Solder paste (or solder cream) is a mix of small solder particles and flux. It is used exclusively in the automated soldering process reflow soldering.
It's very important that the spheres of metal are very regular in size and have a low level of oxide.
Solder paste is deposited on a printed circuit board by screen-printingstainless steel or polyester screen), pneumatic dispensing or pin transfer (where a grid of pins are dipped in solder paste and then applied to the board). (with a
The solder particles are frequently an alloy of tin and lead, with possibly a tertiary metal alloyed, though newer legislation is forcing a move to lead-free solder (see the article on solder for more details).
Solder paste should be stored in an airtight container at low temperatures (above freezing) but should be warmed to room temperature for use. Air exposure to the solder particles in the raw powder form causes them to oxidize so exposure should be kept to a minimum.
Solder paste is Thixotropic ie it's viscosity (thickness) changes with applied shear force eg stirring. cf non-Newtonian Fluid. The Thrixotropic Index is a measure of the viscosity of the solder paste at rest, compared to 'worked' paste. Hence it may be very important to stir the solder paste before it is used.
As with all flux's used in electronics, residues left behind may be harmful to the circuit, and standards (eg J-std, JIS, IPC) exist to measure the safety of the residues left behind.
In most countries, 'no-clean' solder pastes are the most common, whereas in the US, water soluble paste (which have compulsory cleaning requirements) are common.
Printing the paste is only the first step of the soldering process. It is followed by pre-heating and reflow (melting).
As well as forming the solder joint itself, the paste carrier/flux must have sufficient tackiness to hold components while passing through the various processes, or perhaps moved around the factory.
Microscopic evaluation of solder paste reveals the spherical metallic nature of the paste, which to the naked eye appears a grey, plasticine like material.
Source: Wikipedia
