Solder Paste is normally used for connecting the terminations of the components with the land patterns on the PCB The paste is usually applied either by Dispensing or by printing the solder using the stencil. Most of the problems are related to paste printing or dispensing only. So the characteristics of the solder paste, Viscosity needs to be checked periodically to get perfect joints.
Composition
Solder paste is basically comprised of Powdered Metal suspended in thick medium called Flux. Flux is added to act as a temporary adhesive for holding the components until the soldering process. The paste is a gray, plasticine-like material. The composition of the solder paste varies with purpose the paste is used for. For example, with plastic packages on a FR-4 board the solder composition used is eutectic Sn-Pb (63%Sn 37%Pb) or SAC alloys (Sn/Ag/Cu), or if one needs high tensile and shear strength tin-antimony (SnSb) alloys. Generally, solder pastes are frequently made up of an alloy of tin and lead, with possibly a tertiary metal alloyed, though environmental protection legislation is forcing a move to lead-free solder (see also:solder, RoHS).
Solder paste is thixotropic, meaning that its viscosity changes 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. Hence it may be very important to stir the solder paste before it is used.
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.
Classification based on size
The solder particle size and shape determines the paste print-ability. A solder ball is spherical in shape; this helps in reducing the surface oxidation and ensures good joints formation with the adjoining particles. Irregular particle sizes are not used as they tend to clog stencil causing printing defects. The pastes are classified based on the particle size by JEDEC J-STD 005 . This is a standard body governing the electronic industry. The table below gives the classification type of a paste compared with the mesh size and particle size.
| Type designation[JEDEC] | Mesh size in micrometres | Particle size in micrometres | Average size in micrometres |
| Type 2 | -200/+325 | 75-45 | 60 |
| Type 3 | -325/+500 | 45-25 | 35 |
| Type 4 | -400/+500 | 38-20 | 31 |
| Type | -500 | 25-15 | 18 |
According to J-STD-004, solder pastes are classified into three types based on the flux types: rosin based pastes, water soluble pastes and no clean pastes. Rosin based pastes are made of rosin, a natural extract from the pine trees. These fluxes need to be cleaned after the soldering process by using CFC. Due to the ban on this material the usage of rosin fluxes is not predominant. Water soluble fluxes are made up of organic materials of glycol bases. There are wide varieties of cleaning agents for these fluxes. A no-clean flux is made up of resins and various levels of solid residues. No-clean pastes not only save cleaning cost but capital expenditure and floor space. However these pastes need very clean assembly environment and inert re-flow environment.
In using solder paste for assemblies we need to test and understand the various rheological properties of a solder paste. A few of them are explained in this section:
Viscosity: Viscosity of a material is internal property of the material which resists the tendency of flow. In this case, solder paste is desired to have varying viscosities at different stress levels. Such kind of material is called thixotropic. When solder paste is moved by the squeeze on the stencil, due to the application of stress on the paste the viscosity breaks down making the paste thin and helping it to flow easily through the apertures on the stencil and when the stress on the paste is removed it regains it shape preventing it from flowing on the PCB. Viscosity for a particular paste is available from the manufacturers catalog and in-house testing is needed sometime to judge the usefulness of paste after some usage.
Slump: Slump is the characteristic of a material to spread after application. Theoretically it is assumed that the paste side walls are perfectly straight after the deposition of paste and remain like that until the part placement. If the paste has high slump value we see a deviation form the expected behavior, as now walls of the paste are not perfectly straight. Slump in a paste should be minimized as it risks formation of bridges between two adjacent pads.
Working life: Working life is the amount of time paste can stay on a stencil without affecting it printing properties. Manufacturer gives 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 is dispensed pneumatically, by pin transfer (where a grid of pins are dipped in solder paste and then applied to the board) or by jet printing where the paste is distributed on the SMT pads like an inkjet printer.
As well as forming the solder joint itself, the paste carrier/flux must have sufficient tackiness to hold components while passing through the various processes, or perhaps moved around the factory.
Printing is followed by pre-heating and reflow (melting).
As with all fluxes used in electronics, residues left behind may be harmful to the circuit, and standards (eg 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, whereas in the
Storage
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 to the solder particles in the raw powder form causes them to oxidize so exposure should be kept to a minimum.
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 requirements are a gentle rise in temperature (preheat) to prevent explosive expansion (solder balling) and to activate the flux. Thereafter the solder melts and the time in this area is known as Time Above Liquidus. Reasonably rapid cool down is a requirement after this.
A good tin/lead solder joint will be shiny and relatively concave. This will be less so with lead-free solders.
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