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28 The PCB Magazine • February 2015 Figures 4 (a, b & c) show the cross-sections of HASL PCBs after 0, 3 and 14 days, respectively. As discussed previously, a small intermetallic layer is formed between the copper and solder during HASL coating. However, this has grown considerably from 1–2 µm thickness to approxi- mately 2–3 µm thickness after 3 days. After 14 days the intermetallic has continued to grow even thicker, however, rather than growing uni- formly throughout the sample, intermetallic growth is localised to specific areas, in some cas- es growing through to the surface of the solder. The considerable and non-uniform inter- metallic growth in HASL PCBs and their result- ing solder joints is detrimental to the use of the PCBs for a number of reasons: 1) The shelf life of the PCB is reduced. In- termetallics are much more difficult to solder than either copper or solder, and intermetallic growth through to the surface of the PCB panel will create unsolderable regions. This reduces the shelf life of the PCB. 2) The solder joints are more susceptible to brittle fracture, particularly when they are used at high temperatures. Copper/tin intermetallics are more brittle than either tin or copper and as a result solder joints with thick intermetallic layers are more likely to undergo brittle fracture causing failure of the solder joint. 3) Thick, non-uniform intermetallic regions are linked to tin whisker growth. It is believed that stress within the solder join created by the intermetallic growth causes tin whisker growth. Tin whiskers can be catastrophic to the working or a PCB or electronic because of the ability for the whiskers to cause short circuits. In contrast, Figures 4 (d, e & f) show the cross-sections of a HASLEN PCB after 0, 3 and 14 days, respectively. Initially, a thin interme- tallic layer is formed during the HASLEN pro- cess. However, very little (if any) intermetallic growth is seen as the coupons have undergone accelerated ageing and all of the images appear very consistent in their structure. HASLEN: A NEW HIGH-RELIABILITy SURFACE FINISH FOR PCBS continues feature Figure 5: Intermetallic thickness of hASL and hASLEn coatings undergoing accelerated aging at 130°C.