Solder paste

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Crucible with soft solder paste, 88.75% solder content, RoHS, lead-free

Solder paste (also: solder paste) is a pasty mixture of solder metal powder and flux and is mainly used for soldering surface-mountable components ( SMD ) in electronics production using reflow soldering . There are also solder pastes for hard soldering based on copper / zinc and silver and for resistance soldering.

( Soft ) solder paste suitable for SMD soldering in electronics production consists roughly of 90% of spheres of a tin alloy and the rest, i.e. 10 percent, of flux - in each case in percent by mass. Since the density of the solder metal is a multiple of the density of the flux (made of resin , oil, solvent, salt, water), the proportions of these two components are roughly 1: 1 - that is, about 50 percent by volume .

Today, soft solder for electrics and electronics in solder paste is generally lead-free ( RoHS ) and typically consists of 96.5% Sn , 3.0% Ag and 0.5% Cu .


Greatly enlarged image of a solder paste
Circuit board with applied solder paste, before the electronic components are assembled

Solder pastes are classified according to J-STD-005 by the ball size:

class not
greater than
Max. 1%
greater than
min. 80%
Max. 10%
smaller than
Type 1 160 µm 150 µm 150-75 µm 20 µm
Type 2 080 µm 075 µm 075-45 µm 20 µm
Type 3 050 µm 045 µm 045-25 µm 20 µm
Type 4 040 µm 038 µm 038-20 µm 20 µm
Type 5 030 µm (28 µm) 025 µm 025–10 µm (min. 90%) 10 µm
Type 6 020 µm (18 µm) 015 µm 015–5 µm (min. 90%) 05 µm
Type 7 015 µm 011 µm 011–2 µm (min. 90%) 02 µm (max. 1%)
Type 8 011 µm 010 µm 008-2 µm

The solder paste is applied about 150 µm thick (depending on the type of paste, the finer the thinner) in the screen or stencil printing process on the soldering pads (soldering surfaces) of the circuit board and then the electronic components are fitted onto the circuit board (glued or just pressed into the solder paste) . In addition to the typical stencil thickness of 150 µm, thinner stencils of 120 µm, 100 µm and 80 µm are sometimes used, depending on the requirements. In this case, less solder paste is applied during paste printing. If, on the other hand, more solder paste is required, stencils with a thickness of 180 µm or 200 µm can be used.


The assembled circuit board is then soldered using the reflow soldering process , whereby the particles of the solder paste fuse with one another and with the pads and component contacts. The flux facilitates the melting process by lowering the surface tension, preventing oxidation and reducing any oxide residues that may be present . The volatile part of the flux evaporates during the soldering process. The non-volatile content is displaced by the liquid solder and collects around the solder joint. A soldered connection with good electrical conductivity is formed between the components and the soldering pads. The volume of the melted solder is around 50 percent compared to the unmelted solder paste.

Storage temperature

For (soft) solder paste, refrigerated storage at 1… 10 ° C is recommended. It should be tempered to 25 ° C for processing. Avoid heating to a higher temperature. When soldering, heating typically takes place above 200 ° C.

See also


  • Reinard J. Klein Wassink: Soft soldering in electronics . 2nd Edition. Eugen G. Leuze, Saulgau 1991, ISBN 3-87480-066-0 .
  • Wolfgang Scheel (Hrsg.): Assembly technology of electronics . Verlag Technik u. a., Berlin a. a. 1997, ISBN 3-341-01100-5 .

Web links

Individual evidence

  1. Stannol - soldering tin - Sn60Pb40 and Sn63Pb37 technical data sheet,, accessed March 31, 2020. - Density: 8.5 or 8.4 g / cm 3 .
  2. a b According to DIN 32513
  3. Procedures for Handling AMTECH Solder Paste, Rev. 12/05, probably May 2012, accessed March 31, 2020.