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In this article, we will detail hot air solder levelled electroless nickel (HASLEN), a new PCB surface which removes the necessity to coat the nickel with gold, yielding substantial cost benefits while removing the step in which black pad can form. The end result is improved reliability to both solder wetting and the resulting solder joint, and greatly extending the shelf life of the PCB.
This surface finish has been developed by the University of Leicester in conjunction with Merlin PCB and Qualitek (Europe).
Surface protection of copper-tracked PCBs is essential for ensuring consistent solderability during the electronics manufacturing process. This is due to the continued oxidation of copper into copper oxide on the surface of the PCB which is difficult to remove and can prevent the formation of a uniform solder joint after reflow.
The majority of PCB surface finishes involve coating the copper with a metal or organic compound such as solder by hot air solder leveling (HASL), silver, tin or an organic soldering preservative (OSP).
High-reliability, high-value surface finishes often have an additional electroless nickel layer such as electroless nickel immersion gold (ENIG). Where electroless nickel coatings are used, they always have to be coated with an additional layer of an inert metal such as gold or palladium, which serves to maintain solderability for an increased period of time. This is because nickel readily forms a passive oxide layer on its surface that is very difficult to remove. As a consequence, during soldering, no solder fluxes are able to remove this oxide layer, preventing the electroless nickel underneath from coming into contact with the molten solder. This is an environment in which it is very difficult for a uniform a solder join to form. The deposition of a thin layer of gold prevents the nickel oxide from forming and the surface remains highly solderable.
Editor's Note: This article originally appeared ing the February 2015 issue of The PCB Magazine.