Question: I know for tinning stranded wires you should not use Type H or M flux, so we are ordering a No-clean solder Sn63Pb37 with a flux designator ROL0. My question is, would you use that ROL0 solder on the entire solder termination when you solder it into a cup or terminal, or use a higher level solder such as ROMI? My second question is what kind of solder would you suggest for doing repair or adding a component to a surface mount board?
Answer: The type of flux to be used is determined by the surfaces being soldered, such as HASL, OSP, ENIG, Gold, Pd, etc. The more oxides which have to be removed, the more aggressive the flux has to be, therefore if the ROL0 flux works for you, then continue using it as you solder the wire into the terminal itself and any other of your solder joints. Keep in mind, however; that the rosin flux residues may need to be cleaned off and this is where problems can develop. You will have to define the type of solvent to be used to remove the rosin flux residues be it either manually or through a solvent cleaning systems.
With regard to your second question, the solder should be the same alloy as whatever is being used to solder the wires or whatever solder is used on the boards. If the boards were processed with lead-free solder, then the solder to use for the replacement of the components must be lead-free. The alloys of SAC 305 and Sn100C will work together so mixing of those two should not be a problem. The concern, however; is the flux and can it be removed after the replacement of the component? Many times today the industry is using a low-solid content flux (No Clean) to do this soldering and the residues are left behind as they are benign and non corrosive. Verify no additional flux was used to make sure you don’t have flux that was not exposed to soldering temperature as low solid content flux is active until it reaches soldering temperatures.