Soldering is the joining together of two or more metals by melting then putting in a filter metal which has a moderately low melting point onto the joint. Solders falls into three categories, lead alloy solder, lead-free solder and silver alloy solder.
Lead alloy solder can be referred according to the ratio of alloy such as 60/40 the first number represents tin weight and the second number is lead by weight. The 60/40 solder is perfect for general electronics needs, same as 63/37. 63/37 solder serves as an eutectic alloy because of its sharp changeover between liquid and solid states with changes in temperature. Its change of state characteristic also helps reduce cold solder joints as the solder cools. Solder paste 60/40 Tin/lead (Sn/Pb) melts at 188 °C while 63/37 Sn/Pb melting at 183 °C . This alloy however is associated with health risks.
Among the lead-free alloy is am alloy of tin, copper and silver. They are quite expensive compared to lead alloy solders. They have a higher melting temperature and consequently need a higher temperature flux. The tin-silver-copper solder melts at 423 degrees Fahrenheit. They provide very strong but more brittle solder joints.
Silver bearing solder can be lead-free or with lead. Lead alloy solder are topped with silver to prevent silver migration. But, they are quite expensive.
Solder wires for electronics and PCB typically have a core inside the wire containing a flux. It is designed to improve the electrical contact and mechanical capabilities on the solder joints. Acid core utilized in plumbing and Rosin core used in electronics are the two main kinds of flux cores. The size of the solder depends varies some as big as 2mm or 1/16 to as small as .5mm. However, the larger solder the more heat it will take to melt it and often times you end up melting or burning other things before the solder melts. So, choose your solder wire size carefully.
The Kester solder 40/60 lead solder wire has a diameter of .050″ with a combination of Alloy; Sn60Pb40 (60% tin to 40% lead, melted at a range of 361-374°F (183-190°C).The .05 size makes it perfect for most projects. It weighs approximately at 1 lb. The flux action of this solder makes its flow perfect; it quick to work with work. The relatively low melting point also facilitates its absorption into the wire to deliver a solid contact. The Kester brand is characterized by smooth and consistent flow of the solders leaving people satisfied. I know many professionals and hobbyists have strong preference over Kester solder.