Answers the questions for modelers, “What solder should I use?”.
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60/40 solder is made of 60% tin and 40% lead. It has a melting point of around 190°C, depending on the exact composition. Iron tip temperatures of at least 300°C are recommended. It is also very soft, meaning that cracks do not form so readily if the joint moves during cooling.
63/37 solder is made of 63% tin and 37% lead. It has a melting point of 183°C, slightly lower than the more common 60/40 blend. The primary advantage of this solder is not the lower melting point, but its eutectic property. Non-eutectic solders, like the 60/40 solder, have a semi-solid state between solid and liquid. If a joint is moved during this stage, it can result in what is called a cold solder joint. Eutectic solders, like the 63/37, do not have this semi-solid state and are thus considered easier to work with as it produces fewer bad joints. However, these solders typically cost more than their non-eutectic counterparts.
This is made of a half and half mix of tin and lead. Never use 50/50 solder for electronics — it is meant for plumbing. (There are brands[Worthington 50/50 leaded #331887], which is used for electronics and specifically not intended for plumbing). Otherwise, you may end up with failed joints, as the 50/50 solder does not have the same properties as 60/40, having a higher melting point and lower ductility. Also, it is very unlikely to have flux included in it like rosin core solders.