Flux-cored wire for board rework
No-clean flux-cored solder wire is an essential consumable at any PCB repair station. The flux pre-loaded in the wire core lets technicians quickly fix open joints and solder bridges or replace components without cleaning the board after soldering, saving time and chemicals.
However, choosing the wrong diameter, alloy or flux content can leave corrosive residue or dull solder joints. This article helps you choose the right solder wire for each rework task.
Construction of flux-cored solder wire
Solder wire consists of an outer tin alloy and one or more flux cores inside. Flux content typically ranges from 1% to 3.5% by weight. The flux removes the oxide layer on component leads and pads, allowing the molten solder to wet evenly and form a proper fillet.
Choosing the alloy and wire diameter
For lead-free lines, SAC305 alloy (Sn96.5Ag3Cu0.5) produces durable joints that are compatible with the original joints on the board. SnCu0.7 is less expensive and suitable for general repair but requires a slightly higher tip temperature. Wire diameter should be selected according to pad size:
- 0.3–0.5 mm for 0402 and 0603 components and fine-pitch IC leads.
- 0.6–0.8 mm for standard SMD components and connectors.
- 1.0–1.2 mm for through-hole components, wires and large capacitor terminals.
Understanding no-clean flux correctly
No-clean flux leaves very little residue, which is clear and non-conductive once it has been fully heated. What matters is that the tip temperature and contact time are sufficient for the activators to fully decompose. If the tip touches too briefly, residue that has not been fully activated can absorb moisture and cause leakage current on boards operating in humid environments.
Handling tips for the rework station
- Set the tip temperature to around 330–370°C for SAC305, adjusting for the thermal mass of the joint.
- Feed the solder wire into the junction between the tip and the pad, not directly onto the tip.
- Hold the tip for an extra one to two seconds after the solder wets so the flux is fully activated.
- Clean the tip regularly with brass wool instead of an overly wet sponge.
When many points need repair, add a flux pen with the same chemistry as the solder wire, and avoid mixing different fluxes, which can cause residue reactions that are hard to control.
Storage and safety at the repair station
Solder wire reels should be kept in a sealed box, away from dust and moisture that oxidize the surface. Rework stations need a flux fume extraction system positioned close to the work area, as flux fumes can irritate the respiratory tract when technicians work for many consecutive hours.
AMS supplies flux-cored solder wire in a range of diameters, along with matching flux pens and desoldering wick. Contact AMS Vietnam for advice on a repair consumables kit suited to your production line.
