whatever flux you buy, it should be specifically labeled as no clean flux. There are types that require to be cleaned up and can damage components if not cleaned. I don't know if anyone else has mentioned it in here. I use this brand from amazon: Flux . Joshua bardwell's website fpvknowitall.com has a great list of materials and tools and all that which is where i got that amazon link from.

He also recommends 63/37 solder for what it's worth.