For lead free soldering Sn99/Cu0.7 is the most common lead free alloy available from your local hobbyist shop. This is because is the cheapest alloy to make. It has a melting point of 227°C The lead free alloy used in a lot of service and manufacturing companies is SN100C. This alloy is the closest appearance and flow wise to leaded solder. It contain Sn/Cu/Ni and has a melting point of 227°C. SN100C is a eutectic alloy and stays wetter longer to ensure the solder joint wets well. The 030 flux medium is also a very good flux that is used inside the SN100C solder and this helps the solder flow, doesn't stink and is no clean and leaves a clear residue. It's only available in 500g rolls but if you are looking to buy a roll of solder that will last you a while, might as well use one of the better lead free alloys on the market. In Qld the SN100C is sold through RMS Parts http://rmsparts.com.au/productsearch.ews?stocksearch.ewdsearchterm=sn100c In VIC SN100C is sold through Radio Parts http://www.radioparts.com.au/search-products?productsearch=sn100c In NSW SN100C is sold through Okay technologies http://www.okay.com.au/okay2009/ind...sCsid=tqrhuv708lltj28boe20ir6lc7#.VgP1nNzouP8 For leaded soldering 63/37 is the alloy used to manufacture 95% of leaded electronics pcbs around the world. For touch up and rework 60/40 is commonly used as it is commonly available in shops and is slightly cheaper (but not much) to buy as it contains less Tin. Can 60/40 be mixed with 63/37? Sure, both alloys have a similar melting point 63/37 183°C and 60/40 183-188°C. The difference between the two is 63/37 is a true eutectic alloy and has an exact melting point. 60/40 has a ranging melting point and goes through a "cluggy" stage where it is not a liquid nor solid. It's in this stage where any vibration can result in poor solder joint quality. Will 60/40 work as well as 63/37? Very similar and you probably won't see the difference if your are soldering occasionally. But if 63/37 solder wire is available to you then that would be the alloy of choice. Mixing leaded and leaded free solders happens a lot in rework when people are not fans of lead free solder. The 63/37, SN100C etc are all alloys that have been tested to ensure they provide a reliable solder joint. 63/37 leaded solders melting point is 183°C and SN100C and Sn/Cu alloys has a melting point of 227°C Mixing a leaded and lead free alloys will provide you will a solder joint that will look to be okay, but you have now created an unknown alloy composition with a unknown melting point. Mixing of leaded and lead free solders happens all the time in the service industry. But if you are looking to repair/produce a reliable product, then mixing of leaded and lead free alloys is not recommend.