There are two main classes of solder alloys used today for circuit assembly. This consists of both a Leaded and Lead-Free alloy. As the name suggests, leaded alloys contain lead which is then alloyed to tin, and in some cases tin and silver. The most common alloys in this class are the Sn63, which is 63% tin and 37% lead and the Sn62 which is 62% tin 36% lead and 2% silver. These compositions are chosen because they have two key properties, 1) They have a sharp melting point rather than a melting range (such alloys are called eutectic) and 2) they have the lowest melt point for a combination of the various metals. The more metals in the alloy the lower the melt point. Lead-free alloys also as the name implies, do not contain any lead, they still contain tin which is essential to form a reliable solder joint, but the lead is replaced with different metal. The most common alloys in this class are 96SC, often referred to as SAC387, which consists of 95.5% tin, 3.8% silver and 0.8% copper, while the 97SC is 96.5% tin, 3.0% silver and 0.5% copper, this is called SAC305. These lead-free alloys have become increasingly popular as a result of the RoHS legislation in Europe which limits the use of lead in electronics. The most popular particle size distributions are 45-25 microns, which is classified as Type 3 by the IPC definition and the Type 4 powder which is classified as having particle distribution between 38-20 microns.