Solder Paste

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Solder paste (or solder cream) is a mix of small solder particles and Flux (metallurgy). It is used extensively in the automated soldering processes wave soldering and reflow soldering.

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 a mix of tin and lead, 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 causes the solder particles to oxidise so exposure should be kept to a minimum.

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).

Manufacturers Note that this list is not exhaustive.

Interflux Electronics
Indium Corporation
Alphametals (Cookson Electronics)
AIM