Usually, most people who make some microwave stuff think that the best finish they can apply to their boards is gold. It looks good, doesn't change over time and since most HF (high frequency) energy flows through the surface due to the surface effect, it's best to have some protection on the copper traces. In this case, gold.
Some of these rules are right, some are OK and some are simply off.
First of all, let's list the usual finishes available from almost all manufacturers, even the Chinese ones:
HAL (or HASL) - solder is applied to the whole board and then blown off using a very fast stream of hot air
Leadless HAL - the same, just without lead
ENIG (also called immersion gold or soft gold) - two-layer electroplated protection which contains nickel layer directly on the copper traces and thin layer of gold on the nickel layer
And now the less usual ones, or at least less used ones:
Hard gold - almost the same as ENIG, but the layer of hard gold can be much thicker, its crystals can be about 50x smaller than when using soft gold and the gold usually contains nickel, cobalt or both to make it much harder
OSP - organic protection which sticks to the copper and forms organometallic compounds which prevent oxidation, disappears during soldering
Immersion tinning - electroplated tin
No protection, simply just bare copper
ENEPIG - ENIG with additional palladium layer which should prevent "black pads" which can form during electroplating and reduce solderability of the gold layer
EPIG - just like ENIG, however nickel is left out and substituted by palladium
Kapton solder mask - sometimes called generally Coverlay (trademark of DuPont)
Now, let's mention the solderability of the more usual finishes, listed from best to worst:
That may sound strange, doesn't it? You were probably expecting the gold finishes on the top places. And that's the first common myth. Gold isn't the best solderable finish. HAL is usually the best one.
This means repeatability of thickness, surface roughness, resistivity and how precisely it is controlled over the board:
Coverlay / OSP
HAL / leadless HAL
Again, gold isn't the best one. Bare copper and its "covered" variants have the lowest tolerance, roughness and best repeatability. ENIG and its variants suffer from tolerances of the thick nickel layer which usually has rounded edges. For example, let's show an image. Its source is IEEE-Xplore. If you cannot open this because you do not have the magical paywall access, you can find this article on Z Library, just try to find its name. I won't be inserting link to Z Library, because I do not know how prudent HackaDay is when it comes to links to illegal sources of science...
As you can see, the trace is nowhere near the ideal rectangular shape everyone wants to see. If you look closely on the Cu trace, you can see the effects of etching. And the Au/Ni part makes the Cu trace more rounded and thicker than it should be, which is worth noting.
HAL isn't very well controlled process in this point of view. The lead dissolves partially the copper beneath and when blown off using the hot air, it is not very flat. Look at these two pictures.
As you can see, it is not flat at all. That's the main problem with HAL - nonpredictability.
Now, let's list those finishes in order of suitability for microwave stuff:
Coverlay / OSP
HAL / leadless HAL
OK, this is starting to be confusing. Why is ENIG never on the top? Why not even in the top 3? The answer is nickel. Maybe that was too short answer. Let's just state that copper or its covered variants have very low losses on microwaves if the copper is smooth. The reason is that rough surface increases the length of the trace virtually and that increases the losses. HAL is useable, but its impedance may slightly vary due toits non-flat surface....