10CC syringe of Amtech NC-559-ASM-UV (also know as Amtech NC-559-ASM-TF).
Soldering flux is used before and during soldering to prevent oxidation and create better, cleaner solder joints. Amtech is a very reputable brand in the electronics repair industry, used by many professionals.
This No-Clean flux doesn’t require extra additional steps to clean the PCB after use. Some fluxes are water-washable meaning they should be washed off the PCB with distilled water after use to prevent corrosion.
This product is available in a 10CC syringe for ease of use. It does not include plunger or needle. For most basic soldering tasks, these aren’t required. A plunger and needle might help dispense most precise amounts for very delicate repairs.