Solder paste
From Wikipedia, the free encyclopedia
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-printing (with a stainless 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).
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 oxidise so exposure should be kept to a minimum.
Solder paste is Thixotropic. Its viscosity changes with applied shear force (e.g. stirring). 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 fluxes 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.
[edit] Classification
Different solder pastes are distinguished mainly by the type of flux and solder that is used to make it up and by the relative proportions of these components.
Apart from the composition of the metal particles, they are also classified by their size and shape. There are three main classifications for the particles:
- Size A (grade 1)
- -325 +500 mesh (falls through a mesh with 325 threads/centimetre but is caught by a mesh with 500 threads/centimetre)
- 30 micrometres average particle diameter
- 45 micrometres maximum particle diameter
- Size B (grade 2)
- -200 +325 mesh (falls through a mesh with 200 threads/centimetre but is caught by a mesh with 325 threads/centimetre)
- 45 micrometres average particle diameter
- 75 micrometres maximum particle diameter
- Size C (grade 3)
- -100 +200 mesh (falls through a mesh with 100 threads/centimetre but is caught by a mesh with 200 threads/centimetre)
- 75 micrometres average particle diameter
- 150 micrometres maximum particle diameter
Note that Size A is the finest (smallest particles) and that Size C is the coarsest (largest particles).
The paste manufacturer will suggest a suitable reflow temperature profile to suit their individual paste, however one can expend too much energy on this. The main requirements are a gentle rise in temperature (preheat) to prevent explosive expansion (solder balling) and to activate the flux. Thereafter the solder melts and the time in this area is known as Time Above Liquidus. Reasonably rapid cool down is a requirement after this.
A good Tin/lead solder joint will be shiny and relatively concave. This will be less so with lead-free solders.
[edit] Manufacturers
Note that this list is not exhaustive.
