Solder paste

The main advantage of pasta - ease of mechanization of work. Paste is applied with special dispensers or stencil process. The paste can be applied in an even, precisely defined layer using mechanized and automated means, which provides significant savings in solder (30-50%) ^[1] .

Solder Paste Requirements

• should not be oxidized , strongly and quickly stratified;
• it is desirable to preserve its rheological properties for a long time (that is, the ability for viscous flow and deformation );
• should not be spread far beyond the originally applied dose;
• should not leave solid waste residues after soldering;
• must have adhesive properties;
• should not be sprayed when exposed to a sufficiently concentrated heat source;
• should not degrade the technical characteristics of the board ;
• should be washed in standard solvents .

Solder Paste Characteristics

1. Solder composition
   All pastes contain traditional electronics tin solders. In addition to traditional lead solders, lead-free are increasingly common. There are also various alloying additives that improve the quality of soldering, such as silver.
2. Solder Particle Size
   The particle size of the solder has a strong influence on the properties of the paste. The presence of large particles significantly impairs the rheological properties, and a large number of small particles impair the fluidity of the paste. The most used size of solder particles corresponds to type 3 according to IPS classification (25 - 45 microns). Some precision dispensers require the use of fine pastes.
3. Viscosity
   Viscosity of pastes intended for dosing should be in the range of 300 - 450x10 ³ cP. Viscosity of pastes intended for application through a stencil should be in the range of 650 - 1200x10 ³ cP.
4. Particle shape
   The shape of the particles largely determines the ability of the paste to be dosed in one way or another. If the particles have an irregular shape - oblong or in the form of scales, then such paste begins to clog the small holes in the stencil grid or dispenser syringe. For such pastes, the only possible option is dispensing through a metal mask - stencil. Spherical solder particles give the paste the ability to easily pierce through narrow mesh or dispenser openings.
5. Solderability
   Solder solder paste depends on the oxidation and contamination of the surface of the solder powder particles. The amount of oxygen in the thin near-surface layer, which reacts at the very beginning of the process with the flux and the base metal, is important. By international standards, its content should not be more than 0.5% ^[1] . Carbon also affects the surface of powder particles from containers and packaging during storage and transportation. Therefore, at all stages, from the manufacture of powder to soldering, it is necessary to take all measures against the interaction of the powder with oxygen and carbon.

The main physico-chemical properties of solder pastes are determined by the introduction of solder powder 4-15% of the binders. It is they (sometimes with the addition of solvent) that give the paste the desired consistency, prevent its separation and spreading, give adhesive properties, adhesion to the substrate. The binder is neutral to the solder during storage and soldering, and when heated, volatilizes or melts without the formation of hard-to-remove solid residues. Organic resins or their mixtures, diluents and other substances are used as binders. They are added plasticizers , thixotropic substances. The latter prevent the solder powder particles from settling during storage and provide a specified viscosity range.

Solder paste application

Standard application of solder paste is made by screen printing . An alternative to this process is the pointwise application of the drops of the paste with a dispenser ; however, this is less productive.

For screen printing pastes are supplied in containers of 500 grams. For dispensers pastes are supplied in special disposable cartridges (syringes) of 30 or 125 grams.

Store the paste must be in the refrigerator, otherwise it begins to split.

Screen printing

Screen printing machines, according to the principle of operation, differ little from the machines for silk-screen printing works, but the stencils themselves are made of metal sheets ^{[2] [3]} . Metal stencils provide greater accuracy, allow you to cut windows up to 0.1 mm wide. Special metal stencils allow you to set different stencil thickness, applying a paste of different thickness to different parts of the printed circuit board.

Screen printing machines are manual and automated. Machines for high-precision stencils have a four-sided mechanism for tensioning the web, machines simplify the stencil for only two sides. All machines are equipped with means of fine adjustment of the stencil position. To increase productivity and quality, they are sometimes equipped with a stencil cleaning system that prevents the board surface from being contaminated with solder paste.

Pastes for water supply systems have specific requirements, therefore they should not be confused with pastes for microelectronics. First of all, it is a question of sanitary and hygienic requirements.

• Neither solder nor flux should contain toxic substances. Solders do not contain lead and other toxic metals.
• The flux should not be corrosive and should be easily washed off with water.
• To increase the mechanical strength and durability of the compound, copper or silver is added to the solder, which raises the melting point and makes pastes for pipes unsuitable in microelectronics.

• V. Kuzmin, "Materials for soldering electronic components in the production of modern electronic equipment", Electronic Components, No. 6, 2001
• A. Medvedev “Updating Technologies in the Russian Electronic Industry”, Technologies in the Electronic Industry, No. 1, 2006
• A. Bolshakov “Is your paste suitable for dosing? Factors Affecting the Right Choice, Technologies in the Electronic Industry, No. 2, 2005