The rule is simple: Water and electronics don't mix. However, in practice it's closer to "humidity, electronics, and energy". Under the right conditions, washing a piece of hardware poses no risk, and there are occasions when a good immersion is the only way out. For example, Andrew from the channel Cat and Andrew received a motherboard impregnated with dirt and cigarette smoke, a horrendous combination for any equipment...
Imagine you've bought a used computer online, and when you receive it you smell that characteristic cigarette odor. A superficial cleaning will help reduce its impact, but I can say from extensive experience (I grew up surrounded by smokers) that cigarette smoke sticks to everything, and the ideal is a deeper cleaning session.
The question is: How deep? Many enthusiasts and hardware YouTubers have access to large ultrasonic cleaners, but a model with the necessary dimensions can easily exceed 250 euros. The best alternative is to wash hardware components, and Andrew from the Cat and Andrew channel shows us that the main resource is patience:
How to Safely Wash Hardware
This project starts with an MSI H61M-P20 (G3) motherboard, based on the Intel LGA 1155 socket. It's an old platform that doesn't go beyond Ivy Bridge processors, but with a little work it can become a solid terminal. The trained eye will quickly detect the disgusting brown layer covering every square centimeter of its surface. Andrew reports that the motherboard "doesn't work", but the dirt even reached the underside of the processor.
The real reason behind the failure is a series of bent pins in the socket; however, Andrew managed to repair the damage. With the motherboard working again, he removed the battery (very important!), the metal socket support, the chipset heatsink, and a label. Removing all the dust with brushes is always a good idea, but the most impressive part comes with his combination of kitchen degreaser and isopropyl alcohol. The color of the liquid coming out of the motherboard is chilling...
For the actual washing, Andrew uses demineralized water, but beyond the absence of impurities, the important thing is not to rush the process, and that includes the "displacement" of the water with an additional bath of isopropyl alcohol, assisted by a hair dryer on "cold". Metal parts always give us a bit more flexibility, and Andrew takes advantage of it using hot tap water (the last image of the heatsink says it all). To finish, the processor and its fan only receive isopropyl alcohol.
