Solder paste

Solder paste (also: solder paste ) is a paste-like mixture of solder powder and flux and is used mainly for soldering surface mount devices ( SMD) in the electronics manufacturing by reflow soldering. Furthermore, there are solder paste for brazing on the basis of copper / zinc and silver, and to the resistance soldering. For SMD soldering in electronics manufacturing suitable solder paste is for example about 90% of beads of a tin alloy and about 10 percent of flux. Lead-free solder ( RoHS) consists for example of 96.5 % Sn; 3.0% Ag and 0.5% Cu. The percentages relate, however, to the percentage, by mass. The volume percentages are spread equally on 50% to 50 %.

Overview of solder alloys: see Lot ( metal).

Classification

Solder pastes are classified according to J -STD -005 over the ball size.

The solder paste is screen or stencil printing about 150 microns thick (depending on the type of paste, the finer the thinner ) on the solder pads ( solder pads ) applied to the circuit board and then the electronic components are fitted to the printed circuit board ( glued or pressed only in the solder paste ). In addition to the typical stencil thickness of 150 microns partially thinner stencils with 120 microns, 100 microns and 80 microns are used depending on the requirement. In this case, less solder paste is applied when printing. However, is more solder paste is required, templates can be used with a thickness of 180 microns or 200 microns.

Solder

Thereafter, the thus -stocked print plate is soldered in the reflow soldering, whereby the particles of the solder paste and merge with the pads and component contacts. The flux facilitates the melting process, by lowering the surface tension, prevents oxidation and possibly reduces existing oxide residues. The volatile fraction of the flux evaporates during the soldering process. The non- volatile portion is displaced by the liquid solder and accumulates peripherally on the solder joint. It forms a good electrical conductive solder joint between the components and the solder pads. The volume of the molten solder is compared with the non-molten solder about 50 percent.