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Docs Build & Assembly Flux and Solder

Flux and Solder

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The types of solder and flux, when to add extra flux, and how to pick the right diameter.

Flux and Solder

Solder and flux are the two consumables. Choosing the right ones avoids most joint-quality problems.

Solder

  • Rosin-core solder is the only kind to use. The core contains flux that cleans the joint as it melts.
  • Diameter: 0.7-1.0 mm is the sweet spot for this project. 0.5 mm is good for fine work; 1.5 mm delivers too much solder for module pads.
  • Composition:
    • 60/40 lead: easiest to work with, shinier joints. Wash hands after handling.
    • SAC305 lead-free: higher melting point, duller joints, but RoHS-compliant. Work slightly hotter.

Flux

  • Most joints only need the flux in the solder core.
  • Add extra flux (a flux pen or paste) when:
    • Soldering to ground planes and large copper areas that suck heat.
    • Reworking or desoldering.
    • The joint looks dull and refuses to wet.
  • No-clean flux can be left on the board; water-soluble flux must be rinsed. For a field device, no-clean is the practical choice.

A note on lead

Lead solder is fine to use with basic precautions: work ventilated, wash hands after, and never eat at the bench. If you solder regularly, consider lead-free for peace of mind.

How much to use

A correct joint uses the minimum solder that wets both surfaces and forms a concave fillet. If you cannot see the wire silhouette through the solder, you used too much.

Testing a joint

A good solder joint conducts. If a device misbehaves, wiggle test suspect joints with insulated tweezers while watching the multimeter; a loose joint will flicker.