I can't comment about likelihood, but a combination of mechanical impact at some point, with possibly marginal soldering and multiple thermal dilation/contraction cycles could have been contributing factors.

I think the same.

But perhaps it was a cheap soldering process, or the automated process wasn't mature yet.

SMD mounted parts were still kind of new in the 1980s.
Most parts on a PCB used conventional mounting, except computer chips with a huge number of individual pins (if they didn't use sockets).

Those expansion cards weren't meant to last, also, I assume. 5 to 10 years was seen as more than enough in that era.
Standards changed so fast, that 5 years were half an eternity.

With a quality solder, say silver solder with lead and solder flux, it would have lasted much longer. 🙂

yeah it was probably never soldered properly to begin with

adalbert wrote on 2022-07-06, 19:24:

Yesterday I found out that my Trident TVGA8900D started giving a garbled image. It turned out, that a lot of legs on the video c […]
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Yesterday I found out that my Trident TVGA8900D started giving a garbled image. It turned out, that a lot of legs on the video chipset have gotten loose. It was clearly visible under a microscope, the pins were moving, there are also black gaps:

I fixed the issue by re-soldering the chip with addition of a bit of fresh solder and a lot of flux. It works fine now. I documented the process in a video: https://www.youtube.com/watch?v=syAhMC4H5aY

How common is such issue? I remember that some people had similar issue with 3Dfx cards and fixed them by reflowing. I wonder if this is caused by manufacturing defects, or just by the age of solder. I noticed this problem only on the main graphics chipset, with fine-pitch QFP package. The other chip with bigger pins was fine.

I wonder if we should expect this issue to happen more often, like bad capacitor plague or leaked battery issue.

First I agree with what's been said above.

This type of surface mounts seem relatively susceptible to damage caused by (sheer (as in perpendicular)) force (often due to mistreatment for instance) and age and quality of the solder will probably also play a part in this. Bending of the PCB may also be a (part of the) cause of this.

Personally I wouldn't be surprised if cases of such damage have gone unnoticed, as it seems relatively easy to miss with a purely visual inspection. Your diagnostics of the card by pressing on the chip during startup however is well executed, I think your video is really good at showing this particular issue 🙂(much better so than the pic).

Temperature plays a major role in the process of ageing solder. The tin share of the solder degrades over time when components (and their pins) get hot or the pcb heats up.

Any number of things can contribute to that over time, but usually the root cause is just insufficient solder or a poor quality soldering job.
The age of the parts plays a part, as does where the part is in relation to forces exerted on the card when one inserts and removes it.
For example, lots of older Game Boy carts have a SOP-32 mask ROM right next to the structural hole in the middle of the PCB, and the legs of those roms tend to become undone after years of use.
I've fixed dozens of "broken" Game Boy games by just re-soldering them.