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22 The PCB Magazine • February 2015 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 ben- efits while removing the step in which black pad can form. The end result is improved reli- ability 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). Introduction 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 cop- per into copper oxide on the surface of the PCB, by Prof. Karl Ryder and Dr. Andrew Ballantyne, MAtErIALS CEntrE, UnIVErSIty OF LEICEStEr Dennis Price, MErLIn PCB Tom Perrett, QUALItEk (EUrOPE) HASLEN: A New High-Reliability Surface Finish for PCBs 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 com- pound such as solder by hot air solder levelling (HASL), silver, tin or an organic soldering pre- servative (OSP). High-reliability, high-value surface finish- es 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 ad- ditional layer of an inert metal such as gold or palladium, which serves to maintain solderabil- ity for an increased period of time. This is be- cause nickel readily forms a passive oxide layer on its surface that is very difficult to remove. As a consequence, during soldering, no solder flux- es are able to remove this oxide layer, prevent- ing the electroless nickel underneath from com- ing 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. feature

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