMaL (resource efficient material solutions for power electronics), electroplated nickel was substituted with PVD nickel. The main advantage of PVD nickel is a significant reduction of layer thickness compared to the electroplating process. Second advantage of PVD nickel is the limitation of the deposition to areas that are soldered, in contrast to a non-selective electroplated coating. This yields both a reduction of material use and a natural formation of a solder mask due to the selective deposition of solder-wettable nickel. When deposited by PVD at room temperature, nickel exhibits columnar growth patterns, whereas electroplated nickel tends to form a laminar layer. The columnar growth leads to an increase in interface area promoting phase formation behavior. To compare both adhesion layers, we investigate the phase formation after soldering. Instead of a standard soft solder preform, we use a tin-based soft-solder copper-composite material. We investigate a wide range of samples of varying solder time and temperature. We were able to confirm the assumption of enhanced interdiffusion behavior with PVD nickel by faster phase growth and a higher concentration of nickel in the resulting intermetallic phases. There is also a difference in the phase formation dynamics at high temperatures on the physical vapor deposited nickel