What Etep said! Listen, a good connection depends on the CLEANLINESS of the metal, so flux is always a good idea. Rosin-core solder is an absolute MUST for electronics work, as it cleans the joint as the LEAD and TIN bond to the COPPER. Does this make things clear? When dissimilar metals are brought into close proximity, they have a tendancy to produce electrolysis, which ultimately corrodes the copper, which is the softest one. Cleaning and sealing the solder joint will prevent this from happening. O.K.?