What must not be used for no-clean soldering according to the guidelines?

No-clean soldering techniques are specifically designed to use fluxes that do not require cleaning after soldering, thereby simplifying the assembly process and improving efficiency. The guidelines indicate that ORL1 flux materials are appropriate for no-clean applications, but they may not be the ideal choice in all circumstances.

The correct answer pertains to the prohibition of acid-based flux for no-clean soldering. Acid-based fluxes contain aggressive cleaning agents that can leave corrosive residues even if not visible after the process. These residues can lead to long-term reliability issues, such as corrosion of metal contacts and surfaces, which is contrary to the intent of no-clean practices.

Water-soluble flux, while suitable for some soldering processes, requires cleaning because it leaves residues that can attract moisture and cause reliability problems over time. Rosin flux, on the other hand, is specifically formulated to do well in no-clean applications; it creates minimal residues that are non-corrosive.

Understanding the different types of flux and their compatibility with no-clean soldering processes is crucial for ensuring the long-term reliability and performance of soldered joints in electronic assemblies.