Usually automatic, but don't count on it.

It depends on the hardware, but in most cases the BIOS auto-detects soldered RAM changes via SPD data. The memory controller handles rank and density on its own. You don’t need to flash a custom BIOS just to upgrade a soldered chip.

That said, there’s a catch. Some devices have locked memory training or strict SPD checks – think ThinkPads, MacBooks, or certain soldered-only Chromebooks. If the BIOS rejects the new RAM at boot (black screen, beeps), that’s when you need a custom or patched BIOS to bypass the whitelist or force training. Not common, but it happens.

Bottom line: Try the upgrade first. If it doesn’t work, then start hunting for a modified BIOS. Don’t preemptively flash something you might not need.

Just know that soldered RAM upgrades are risky – one bad solder joint and the whole board is toast. So if you’re already at that level, a custom BIOS is the least of your worries.