Issue link: https://iconnect007.uberflip.com/i/239245
the pros and cons of pcb surface finishes continues Copper and tin, however, have a strong affinity for one another. The diffusion of one metal into the other will occur inevitably, directly impacting the shelf life of the deposit and the performance of the finish. The negative effects of tin whiskers growth are well described in industry related literature and the focus of several published papers. Advantages: • Flat surface • No Pb • Re-workable • Top choice for press fit pin insertion Figure 1: Printed circuit board with HASL/lead-free HASLs surface finish. Advantages: • Low cost • Widely available • Re-workable • Excellent shelf life Disadvantages: • Easy to cause handling damage • Process uses a carcinogen (thiourea) • Exposed tin on final assembly can corrode • Tin whiskers • Not good for multiple reflow/assembly processes • Difficult to measure thickness OSP/Entek Organic solderability preservative (OSP) or anti-tarnish preserves the copper surface from oxidation by applying a very thin protective Disadvantages: • Uneven surfaces • Not good for fine pitch • Contains lead (HASL) • Thermal shock • Solder bridging • Plugged or reduced PTHs (plated through holes) Immersion Tin According to IPC, immersion tin (ISn) is a metallic finish deposited by a chemical displacement reaction that is applied directly over the basis metal of the circuit board, that is, copper. The ISn protects the underlying copper from oxidation over its intended shelf life. 32 The PCB Magazine • January 2014 Figure 2: PCB with immersion tin surface finish.