Saturday, June 15, 2024

What are the disadvantages of lead-free solder vs. lead solder?

What are the disadvantages of lead-free solder vs. lead solder?

The major disadvantages in no particular order are:

A "good" lead-free solder joint is grainy and rough.

Please note that both of these solder balls are cold, yet look how shiny the leaded solder is next to the rough lead-free solder:

I have found that lead-free solder often requires tight temperature regulation on your soldering iron, depending on the exact mix of metals. For many years, I and others have used unregulated irons or those with interchangeable tips that are at 600F, 700F, or 800F with 63/37 and 60/40 solder without problems. When I switched to lead-free, I had to get a digital adjustable iron because many lead-free solders have only a 10 or 15F working range.

The flux in lead-free solder is much more active than the rosin (literally pine sap) flux in leaded solder. In addition, you -must- use a lot of added flux when soldering lead-free solder. You should not be breathing in smoke from any kind of soldering, but the lead-free flux is much more irritating to your eyes and respiratory tract. And unless you use no-clean flux, the flux must be cleaned from the board or it will corrode the metal.

Tin whiskers! Lead was originally added to solder because of tin whiskers. No one is really sure why they grow, but they can grow at prodigious rates and cause failures. There is no clear, repeatable correlation, but elevated temperature, humidity, and voltage seem to have some correlation. Tin whiskers have been known to grow right through conformal coatings. This is especially a risk in modern high density, low current circuit boards.

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The major concern from solder smoke is the flux, not lead. Lead does not tend go into vapor at soldering temperatures. For that, lead mining and smelting are the main pathways, and leaded gasoline for countries that still allow it. Leaded paint being an ingestion risk for children and amateur home renovators. Keeping in mind, of course, that lead absorption is cumulative as it is slow to leave the body.

As a side effect of lead-free solder, much more aggressive fluxes must be used, and in much greater quantities. In my many years of soldering, I've rarely added flux when using leaded solder, but -always- end up adding flux for lead-free joints. In either case, the smoke is very bad for you and should be aggressively removed. I've found the common tabletop smoke fan is a huge waste of money. I have a 200mm fan that is in a flex arm lamp in place of the magnifying lens. It is like a mini fume hood.

As for skin absorption, I've commonly used empty solder wick rolls to hold the solder I'm using. And wash my hands before doing anything else.

Lead (Pb) Toxicity: How Are People Exposed to Lead?

The Problem with Solder Smoke

Coal burning releases a lot more lead into the environment and has been cited as the major source of lead found in children's blood.
Lead in Children’s Blood Is Mainly Caused by Coal-Fired Ash after Phasing out of Leaded Gasoline in Shanghai

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