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What are the different tinning techniques for PCBs, and what are the advantages of one technique over the other?

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The easy answer is that you should look at the options your board house offers and then research that.

Common options I've seen are:

  • HASL, Hot Air Solder Leveling: They bascially coat the entire board in solder and use a air knife to blow the excess off, it's cheap but it can leave a slightly uneven surface, which might mean something if you are doing BGAs.

  • OSP, Organic Solderability Preservatives: A very thin coating that keeps oxygen away and evaporates when soldering, it looks as though there is just bare copper on the board.

This page has a good description of these and other finishings: http://www.multicircuits.com/pcb/tech/surface_finishes.html

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Immersion silver seems to be getting popular for RoHS boards. It gives a very flat surface, and is fine if boards are soldered quickly. Some companies use immersion gold, even on low-cost boards. Silver can oxidise over time, which apparently can affect solderability, so gold might be better if boards are to be stored. I've had no problems hand-soldering boards finished with both immersion silver and immersion gold (lead solder), and neither has the company I use sometimes for surface-mount board assembly, using lead-free solder paste.

Gold plating has been used on connector pins for years, and has never caused soldering problems.

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  • \$\begingroup\$ Silver surprises me. We once had a batch of boards which were accidentally completely gold finished, and I found them impossible to solder, at least by hand, but I think reflow wouldn't have worked either. Gold has a melting temperature over 1000°C (silver is not much less), and soldering occurs at temperatures around 300°C. \$\endgroup\$ – stevenvh Apr 21 '11 at 9:54
  • \$\begingroup\$ @myself :-) - Apparently there's a binary eutectic Ag alloy with a low melting point, but it's Ag-Pb, so wouldn't be RoHS compliant. \$\endgroup\$ – stevenvh Apr 21 '11 at 10:21
  • \$\begingroup\$ I've tried to solder (lead-tin) on a gold plated connector, but the solder refused to flow over the gold. Maybe because gold on a connector is thicker than the immersion gold. \$\endgroup\$ – Federico Russo Apr 21 '11 at 11:11
  • \$\begingroup\$ @federico - thickness has nothing to do with it. Below the melting point the material is not permeable for the solder, even when only a few tens of atoms thick. Soldering is a metallurgical process. I don't understand how it can work for Leon. \$\endgroup\$ – stevenvh Apr 21 '11 at 11:39
  • \$\begingroup\$ @stevenvh - You can solder to copper - which melts at 1085°C \$\endgroup\$ – MikeJ-UK Apr 21 '11 at 12:55
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Most of the work that I did in recent years have called for ENIG - Electroless nickel immersion gold. The plating comes out nice and flat, which is not the case with HASL. It also looks pretty, but that's a subjective thing. :)

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