TABLE OF CONTENTS Palladium (Pd) is an attractive, cost-effective alternative finish for printed wiring boards (PWBs) that is both solderable and wire bondable. Similar to gold, Pd is a noble metal; therefore, it is stable against many chemical reactions, such as oxidation. In addition, Pd exhibits several advantages over Au as a potential surface finish constituent:
Pd is cheaper than Au.
Pd exhibits a density 38 percent lower than Au (12.02 versus 19.32 g/cm 3), thus further reduces the cost of Pd needed for surface finish applications.
Pd displays a tensile strength about 35 percent higher than Au.
Pd exhibits a hardness at 250 to 290 Vickers, about twice that of copper and three times that of gold, thus making it more suitable for contact purposes.
Pd dissolves in molten 60Sn-40Pb at a much lower rate than Au (about 0.01 versus 5 m/s), thus it is less prone to contaminate the solder pot.66 ,67
Electrolytic Pd or electrolytic Pd with Au flash provides a thin deposit on top of copper. The Pd is less than 0.5 m, and is typically 0.25 m in thickness. The Au flash is 0.025 m in thickness. The fabrication process is described in the following subsection.
Fabrication Process. The electrolytic Pd plating process is integrated with the PCB patterning process as follows:
Cu plate
Pd plate
Resist strip
Cu etch
Solder mask application
During the plating cycle, Pd is applied immediately after acid copper...
