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30 The PCB Magazine • February 2015 unsolderable using current solder fluxes. When this happens, there is often still full coverage of gold across the surface, making it extremely dif- ficult to tell non-destructively whether there is a problem with the coating. In addition to the surface of the PCB ap- pearing to have a good finish, once the PCB has gone through an assembly process, the solder joint appears to have formed correctly but is in fact very weak and is thus likely to fail quickly when used as a product. HASLEN, while possessing the benefits of having an electroless nickel barrier to the un- derlying copper, does not require the corrosive immersion gold layer and as such the process responsible for black pad has been removed. As a consequence, PCBs with the HASLEN surface should be more reliable than their ENIG coated counterparts. Conclusions We have detailed the new surface finish HASLEN. This is a process that combines elec- troless nickel coating of PCBs with hot air sol- der levelling, very common processes used by most PCB manufacturers in the world. Solder levelling of electroless nickel is very difficult as very few solder fluxes are capable of removing the oxide layer from electroless nickel. This has been enabled by solder fluxes based on the nov- el liquids' deep eutectic solvents. HASLEN provides a surface finish which has a higher shelf life than conventional HASL, should produce more reliable electronics post assembly, particularly when used in high tem- perature environments, and additionally, re- duce tin whisker formation. When compared to ENIG it provides many of the same func- tionalities while not requiring the costly im- mersion gold process. The gold deposition process is linked to black pad, the source of a major reliability problem with ENIG PCBs. HASLEN offers an alternative coating method to provide a solder join with the same proper- ties as that of ENIG but with significantly less reliability problems. Acknowledgements This coating technology has been developed in conjunction with Merlin PCB and Qualitek (Europe) and special thanks go to both Dennis Price and Tom Perrett for their ongoing help and guidance with its development. Addition- ally, thanks go to IeMRC for funding. For more information on this new technolo- gy, please visit www.haslen.org. To obtain a sam- ple HASLEN coated PCB, please contact Andrew Ballantyne, Dennis Price, or Tom Perret. PCB References 1. Abbott, A. P., Capper, G., Davies, D. L., McKenzie, K. J.. Obi, S. U., "Solubility of metal oxides in deep eutectic solvents based on cho- line chloride," Journal of Chemical and Engineer- ing Data 2006, 51, (4), 1280–1282. 2. Smith, E. L., Abbott, A. P., Ryder, K. S., "Deep Eutectic Solvents (DESs) and Their Ap- plications," Chemical Reviews 2014, 114, (21), 11060–11082. 3. Ballantyne, A. D., Zaleski, C. J., Harris, R. C. H., Price, D., Ryder, K. S., "Sustainable Solder Flux from Novel Ionic Liquid Systems: Greener, Cleaner, Cheaper," The Journal of the Institute of Circuit Technology 2014, 7, (2), 6–10. karl ryder is professor of physical chemistry at University of Leicester. dr. Andrew Ballantyne is research associate at University of Leicester. dennis Price is business development director at Merlin Circuit technology Ltd. tom Perrett is sales manager at Qualitek Europe. HASLEN: A NEW HIGH-RELIABILITy SURFACE FINISH FOR PCBS continues feature