Tinning is a method of replacing the existing surface finish of a Solderable termination with a pristine solder finish of the desired alloy. The tin that is in both the tin lead and lead free solder alloys, is a very aggressive metal considering its ability to dissolve other metals. When a sample is dipped into the molten solder bath, the tin in the bath will first dissolve and remove the surface finish and then will dissolve some of the basis metal as well.
How much of the basis metal is removed is a function of the duration of the immersion, whether the solder bath is static or dynamic, and to a lesser extent, the temperature of the bath. When the sample is removed from the solder bath it will have a fresh coating of the solder alloy in place of the original surface finish. The removal of the basis metal (dissolution) is a greater concern when using lead free alloys because the higher tin percentages (usually approximately 97%) are more aggressive than the tin lead alloys.
In 2003 legislation was introduced in EU to promote the collection, treatment, recycling, and recovery of waste from electrical and electronic equipment. This legislation is known as the “Waste Electronic Equipment (WEEE) Act’ and is formally dictated by directive 2002/96/EC of the European Parliament.
A complimentary directive, the Restriction of Hazardous Substances (RoHS), was also introduced in 2003 given by 2002/95/EC of the European Parliament. Beginning July1, 2006, RoHS legislation restricted the amounts of Lead (Pb), Cadmium (Cd), Mercury (Hg), Hexavalent Chromium (Cr6+), Polybrominated Diphenylethers (PBDEs) and Polybrominated Biphenyls (PBBs) in electronic and electrical equipment. These chemicals are known to present a risk to human health and the environment. Each European Union member state will adopt its own enforcement and implementation policies using the directive as a guide.
Manufacturers outside the European Union may wonder what RoHS has to do with them; after all, it’s a European directive. The reality is that RoHS directive has a global effect. The directive indicates that anything covered by RoHS entering the European Union must be compliant, whether they include cables made in China, parts molded in the USA, or PCBs made in Japan.
If a product is eventually destined for the EU, it is impacted by RoHS. In addition, the EU is not the only global entity taking steps to reduce the toxins in electronic devices. California’s Electronics waste Recycling act of 2003, for example, echoes the RoHS directive and has been implemented since January 1. 2007. Japan, China, and Korea are also expected to follow with laws of their own.
All this legislation has a huge effect on consumer electronics, while the defense and aerospace industry continue to use lead bearing solders and avoid the use of tin plating. The main reason that most of the high reliability industry has refused to embrace the use of lead free materials is the risk of tin whiskers. While there is no definitive answer to what causes tin whiskers, we do know what will inhibit the growth of tin whiskers... Lead. As little as 1% lead added to pure tin materials can drastically reduce the occurrence of tin whiskers.