Electroless Plating of ENEPIG and EPAG – Comparing the Impact of Pd-Bath Type and Au-Bath Type on the Deposit Properties and Manufacturing Conditions

For high performance and high reliability applications like they are present in the HDI or IC substrate industry, the final finish Ni/Pd/Au, also known as ENEPIG (Electroless Nickel, Electroless Palladium, Immersion Gold) is well-established. It provides a high solder joint integrity and is wire bondable with different types of metal wires. For some years already the nickel free surface finishes Pd/Au or Au/Pd/Au are considered as potential alternatives when it comes to shrinking feature sizes, smaller lines and spaces or high frequency applications where nickel is considered as limitation. This paper focuses on key differences and similarities of Pure Pd and PdP-type ENEPIG layers as well as different types of gold electrolytes with immersion or autocatalytic dominated reaction mechanism. The study is split into three sections with the first focusing on the PdP vs. Pure Pd ENEPIG comparison, the second on the gold bath evaluation and the third on the comparison of Pd/Au vs. ENEPIG. In section one the comparison of different palladium plating solutions includes the study of the initial plating reaction of Pure Pd vs. PdP plating electrolytes and the performance related to background and excessive plating. It is shown, that depending on the type of electrolyte different sensitivity towards process contamination is observed which affects the deposition reaction. The palladium electrolytes are also compared in bath composition and Pd-content and the resulting effect on the cost contribution to plating process cost. Real data from mass production environment are compared with cost estimations backing up the estimated savings of a modified Pd-plating process. In the second part, the type of gold electrolyte and plating mechanism are studied to compare the impact on plating homogeneity and achievable layer thickness. The plating mechanism for immersion and autocatalytic gold electrolytes are described and solutions are presented to prevent the additional usage of potassium cyanide for stabilization. In the final section certain perspectives of the final finish Pd/Au are compared to ENEPIG looking in particular into the capabilities in the solder joint integrity and the signal loss in high frequency applications. Beside that, different aspects of the required production environment and equipment are highlighted which is needed to implement ENEPIG or Pd/Au plating capabilities at a PCB manufacturing plant. The paper gives insights into the potential solutions for process modification of the well-established final finish ENEPIG to target and fulfill the future demands in HDI and IC substrate manufacturing by widening the plating capabilities.