TABLE OF CONTENTS Electroless Ni/Pd with or without an overcoat of immersion Au is a cheaper alternative to ENIG (Ni/Au). A thin layer of Pd on copper is sufficient for delivering a good solderable finish. However, in cases where Au wire bonding is required, an undercoat of Ni will be required for best performance. For soldering and Al wire-bonding applications, the thickness of an EN underlayer is typically 100 to 200 in (2.5 to 5.0 m). On top of that is the electroless Pd layer, with a thickness of 5 to 10 in (0.125 to 0.25 m), and more commonly 6 to 8 in (0.15 to 0.2 m). The overcoat immersion Au flash typically is less than 1 in (0.025 m). If the Pd layer is increased to 15.0 to 20.0 in (0.375 to 0.50 m), the surface finish is then solderable and gold-wire-bondable.
Fabrication Process. The EN process is the same as that described in Sec. 14.4.2. After the EN process, the surface to be Pd plated is catalyzed with an immersion deposit of Pd before electroless Pd plating can proceed. This catalyzation restricts the deposit to the metallic substrate without extraneous plating on the mask or laminate.
Electroless Pd is an autocatalytic process that involves using a chemical reducing agent to plate palladium metal out of solution. The reducing agent varies and may incorporate other elements in the deposit, such as phosphorus if sodium hypophosphite is the reducing agent. The thickness of the autocatalytic deposit...
