DaveJustDave wrote:

I'm looking for an inexpensive setup. something big enough that i can dunk a motherboard in.

Define "inexpensive".

I also have a feeling that the size requirements to clean an MB (ATX size?) will likely preclude anything cheap.

Most cheap ultrasonics are quite small; they're aimed at cleaning glasses or jewelry. Anything much larger and you're getting into commercial territory.

You at least in need of a 12lt unit (smaller ones exists but their tank sizes are not enough for a motherboard). This is pretty much an industrial unit, and probably will cost at least more than a thousands of USD. Chemicals (more like powerful detergents really) are generally available from the equipment suppliers and need to be determined according to the materials that will be cleaned.

I have no idea about interaction of chemicals and a PCB though (any destructive side effects?) since I never used or heard of ultrasonic cleaning in such a job. If there is such a process used in industrial scales, it's my bad not hearing any of it but, actually I'm rather familiar with the process (all the factories that I worked for for the last 20 years have ultrasonic degreasing lines installed) and also with PCB cleaning (managed an industrial maintenance team for years).

One reminder, any metal that is not protected against corrosion (i.e. uncoated, unplated, unpainted, steel) will be corroded in a matter of hours after the cleaning, be aware of that fact.