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62 SMT Magazine • June 2015 ponents for whisker growth. The numerous past failures caused by tin whiskers were related to satellites, heart pacemakers and the shutdown of nuclear power stations. The Alter Technol- ogy test samples included surface mount devic- es having two tin-plated terminations (chips). These results will be published during 2015. However there is some controversy concerning the JEDEC standard (only whiskers greater than 40 microns are considered potentially harmful and whiskers shorter than 40 microns are ac- ceptable). As this is a short-term accelerated test it is not suitable for space electronics. Out of three lots of testing, the first lot showed three parts having one whisker each, with lengths of 99, 57 and 112 microns, respectively. The second lot showed two parts with whiskers up to 122 microns and the third lot showed 41 whiskers of up to 192 microns in length. Reliability test- ing was considered essential whenever tin fin- ishes are envisaged. For long-life electronics the JEDEC standard is probably not suitable. The best strategy, considered by Alter Technology, is to avoid using pure tin-plating on any com- ponent parts. Morilla also discussed mitigation strategies that included conformal coating, the use of a nickel barrier layer and component bak- ing treatments. It should be noted that the test components did include a nickel barrier layer between substrate and tin-plating but this was found to be an ineffective mitigation method. Reference was also made to the Guidelines for a Lead-free Control Plan, published by ESA. The final paper also concerned tin whisker growths and was given by Martin Wickham of the National Physical Laboratory, Teddington, UK. Wickham compiled the results of many whisker growth studies performed at NPL, and particularly the study of various families of con- formal coatings applied to tin-plated plates, and their ability to prevent short circuits. All coatings are better than no coatings; whisker growth is greatest at edges where coatings may be thin—it was noted that whiskers can grow at any time to cause electrically intermittent contacts. Other studies were made using "real assemblies" that included integrated circuits having tin-plated Olin 194 lead frames where the leads were heat treated for 15 minutes at various temperatures up to 150 o C; some were reflowed at 230 o C. It was noticed that increas- ing the temperature of heat treatment (and also reflow) delayed the time for the first whisker to appear. These tests are continuing, but it is re- ported that the uncoated control samples have "shorted extensively," and all samples will be revisited/inspected biannually. Wickham en- quired whether additional companies might support these studies, particularly when con- eVenT ReVIeW: (eMPS-6) continues artICle figure 3: pure tin stripping up to package body without removing package identification, for space and military circuits using a standard chemical process; leads are then pre-tinned with eutectic solder [1] . figure 4: Modification of interconnection design to ensure absence of thermal fatigue under severe thermal cycling environments [1] .