Flux-cored wire: how to choose diameter, flux and alloy
A flux-cored wire is defined by three choices: the alloy, the flux contained in the core and the diameter. Getting one wrong means cold joints, too much solder on the pads or residues to clean. This guide summarises the criteria for choosing the right wire for hand soldering, rework and soldering robots.
Dickmann Srl · updated
1. The diameter
The diameter should be chosen according to the pad and the thermal mass of the joint. A wire that is too thick deposits excess alloy and creates bridges; one that is too thin slows down work and forces you to keep the tip on the joint for longer.
| Diameter | Typical use |
|---|---|
| 0.3 – 0.5 mm | Small SMD components and precision work |
| 0.5 – 0.8 mm | SMD and small THT components |
| 0.8 – 1.2 mm | THT components and general use |
| 1.5 – 2 mm | Wiring, connectors and large cross-section conductors |
2. The flux
The most common versions contain 2 to 3 % flux by weight, in a single core or in several micro-cores for more uniform delivery. More important than the percentage is the class: for electronics, low-residue ROL0 fluxes are used; for difficult surfaces, more active fluxes with cleaning of residues. See the guide to IPC J-STD-004 classification.
| Requirement | Flux | Flux-cored wires in the range |
|---|---|---|
| Electronics, minimal residues | No-clean ROL0 | Dixtra No Clean, AIM Glowcore |
| Electronics, rosin flux | Rosin ROL0 | Alfa C, Alfa E |
| Difficult surfaces, fast soldering | Rosin ROM1 | Impetol B |
| Structural soldering on copper, brass, iron | Organic | Dixtra Gel, Dixtra Glico |
3. The alloy
Equipment subject to RoHS requires lead-free alloys: SAC305 as the standard, low-silver SAC0307, silver-free Sn99.3Cu0.7 and 99SnCuNiGe. Tin-lead alloys, such as 63Sn37Pb and 60Sn39PbCu1, remain for uses outside the scope of RoHS or covered by exemptions. Alloys with a small addition of copper reduce soldering iron tip wear.
4. Tip temperature
With lead-free alloys the tip typically works between 330 and 370 °C, with tin-lead alloys between 300 and 340 °C. A tip of the right shape and mass for the joint is better than a higher temperature: excessive heat degrades the flux before it has done its job and shortens tip life.
Soldering robots
Flux-cored wires are also used on the wire feed systems of soldering robots: diameter and reel depend on the system. For automatic top-up of solder pots, on the other hand, solid wire without flux is used.