Re: Soldering SMD
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Lead-free solder has three disadvantages compared to 63/37 solder (Sn63Pb37) with lead. Its melting point is significantly higher (220C vs 183C), which means that your soldering iron needs to be hotter and there is less separation between enough heat to make a joint and too much heat causing damage to components. It also isn't quite as good at wetting a joint, which means that the solder is a bit slower to spread across the joint and make a firm connection. Finally, the most common alloy (SAC305) is not a eutectic?alloy; it has a small plastic range between 217C and 220C. (In that regard it's worse than the eutectic 63/37, but a bit better than the 60/40 alloy that was in common use for many years.) But if you prefer to work without lead in the shack or you live somewhere that has banned lead solder, you can do quality work with lead-free. (The US has not banned lead solder. Some states such as California have banned the sale of consumer electronics that contain it; because of that and the international restrictions, just about everything electronic you can buy in stores is the US is built with lead-free solder. But residents can buy lead solder for personal use.) I have done most of my kit building for the past 20 years with lead-free solder, including building an Elecraft K2 with most of the options as well as a number of QRP Labs kits. The best lead-free alloys contain silver so they will be more costly than tin/lead solder. (I have not yet tried the new no-silver germanium-doped solder alloy, SN100C. The full formula of that one is Sn99.3Cu0.7Ni0.06Ge0.005; I know, that adds up to a bit more than 100%.) The most common lead-free solder is SAC305, which is an abbreviation for Sn96.5Ag3Cu0.5, the full formula of the solder; that is, 96.5% tin, 3.0% silver, and 0.5% copper. You can get it as wire solder, or as paste for hot air or reflow soldering of SMD components. For hand soldering I prefer an alloy that also contains antimony. Sparkfun sells what they call their "special blend", which adds 0.15% antimony and removes that percentage of tin. I have a roll of solder with antimony that I got at a hamfest years ago; I don't know its exact?formula. The addition of antimony makes the wetting comparable to 63/37, though the melting point remains higher. Another solder that is useful for lead-free SMD work, especially board rework, is a bismuth alloy. The common alloys are Sn42Bi58, or versions that replace a small amount of the bismuth with silver (Sn42Bi57.6Ag0.4 or Sn42Bi57Ag1). The version without silver can lead to brittle joints. Bismuth-bearing solder is rarely seen in wire form because it breaks too easily,?though ChipQuick does sell?it. Bismuth solder has a much lower melting point (138C); adding some to a joint makes desoldering much easier, and if you need to rework a tight SMD board that requires hot air rework rather than using a soldering iron (either because it is too tight for an iron or it involves components with solder points underneath), putting in the new component with bismuth solder lets you do it without disturbing nearby components. BUT... approach the use of bismuth solder on a board that contains lead solder or board plating with lead with extreme caution because an alloy of tin/lead/bismuth can have an EXTREMELY low melting point well below 100C. If you're going to put on a new component, clean off all the existing solder you can before putting on the new one so that there will not be a significant amount of lead present in the reworked joint. Finally, I should mention that some builders like a lead alloy with silver; that's usually Sn62Pb36Ag2. It's lovely stuff to work with, even nicer than 63/37, but the addition of silver makes it more expensive. The website that is what remains of Radio Shack still sells it, and you can get larger rolls of that alloy from ChipQuik. On Sun, Oct 31, 2021 at 4:05 AM Ellis Birt (G7SAI) via <ellis.birt=[email protected]> wrote:
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Shirley Dulcey KE1L