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Solder Paste – Flux

The second main ingredient in solder paste is the flux (vehicle). Flux is a very complex group of chemicals/materials that must be able to do a number of things, some of which must happen simultaneously (a partial list is below). This requires the knowledge of experienced chemists and material scientists.


1)     The flux must not react with the powdered solder while in storage (shelf life). This is aggravated by the wide variety of solder alloys available in paste form. Each alloy family typically requires it own unique flux formulation.

2)     The flux must effectively remove any surface oxides present on the solder powder itself and mating surfaces prior to and during the melting of the solder.

3)     It must effectively prevent re-oxidation of the solder powder and mating surfaces at the elevated temperatures associated with reflow soldering, especially when performed in an air environment (air is ~21% oxygen).

4)     The performance of the flux must be unaffected by a wide range of temperature and humidity conditions.

5)     If the flux is “no-clean”, the residue must be non-corrosive and non-conductive per J-STD-004.

6)     If the flux is RMA, the residue must be easily cleaned with commercially available chemicals.

7)     If the flux is water soluble/washable, the residue must clean thoroughly with heated and pressurized deionized water.

8)     In a stencil printing application the flux must adequately fill and release from the apertures of a wide variety of sizes, shapes and stencil types, not stick to the squeegee, not dry out too quickly on the stencil, retain a brick like shape both at room temperature and elevated reflow soldering temperatures (minimize slumping), provide sufficient tack to hold components in place, not spatter during soldering (boiling of flux solvents), not outgas excessively (voiding)  and provide effective wetting of the solder to a wide variety of board metallizations, component lead platings and package bumps.

9)     In dispensing applications, the paste must dispense smoothly and consistently through a variety of needles sizes either through manual application or a variety of dispensing equipment types and technologies without clogging (and do many of the things listed in item 8).

10) Provide a cosmetically appealing solder joint and flux residue (if “no-clean”).