Soldering with Solder Grease: What to Look for

Solder grease or so-called "flux" is not always used during soldering. When fluxes can be used, and what you always have to pay attention to when using solder grease, read this article in detail.

Use of solder grease

Basically, the solder grease (technically correct called "flux") causes a better wetting of the solder joint with the solder. A solder joint is thus more durable.

Solder grease must not be used when

  • soldering boards
  • lack of possibility to wash the flux again
  • a residue-generating flux is not desired

In general, fluxes are always used where parts are already oxidized or there are generally unfavorable conditions for a solder joint. If a joint is no longer bright, clean it roughly and wet it with flux. Always use as little flux as possible.

Mode of action of fluxes

A flux should remove any oxide layer from the metal before soldering . Such oxide layers are formed mainly in older, exposed metal parts. Copper oxidizes quite fast, the oxide layer prevents a good connection with the solder.

Removal of the oxide layer can happen in different ways - many fluxes are highly acidic, others work with other chemical processes.

Designated fluxes

Each lot has its own designated flux . Other fluxes should not be used, even if they are present.The table below contains suitable fluxes for individual solder alloys:
Alloy Solder

Suitable fluxSn60Pb40 (tin-lead)
F-SW 26, F-SW 21Sn62Pb36Ag2 (tin-lead-silver)
F -SW 32All tin-silver, tin-copper and tin-silver-copper alloys
F-SW 32Exception Sn99Cu1 and Sn95Ag4Cu1
mod. RosinException Sn95AgCu0,7
F-SW 34Important during processing

Solder grease must never be used if electronic components are to be soldered together. The possible residues can cause serious problems later. If absolutely necessary, use special fluxes for electronics - they are specially designed for this purpose and guaranteed to be residue-free.

Always pay attention to the acid resistance of the parts with particularly acid-containing fluxes - otherwise corrosion may later threaten under certain circumstances.

Tips & TricksIf you need to solder problematic areas together, you can also tin them first: Apply a thin coat of soldering grease and let the soldering tin run separately over the joint on each side. Then you can easily hold both parts together and heat the two joints together until the solder runs into each other.

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