A solder paste is essentially powdered metal solder suspended in a thick medium called flux. Flux is added to act as a temporary adhesive, holding the components until the soldering process melts the solder and makes a stronger physical connection.[dubious – discuss] The paste is a gray, putty-like material. The composition of the solder paste varies, depending upon its intended use. For example, when soldering plastic component packages to a FR-4 glass epoxy circuit board, the solder compositions used are eutectic Sn-Pb (63 percent tin, 37 percent lead) or SAC alloys (tin/silver/copper, named for the elemental symbols Sn/Ag/Cu). If one needs high tensile and shear strength, tin-antimony (Sn/Sb) alloys might be used with such a board. Generally, solder pastes are made of a tin-lead alloy, with possibly a third metal alloyed, although environmental protection legislation is forcing a move to lead-free solder.
Solder paste is thixotropic, meaning that its viscosity changes over time with applied shear force (e.g. stirring). The thixotropic index is a measure of the viscosity of the solder paste at rest, compared to "worked" paste. Depending upon the formulation of the paste, it may be very important to stir the paste before it is used, to ensure that the viscosity is appropriate for proper application.
The size and shape of the metal particles in the solder paste determines how well the paste will "print". A solder ball is spherical in shape; this helps in reducing surface oxidation and ensures good joint formation with the adjoining particles. Irregular particle sizes are not used, as they tend to clog the stencil, causing printing defects. To produce a quality solder joint, it's very important for the spheres of metal to be very regular in size and have a low level of oxidation. Solder pastes are classified based on the particle size by JEDEC standard J-STD 005. The table below shows the classification type of a paste compared with the mesh size and particle size.
Type designation [JEDEC]
Mesh size in lines-per-inch
Max. size (no larger than)
Max. size (less than 1% larger than)
Particle size in um (80% min. between)
Avg. size in um
Avg. size in um (10% max. less than)
According to JEDEC standard J-STD-004 "Requirements for Soldering Fluxes", solder pastes are classified into three types based on the flux types: Rosin based pastes are made of rosin, a natural extract from pine trees. These fluxes need to be cleaned after the soldering process using chlorofluorocarbons (CFCs). Due to the ban on CFCs, rosin fluxes are no longer predominant. Water soluble fluxes are made up of organic materials and glycol bases. There is a wide variety of cleaning agents for these fluxes. A no-clean flux is made with resins and various levels of solid residues. No-clean pastes save not only cleaning costs, but also capital expenditures and floor space. However, these pastes need a very clean assembly environment and may need an inert re-flow environment.
In using solder paste for circuit assemblies, one needs to test and understand the various rheological properties of a solder paste.
Viscosity The degree to which the material resists the tendency to flow. In this case, varying viscosities of solder paste are desired at different levels of shearing force. Such a material is called thixotropic. When solder paste is moved by the squeegee on the stencil, the physical stress applied to the paste causes the viscosity to break down, thinning the paste and helping it flow easily through the apertures on the stencil. When the stress on the paste is removed, it regains it shape, preventing it from flowing on the circuit board. The viscosity for a particular paste is available from the manufacturer's catalog; in-house testing is sometimes needed to judge the remaining usability of solder paste after a period of use.
Slump The characteristic of a material's tendency to spread after application. Theoretically, the paste's sidewalls are perfectly straight after the paste is deposited on the circuit board, and it will remain like that until the part placement. If the paste has a high slump value, it might deviate from the expected behavior, as now the paste's sidewalls are not perfectly straight. A paste's slump should be minimized, as slump creates the risk of forming solder bridges between two adjacent lands, creating a short circuit.
Working life The amount of time solder paste can stay on a stencil without affecting its printing properties. The paste manufacturer provides this value.
Solder paste is typically used in a screen-printing process, in which paste is deposited over a stainless steel or polyester mask to create the desired pattern on a printed circuit board. The paste may be dispensed pneumatically, by pin transfer (where a grid of pins is dipped in solder paste and then applied to the board), or by jet printing (where the paste is sprayed on the pads through nozzles, like an inkjet printer). As well as forming the solder joint itself, the paste carrier/flux must have sufficient tackiness to hold the components while the assembly passes through the various manufacturing processes, perhaps moved around the factory. Printing is followed by pre-heating and reflow (melting).
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 requirement is a gentle rise in temperature to prevent explosive expansion ("solder balling"), yet activate the flux. Thereafter, the solder melts. The time in this area is known as Time Above Liquidus. A reasonably rapid cool-down period is required after this time. A good tin/lead solder joint will be shiny and relatively concave. This will be less so with lead-free solders.
As with all fluxes used in electronics, residues left behind may be harmful to the circuit, and standards (e.g. 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; in the United States, water-soluble pastes (which have compulsory cleaning requirements) are common.
Solder paste should be stored in an airtight container at low, but above freezing, temperatures. It should be warmed to room temperature for use. Exposure of the solder particles, in their raw powder form, to air causes them to oxidize, so exposure should be minimized.
- Teardown of iPad 2
- JEDEC Solid State Technology Association (formerly the Joint Electron Devices Engineering Council)
- Solder Paste Task Group (January 1995). "J-STD-005 Requirements for Soldering Pastes". Arlington, Virginia: Electronic Industries Alliance and IPC.
- Tarr, Martin. "Solder paste basics". Online postgraduate courses for the electronics industry. University of Bolton. Retrieved 2010-10-03.
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- Restriction of Hazardous Substances Directive (RoHS)
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