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This question already has an answer here:

Soldering in a RoHS-compliant way normally means using a solder that does not contain lead - the metal that made up most part of common solder before.

Handsoldering using lead-free solder seems to be harder. Possibly it also makes industrial soldering methods "harder" (as opposed to just different)?

  • What are the practical difficulties that arrise in comparison to using "classic" solder?

  • What causes these difficulties?

  • How can they be handeled or worked around?

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marked as duplicate by Daniel Grillo, PeterJ, Ricardo, Dave Tweed Apr 23 '15 at 12:22

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Lead-free solder requires higher temperature. Heating up parts requires more attention to details; the tip must be clean and sized for the work at hand, the surfaces must be clean, etc. Basically, the same as leaded but you can't be sloppy.

If you never soldered with leaded, all this will easily be learned in the beginning (I started soldering as a kid many years ago, using pure tin). Once you know how to do it there will be no difference. I switched to LF-only some years ago so that I don't need to maintain separate lead-free environment in my place of work.

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What are the practical difficulties that arrise in comparison to using "classic" solder?

  • Lead free solder typically melts at higher temperatures than tin-lead. This means the solder process puts more stress on the parts being soldered. It probably also relates to it being harder to get a consistent fillet and good wetting when hand-soldering.

  • Lead free solder is prone to "tin whiskers" which can cause short circuits some time after a board is built.

What causes these difficulties?

Mianly the higher liquidus temperature and lower ductility of the common lead-free solder alloys.

How can they be handled or worked around?

Among other things,

Careful attention to part selection: Make sure all the parts in your design are suitable for use with lead-free assembly temperatures.

Attention to design rules: Provide necessary pad dimensions to provide adequate solder volume and toe-room for fillets. Provide additional clearance to avoid tin-whisker shorts. For really high volume products, arrange parts to allow consistent heating across the board (avoid finicky assembly areas near high-thermal-mass parts, for example)

Assembly process control: Use an assembly shop that will tune the heating profile to ensure good quality soldering. And will inspect outgoing product to catch solder defects and feed back for process improvement.

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  • \$\begingroup\$ "Careful attention to part selection: Make sure all the parts in your design are suitable for use with lead-free assembly temperatures." - you won't miss it. Parts for leaded process are quite a bit more expensive there days. \$\endgroup\$ – Oleg Mazurov Apr 22 '15 at 0:58
  • \$\begingroup\$ @OlegMazurov, for something like BGA, that's true. But you could still get tripped up on some precision through-hole part or something. \$\endgroup\$ – The Photon Apr 22 '15 at 1:05

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