I'm curious about the historical origin of the use of the word "card" in "expansion card."
"Expansion board" makes sense, given it's a PCB, and a PCB is a board (of fibreglass for example).
But "card" seems a bit esoteric. Did this terminology originate from the physical resemblance to punch cards, index cards, or something else - possibly simply the fact they're rectangular and most type of cards are too!?
'Boards' of various size and style are a later development.
I'm curious about the historical origin of the use of the word "card" in "expansion card."
Asking for a 'why' when it's about language is rarely useful. Still, in this case it can be said that "card" existed even before it was used for expansions :))
"Expansion board" makes sense, given it's a PCB, and a PCB is a board (of fibreglass for example).
Well except that early cards were not PCBs, as the first generation didn't carry any circuit, especially not a 'printed' one, nor were they fiberglass, but made of resin soaked paper.
PCBs as we know today underwent a long development. Electronics started with frames mounting components connected by wires and other components, called point-to-point, a technology taken from previous radio design, as seen here:
(Taken from Wikipedia)
Early computers were built the same way except, due to their repetitive nature, a more structured modular approach was possible. See this IBM Module used in 700 series mainframes in the 1950s:
(Taken from this CHM page)
There is nothing even close to a PCB here. As early as the 1920s the first radio equipment manufacturers started using early forms of circuit board made from resin-soaked and pressed paper. But those boards did not feature any traces, nor any other components - only holes to hold wired components. This use solved a major problem of point-to-point wiring by keeping wires and components apart from each other by a non-conductive carrier.
The first circuit board with wires embedded showed up as early as the 1900s (even Edison tried to build some). They were ahead of their time: there was no real need, as the few components that made up a radio or similar did do well without. In areas where complexity and density were required, there was some limited use. Even some multilayer boards were used by German manufacturers during the war. Neither were of much influence for early computing (*1)
In radios, PCBs started to appear in Europe during the late 1930s/early 40s, while the US still held the technology a war secret until 1949, somewhat slowing development. It wasn't until the mid 1950s that the first PCBs were used in computers. Although DEC's flip-chip modules are not amongst the earliest examples, they may be the best-known ones:
(Taken from Wikipedia)
Since they were targeted at a low price point, they show many features of earlier boards (and quite low density). From there to today's 20+ layer and 50µm trace size was still a very long way.
But "card" seems a bit esoteric. Did this terminology originate from the physical resemblance to punch cards, index cards, or something else - possibly simply the fact they're rectangular and most type of cards are too!?
Rather from simply being just cards made of paper. There's no need to borrow any meaning from other media.
In fact it's, pretty much the reverse of your assumption: being called 'board' is a result of distinguishing them, as the main unit, from cards, which are parts thereof.
*1 - One notable intermediate step for complex boards, used especially on the Allied side, were boards holding riveted rows of solder points. Its use in several US products of the late 1940s was due to the fact that the US considered PCBs a war secret, and only released in 1949.
(Taken from Wikipedia)
*2 - Thin S100 or ECB.
*3 - The notable difference between motherboards and earlier incarnations of backplanes is that a motherboard contains active components, often quite expensive ones like CPU, Memory and/or basic I/O. Back in the early days no one would have put either on a fixed layer burrowed under everything else. Not just because of the size needed, but also as cost in case of failure would have killed any company, regardless of size. It only became a thing when most components of a machine became small enough to fit on a single reasonable-sized board.
Without answering the 'why' - this is standard electronics usage. I have known 'card' in this sense since the early 1970s, when I first learned which end of a soldering iron to pick up.
Here is an arbitrary example - the Aurora sound-to-light project from Practical Electronics magazine in 1971. See page 400 in the May issue (page 50 in the PDF file).
All circuits are mounted on individual cards...
The Aurora was built in two boxes, each of which had 8 or so cards in it. (I am familiar with it, since a friend built one. I think I helped with grunt work like winding pulse transformers).
In my experience, a board may be a standalone thingy, but cards carry the sense of being plugged into something, or connected together. It's a vague distinction,sure.
...possibly simply the fact they're rectangular and most type of cards are too!?
That strikes me as the most plausible answer - as well as the fact that "cards" are commonly something inserted into (or at least matched with) some kind of holder.
I don't think the word "board" has any profound meaning either beyond the idea of a flattish rectangular lump of something, except the tendency that "boards" are heavier and less mobile than "cards".
So you get formulations like a "motherboard" with "expansion cards".
With all due respect to the other answers here attempting to explain the terminology from an electronics point of view, my instincts tell me that's likely not the full picture of it.
Terms for new pieces of equipment are often hit upon by analogy to familiar, everyday objects, and the way that expansion cards are mounted into both PC form factors and, before that, rackmount industrial equipment likely reminded early adopters of the handling of literal paper cards in some manner. (Possibly playing cards, possibly not.)
For example, here's an image of a 1978 Milton Bradley game called "Rack-O" that I think we can all agree bears more than a passing resemblance to the sort of equipment we're talking about:
This particular set is ca. 1978, but the game was in production since at least the early 1960s.
I'm NOT in any way claiming that this exact game gave rise to the term "expansion card". Merely that the image of sliding literal cards into slots, or lining them up in racks, was probably a fairly common one 3/4 of a century ago, much more common than it would be today. The peripheral cards in a modular PC chassis, or a rackmount card cage, were likely to have been highly reminiscent of actual paper cards, to the people working with them.
Wiktionary says that "card" can mean any flat rectangular piece of stiff paper, plastic etc. Which a PCB is.
It also mentions that paper card is thinner and more flexible than a paper board (e.g. book covers, board games). Also paper cards are thinner than cardboard.
Before the era of printed circuit boards, one method of making circuits was to mount components on on a 63mil thick slice of plywood.
Likely why PCBs are still called a board and still 63 mil thick.
