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26 The PCB Magazine • February 2015 HASLEN: A NEW HIGH-RELIABILITy SURFACE FINISH FOR PCBS continues feature Figure 4(d) shows a cross-section of a HASLEN coated PCB where a thin electroless nickel layer can be seen above the copper, visible as a dark grey band. Above this is the solder, visible as the light grey metal. As with all conventional HASL PCBs there in an intermetallic layer formed but in this case the intermetallic layer is formed be- tween the solder and electroless nickel rather than the copper. This has important consider- ations for PCB longevity and use as the rate of intermetallic growth for and electroless nickel solder joint is much lower than that of a copper solder joint. Improved Lifetime and Reliability vs. HASL The differing nature of the intermetallic growth between HASL and HASLEN solder joints is highlighted through accelerated aging studies as shown in Figure 4. Conventional HASL and HASLEN PCBs were placed in an oven at 130°C for varying periods of time, cross-sectioned and imaged under a microscope. Figure 3: (a) Cross-sectional diagram of a hASLEn coating and (b) image of the resulting hASLEn surface finish. Figure 4: Cross-sectional images of hASL (a, b and c) and hASLEn (d, e and f) that have been in an oven at 130°C for 0, 3 or 14 days.