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I've been looking around online for information on when I should use different finishes. I've found plenty of pro's and con's about each finish, but very little information on when I should choose one finish over another. The rule of thumb we have been using is if you need a high solder reliability use ENIG gold, otherwise a RoHS friendly finish (either lead-free or silver or whatever the manufacturer recommends).

However, a lot of what I have been reading seems to suggest the ENIG is great if we need to store blank boards for a long time, or if using BGAs which need to have a very flat surface to help promote soldering correctly. I have found very little information supporting ENIG as making a high reliability solder join.

My question then is when should I specify ENIG, or HASL, or some other type of finish?

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    \$\begingroup\$ We specify silver finish for boards that are screwed down with metal fixings. Nickel plated screws, washers and pillars are used too. This lessens the electropotential difference of the dissimilar metals and reduces corrosion over time. This is especially important if the board is subject to Pollution Degree 2 or 3 (condensation can/will occur). Immersion silver is flat enough for BGAs, but will not store for as long as gold finish. \$\endgroup\$
    – Matthew
    Commented Oct 26, 2016 at 17:19
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    \$\begingroup\$ I have always used ENIG for consumer products. I am not aware of any downside to ENIG other than cost. \$\endgroup\$
    – user57037
    Commented Oct 26, 2016 at 17:23

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I think it all comes down to corrosion, the flatness of the finish and cost.

HASL isn't flat so you can rule this out for fine pitch ICs or BGA, the gold over nickel ENIG is as flat as it gets and the gold surface is the least reactive so won't really corrode and last for years on the shelf. I don't think its any better for the quality of the soldered connection as the gold pretty much vanishes as soon as you solder it.

Ideally we'd always use ENIG but it's expensive so we tend to use the silver finish which offers a good compromise for cost and shelf life, it's also fairly flat so I believe it's suitable for BGA. Our supplier rates it for 12 months shelf life but I've assembled on silver finish PCBs that are at least 3 years old OK.

Mathews comment about dissimilar metals corrosion (galvanic action) is also worth bearing in mind here if your reliant on the fasteners for earthing.

Update

Thought I would just update as I've been reading a bit recently about the reaction of silver immersion with atmospheric sulphur whereby silver sulphide can develop on the PCB and 'creep' across the board. This could be an issue when operating in environments with elevated levels of sulphur and high humidity. See this article. Since writing this answer I've noted that immersion silver finish is no longer a popular choice.

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    \$\begingroup\$ For non-BGA. at what pitch would you say the flatness becomes an issue? 1mm? 0.5mm? Less, more? \$\endgroup\$
    – CraigC
    Commented Oct 26, 2016 at 21:25
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    \$\begingroup\$ You mean what pitch to go down to with HASL? I absolutely wouldn't go less than 0.5, maybe 0.8. \$\endgroup\$
    – SpaceCadet
    Commented Oct 26, 2016 at 21:41
  • \$\begingroup\$ Is ENIG better for edge connectors such as game carts? I've seen pics of HASL edge connectors becoming scratched very quickly. \$\endgroup\$
    – cbmeeks
    Commented May 25, 2018 at 14:34
  • \$\begingroup\$ Edge connectors usually undergo an extra electroplating step to deposit more material. \$\endgroup\$ Commented Jan 5, 2021 at 13:52

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