I have two PCBs that I need to connect together with holes or notched pins that I have to solder by hand.
In order to reduce soldering time, I've placed a few 2mm square pads on the bottom PCB and I'm thinking of placing another one on the top PCB, then after connecting the PCBs together, I hope these pads will be connected without having to solder them.
Illustration:
Is that good practice? Does it logically have a success rate of at least 98%?
No, absolutely not. The soldermask alone is not perfectly flat, so that will prevent the pads from coming into direct contact with each other. If the board finish is HASL, it's possible that it could be thick enough that the pads come into contact, but not guaranteed. This would not be a gas-tight connection, however, meaning that the finish would oxidize and the connection worsen over time.
You could do this if you applied some solder paste, just like soldering any LGA part, however. You'd need to reflow it with either a reflow oven or a hot-air station, though.
Castellated holes are popular for this style of direct board-to-board attachment with a conventional soldering iron (and you can even use castellated holes to fit into some PLCC sockets for a solderless connection).
If you need to make repeated connections between boards like this without soldering (this is very common in my work as a test engineer, to attach a DUT to a test fixture), you need some kind of connector. Standard pin headers can be used if you don't mind having a connector on both sides of the connection, or you can attach pins to give your PCB the same pin size and layout of common through-hole packages (e.g. TO-220, TO-247, DIP) and then use a test socket for that package.
Alternatively, and probably most usefully, you can use pogo pins to avoid having to solder anything to your movable boards. In the latter case, the movable boards should have a gold-plated finish (ENIG/ENEPIG or hard gold) to ensure connectivity, unless you know you'll be testing the boards very shortly after they're manufactured (before they've had time to oxidize). You should also ensure that there's some good way of holding the movable board in position, so that the pins are properly aligned with the pads; this can be as simple as two or three posts on the test fixture that go through mounting holes on the DUT, or as involved as a 3D-printed clamping mechanism.
A card-edge connector is also an option; your movable board would need to have an appropriate pattern on one of its sides (which should be plated with hard gold if intended for repeated connection and disconnection, or ENIG/ENEPIG for one-time use. Many fabs offer selective hard gold plating ("gold fingers"), which is cheaper than full-board hard gold, specifically for this purpose.), which can then be inserted into a card-edge connector on the other board.
In addition to Hearth's answer, there are also one piece spring loaded contacts and solder-able pads. AVX (Kyocera), Mill-Max and Samtec supply such connectors.
