What flux should I use when replacing PCB components?

TraceTech No-clean Flux Pen is formulated to perform at the elevated temperatures of lead-free soldering but is also effective for tin-lead soldering as well. It's designed with a low surface tension to prevent bridging. Post-soldering cleaning is optional because the light residue left after soldering is barely noticeable is non-corrosive and halide free. Flux pens are an excellent way to dispense flux for benchtop soldering. You just hold the pen vertically and briefly depress the tip to start the liquid flow. This will saturate the tip of the pen with flux. Draw flux on the area to be soldered. Gently press the tip again when more flux is needed to keep the tip damp with flux.