Just a few comments that might fill in the blanks on this question. I’ll try not to be long-winded: There are numerous lead-free alloys. In fact, our most common solder standard list nearly 100 different solder compositions. At this time, since 2010, most commercial electronics are lead-free, so this is certainly nothing new, unless you are working with defense, avionics or space hardware. The most common lead free solder used in the commercial electronics industry is what we call a SAC305. That is, SN96.5 AG3 CU0.5 (3% silver, .5% copper and the rest tin). This particular alloy has a melting point approximately 40 degrees higher than the traditional SN63 (63% tin, 37% lead). Be aware that other alloys may be very high temperature but you probably won’t see them on circuit boards. The only one reason lead free is used is for ROHS (Restriction of Hazardous Substances), which is mandated my many parts of the world. By the way, the reason lead free was introduced was due to the elimination of carcinogens, unless the flux in the core contain toxics that might be harmful. When we made the switch from tin/lead to lead-free solders around 2006, it came with some challenges but the electronics manufacturing industry, which I’ve worked in for 40 years stepped up to the plate and made the changes. Many comments have stated that the soldering irons need to be a different brand, longer dwell times and larger tips, all of which are justifiable to consider but most people can accomplish reliable solder joints by simply observing the following; 1. Realize there is a delta in the melting points so you might consider increasing the tip temperature. There is absolutely no way to recommend a temperature since there are so many variables, such as heat-sensitive components, board thicknesses, and the actual alloy itself. For that reason, you might consider staying on the joint a bit longer than what a person with tin/lead soldering history is used to. Probably not longer than 6 seconds unless you are doing rework. That is a totally different subject. 2. The use of additional (auxiliary liquid) flux will promote wetting. This is due to the increased melting temperature, which promotes oxidation, thus the need for additional flux that has not burned off, to accomplish the removal of oxidation. 3. Wait for wetting; It is necessary to use the proper tip temperature (again this is not something anyone can recommend due to various thermal masses but it could be estimated to be around 725-750 degrees F in the average electronics assembly) and dwell time but WAIT for evidence of wetting. This is the intermetallic bonding as evidenced of a “flow of solder” to a low contact angle where the solder tends to be drawn out as far as the mass of solder allows. In hobbies like ours, using a tin-lead solder to rework a lead-free assembly most likely may not affect the reliability if it has been soldered properly. However, this would be totally inappropriate in my industry since our product is sometimes under contract to not contain lead, mercury, cadmium, etc. As I said, most things sold on the commercial market are already lead-free and most (not all) of the solders you will find sold at retail stores these days will be lead-free. Sorry, I tried to be brief. I will be happy to answer question if you have any. A reliable solder connection is a simple process. Rework can be a little tricky at times. One more thing... the soldering iron is critical. We can talk about tip temperatures and dwell times but if the iron is not capable of heating the connection area rapidly and maintaining sufficient soldering temperature range at the connection throughout the soldering operation, it will not do the job. Weller, Hakko, Edsyn, Metcal, PACE, JBC are all excellent manufacturers but it also depends on the model. Of course there are other brands I didn't mention. The key is to have a reliable iron that will do what is mentioned above, not to mention protect ESD sensitive devices and not degrade other features. I hope this helps someone.
