Bureaucrats noticed that there was some lead in the ground water from where old electronics was disposed. The real source of this lead was from the glass in CRTs in TV sets and monitors, but the smartest in the field do not work for the government - they painted with an overly wide brush. The lead in solder comes from lead oxide ores. Lead oxides are poisonous and once smelted into lead, become less toxic. Once alloyed with Tin the solder is even less toxic. If this solder is now buried back in the ground it is in a safer form than when we started.
The politicians don't like lead because of the problems with lead-oxides in paint. Once the politicians made the mistake in banning lead from solder, they can never undo the ban without admitting they made a mistake. Politicians never admit to error, so we have to live with no-lead solder in electronics.
No-lead solders do not make as good a joint and subject the parts to more heat stress. Both of these problems shortens the life of the electronics and increase the disposal rate of electronics. Many parts are no longer available in leaded versions even with the exemption for life critical applications, so they are used in life critical applications. Many electronic gadgets can play life critical roles (a cell phone calling 911). Yes, people will die from these policies. (BTW - the silver in the new solders is toxic to many forms of aquatic life.)
There is no reason to expect these policies to change and they are in the process of being adopted in the USA (2008). We have to learn how to best live with the political idiocy.
It appears that Tin-silver-copper alloys in the range 3-4% silver and 0,5-1,0% Cu melt in the 217 to 220°C range system is the standard. No more Tin-lead with its nice 183°C melting temperature; we now have to go up to 220C to melt the new solder. To do so with out damaging the components requires careful heat profiling - lots of preheat. It is also important not to go above 260°C or remain melted for longer than 3-seconds total. Soldering irons should be set for less then 350°C and the joint needs to be made in 3 sec. If you re-solder - you need to remove the solder and start over - the 3 seconds is for one time and one time only!
Other trace alloys can help the solder - a bit of nickel can help flow. Antimony? Indium, Gold, Bismuth would be used except for cost. A number of SAC + bismuth alloys have been patented - and patents litter the field making electronics more expensive and less reliable. The finest micro-structure with Bi is 93.3Sn-3.1Ag-3.1Bi-0.5Cu, with a melting range at 209°C to 212°C, and a rather small pasty range.
Each percent of indium (up to 12%) lowers the melting temperature about 1.8°C. Sn4.1Ag0.5Cu8In melts at 195°–201°C, Sn4.1Ag0.5Cu12In melts at 185°–195°C. Indium enhances fatigue life Indium blends are locked up with patents.
Sn3Ag0.5Cu8In is offered as 'a' best compromise, melting at (196–202°C), narrow pasty range and acceptable wetting characteristics. But the price is too high. There is also some concern about the growth of a tin-indium eutectic that melts at 117°C.
It appears that most will use something of a compromise.
The precise ternary eutectic of SAC is Sn-3.5Ag-0.9Cu and melts at 217°C.
Designs of the circuit board and leads should take in account the less flexible joint.
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