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While it is not used as much nowadays, wave soldering was a big advance in the productivity of manufacturing electronics, compared to soldering all the components by hand. It consists of placing all the components on the board, then passing the board over a standing wave of solder.

The part I have not been able to find an answer to: intuitively that seems like it should uniformly coat the entire underside of the board with solder. How does the stuff know it is supposed to only stick to the components?

(I don't know whether this technology has changed significantly over the decades, but if it matters, I am particularly interested in how it worked in the sixties and seventies when it was replacing hand soldering, and with through-hole rather than surface-mount components.)

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(There is a rather complete Wiki-article about)

While it is not used as much nowadays,

Still widely in use, just not so much with fine electronics.

then passing the board over a standing wave of solder.

It's rather the edge of a 'solder-fall', but there were various designs over time.

How does the stuff know it is supposed to only stick to the components?

Solder isn't a glue, but joins by diffusing into the surface of another metal (*1,*2), thus it doesn't stick to anything. So it does not stay on a PCB just by touching it. Old radio boards (but even early computer boards, most notably SWTPC or OSI) are made that was. quite visible by having 'thick' and 'round' traces.

To avoid solder on such and parts that should not carry any, a solder mask is used. That's essentially a layer of lacquer printed on top of the traces, wherever no solder needs to go. It also helps increasing fine details

In addition the PCBs are preheated, so the solder moves in right away. Naturally the traces heat better than the PCB. In fact, heating (degree and speed) - and later cooling - is a major part of making a wave soldering work.

I don't know whether this technology has changed significantly over the decades,

It did, but that doesn't matter much.

*1 - That's what soldering is about, melting just one of the metals. In contrast to welding were both sides (usually the same metal) that get melted and crystalize again in a unified area - and no, sintering is again different, but brazing is essentially the same a soldering.

*2 - It's not any 'metal', but a specific one. Solder is especial formulated to interact at a certain temperatures with a certain metal.

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    Careful there - most metallurgists are pretty uptight about anyone saying brazing and soldering being essentially the same. Even when you are their boss. And, no, I feel the same as you, but they are more precise.
    – Jon Custer
    Nov 21, 2020 at 21:16
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    @JonCuster "Even when you are the boss" ... like that. As so often this depends on how to define it. Soldering,as well as brazing is defined (on a practical level) as connecting two metal pieces by using a third metal with a lower melting point in a way that the connecting material fully melts but the connected pieces not. Soldering and brazing differ in temperatures (and thus materials) used, with an artificial borderline. In hindsight it's a historic distinction, depending on local culture - like in German soldering is called 'Löten' while brazing is 'Hart-Löten', literally 'hard-soldering'
    – Raffzahn
    Nov 21, 2020 at 21:29
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    Agree - to me they were the same, but some took umbrage at it. As normal, in any particular sub-field people use highly nuanced language to make fine distinctions that matter to those in that sub-field. But, we solder microelectronic, that is for sure.
    – Jon Custer
    Nov 21, 2020 at 21:48
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    Just to add more verbiage to the comments... the American Welding Society pretty much defined a semi-arbitrary cutoff between brazing and soldering- if the liquidus temperature is under 450C it is soldering (between Zinc and Aluminum).
    – user11599
    Nov 22, 2020 at 6:41
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    To muddy things slightly more, carbide tool tips bonded using a brassy alloy into a stell carrier are usually referred to as "cemented".
    – Mark Morgan Lloyd
    Nov 22, 2020 at 14:00
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The PCB is coated with a solder mask. That's usually the green shiny layer on the surface of the PCB. There are openings in the solder mask on places that need solder. So the solder only sticks to exposed copper pads and component legs.

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