Is there a chemical difference between alloys in solder wire and alloys solder paste that make solder wires solid and solder paste fluids?

For instance, this solder wire and this solder paste both have the composition Sn96.5Ag3Cu0.5, but the wire is solid and the paste is fluid. I know that the solder wire has a core of flux inside, but apart from that I don't understand how at the same temperature one alloy is solid in the wire and fluid in the paste.

  • \$\begingroup\$ If the alloy in the solder paste is liquid, why is it solid after heating by soldering? The alloy in both solder wire and solder paste is solid before and after heating, The alloy is liquid only when heated above the melting point. \$\endgroup\$
    – Uwe
    Apr 11, 2020 at 8:49
  • \$\begingroup\$ its a paste the same way mud or quicksand is a paste. it doesnt mean the sand or dirt itself is ever a liquid \$\endgroup\$
    – DKNguyen
    Apr 11, 2020 at 19:33

2 Answers 2


Solder paste is actually composed of tiny solid balls of solder suspended in flux, so it's not a liquid it just seems like it without a magnifying glass or microscope. The wiki article goes into more detail about the different sizes of solder balls if you're interested in reading further

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  • 4
    \$\begingroup\$ Like how mud is a suspension of solid particles in water, but behaves like a liquid. \$\endgroup\$
    – Vikki
    Apr 10, 2020 at 20:55

The solder granules in solder paste are actually solid, and can be of the same alloy as wire solder.

The flux itself is what is liquid, to facilitate its application onto the circuit board and hold the SMD components in place as hot air is applied to solder them to the board.

  • Soldering paste is meant for soldering SMD parts on a circuit board. Only hot air (or another heat source that doesn't touch the paste) is required.

  • Wire solder is meant for hand soldering other, larger jobs using a soldering iron.

  • \$\begingroup\$ @cjs Thank you. Answer edited. \$\endgroup\$ Apr 12, 2020 at 18:07

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