Mixing RAM voltages is asking for trouble.

Don’t do it. The system will likely fail to boot, run unstable, or — in the worst case — damage the memory controller or the soldered RAM.

Soldered RAM is fixed. The motherboard expects that voltage. If you install a replaceable module with a different voltage (1.2V vs 1.35V), the memory controller has to negotiate between them. Most laptops and soldered-RAM boards don’t support this well. They either refuse to POST, crash randomly, or force both sticks to the same voltage — which can stress the module that wasn’t designed for it.

Even if it boots, expect random crashes, blue screens, or corrupted data. You’re not saving money here.

Stick to the exact same specs as the soldered RAM, or don’t upgrade at all.