Which is better soldering wire (lead-free or leaded)

[madires]

More to read:
- Reliability of Joints Formed with Mixed Alloy Solders
  http://publications.npl.co.uk/npl_web/pdf/matc85.pdf
- Board level reliability
  http://www.infineon.com/dgdl/board_level_reliability_tin_plated_components_v1_1.pdf?fileId=db3a3043132679fb01132f4a7131049c
- Database for Solder Properties with Emphasis on New Lead-free Solders
  http://www.msed.nist.gov/solder/NIST_LeadfreeSolder_v4.pdf

[Sigmoid]

no we dont.
i dont - i say that the right lead-free solder performs just as good as leaded when soldering and the joints are stronger afterwards.
that's a scientific fact btw.

Nope. A scientific fact is, for example, the resilience of solder joints after a standardized, controlled application.

Which solder is "better" is an engineering decision. Sometimes lead free can be better, for example in reflow process. (I'm not saying it IS though, hardness is one thing, flexibility is another - something can be more rigid and less resilient, like a bamboo and an oak tree compared...)

However, "better" is not synonymous with "gives the best results under ideal circumstances". In that spirit, asm IS the "best" programming language ever. In reality it's only feasible for a minute subset, like .01% of all software development problems.

Leaded solder is more likely to provide consistently good results under a wide range of circumstances. Lead free provides inferior results under a significant part of that range (eg. less that skilled workforce hand soldering).

Engineering means taking the scientific facts, and making the right compromises. Without environmental concerns, as an engineer, I say leaded is far superior in hand soldering, because assuming an average factory with average workers, it's less likely to result in defects. As for prototyping, it's less likely to result in lost time, ruined parts and a very pissed-off engineer.

[stj]

Leaded solder is more likely to provide consistently good results under a wide range of circumstances. Lead free provides inferior results under a significant part of that range (eg. less that skilled workforce hand soldering).

"chicken shed manufacturing" as we used to call it, is not my concern - it shouldnt exist.

[CatalinaWOW]

More to read:
- Reliability of Joints Formed with Mixed Alloy Solders
  http://publications.npl.co.uk/npl_web/pdf/matc85.pdf
- Board level reliability
  http://www.infineon.com/dgdl/board_level_reliability_tin_plated_components_v1_1.pdf?fileId=db3a3043132679fb01132f4a7131049c
- Database for Solder Properties with Emphasis on New Lead-free Solders
  http://www.msed.nist.gov/solder/NIST_LeadfreeSolder_v4.pdf

Good stuff.  Some interesting points from the linked material

In the first paper-

"Joints with lower ultimate shear strength generally give the better first failure performance."

"Mixed alloy joints in this study generally gave better or equal first failure performance to the original alloys."  Implies that at least from a reliability standpoint there is still work to be done on finding the "best" lead free solder.  Apparently the ad-hoc alloys created during these rework operations are pretty good.

The third paper is an excellent survey showing the various properties that must be considered in selecting the "best" solder for an application.

[jitter]

Look at this paper's conclusion on different solder alloys and thermal cycling. It contradicts the commonly held belief that lead containing solder gives the better joint.

Conclusions
The data generated from TG1 and TG2 suggests that all three lead-free alloys have a greater characteristic lifetime compared to eutectic Sn/Pb in 0/100 °C temperature cycling. The Sn/Ag alloy showed the highest Eta, which was twice as great as that of Sn/Pb

Leadfree solder may have some different issues than lead containing solder, but when the leadfree soldering process is done right, it gives longer lasting results.

[tooki]

Lead is not bad for the environment.

Lead does not significantly vaporize at soldering temps. When you get lead closer to its BOILING point than its melting point, then you can start worrying about lead poisoning from lead vapor.

Lead is highly insoluble in water. It does not transfer to your skin and into your mouth and poison you. As long as you don't let your solder corrode into lead oxide, it is not dangerous.

the MSDS for lead disagrees.
http://www.sciencelab.com/msds.php?msdsId=9927204

Lead is extremely bad for the environment if a large amount gets into the water supply, hence why it's labeled as an environmental hazard, the flint crisis is a good recent example of this.
http://www.cnn.com/2016/01/11/health/toxic-tap-water-flint-michigan/

Lead is VERY TOXIC and should be handled with extreme care.

You do realize that MSDSs are the Chicken Littles of occupational safety? The MSDSs for common household chemicals are terrifying.

And you realize that even the Flint incident is far more complex than "Lead's baaad, mmmkay"? Lead pipes tend to be quickly passivated thanks to the layer of limescale that builds up almost instantly. Flint's mistake was to allow the water to become very acidic, which dissolved the limescale layer, exposing the lead itself. Lead water pipes are common worldwide, because it normally causes no problems, so you just swap it when the pipe fails.

Environmental lead isn't from electronics, it's from tetraethyllead, lead paint, and car batteries. (And in mining dross.) And remember also that in e-waste disposal, the vast majority of the lead we're dealing with isn't in solder, it's in the heavy lead glass of CRTs. One CRT contains far more lead than all the solder in all the (contemporary lead-bearing) electronics you owned at the time.

It's years after RoHS and we've basically only now figured out how to make it perform as well as leaded solder. Most likely, lead-free solder was a complete waste of time and net loss for the environment.