Palladium deactivation pretreatment process for electroless nickel immersion gold process (ENIG): Elimination of metal plating at non-plated through holes (NPTH)

摘要

One of the most challenging issues for printed circuit board (PCB) production is the metal plating at non-plated through holes (NPTH) during electroless nickel and immersion gold process (ENIG). It is believed that the undesired metal plating was induced by the excess palladium catalyst, which was adsorbed on the rough surface of the NPTH walls at the stage of make hole conductive process (MHC). The defect at the NPTH can lead to serious problems during product manufacturing. Redundant metal plating at the NPTH will alter the dimensional tolerance of the hole and affect the assembly of a device. If the device is unrecoverable, it will turn into very expensive scrap. Since ENIG is the final finishing step, it is crucial to eliminate metal plating at the NPTH to ensure the quality of the device before assembly. A general solution to metal plating at NPTH is pretreating the PCBs with thiourea and hydrochloric acid prior to ENIG process. However, thiourea is known to be carcinogenic and toxic to aquatic organisms. Additional time on extra process and costly waste treatment would be disfavored by many manufacturers. A palladium deactivation pretreatment process has been developed which can be integrated into the first two steps of ENIG process: acid cleaning and microetching steps. A NPTH Cleaner is developed for the deactivation of palladium residue at NPTH, replacing thiourea with non-carcinogenic and less toxic substances. Then a Modified Microetch will be applied for copper etching to ensure a bright metal finishing. The novel products are completely compatible with existing ENIG process where there is no additional operating time nor equipment installation required. The innovative pretreatment process can further expand the range of process window for ENIG, and result in higher efficiency.