Thin film SMD resistor reliability
Where I worked at we coated ALL circuit boards with a conformal coating, usually a polyurethane type of spray. Then all of those issues about humidty and self-etching and trace corrosion and capacitor film degrading go away, not to mention creapage currents due to humidity and dust build-up on the board, especially if the board has high voltage greater than 48VAC on it. Thin film resistors and film capacitors have this corrosion issue, which is why the coating is mandatory for many manufacturing plants. The last thing they want is latent failures or worse yet, a burned board that left soot all over the place.