Solder paste is not a commodity, and it is important to understand why. Solder paste is likely the most complex material used in any manufacturing process. I think most people know that solder paste is a combination of solder powder and flux. At first blush, it seems quite simple. However, even the solder powder is not a commodity. In superior solder paste, the powder size distribution is well controlled, the solder particles are spherical and not oxidized. Poor solder powder has powder particles that are non-spherical and oxidized.
Many of you may be familiar with type three, type four, and type five powders. All of these powders have certain size distribution requirements. Superior solder paste will have powder sized distributions consistent with the requirements. Starting with a poor solder powder makes guaranteeing the right powder size distribution difficult, and not all fluxes are equal.
The flux is a complex mixture of solvents, activators, gels and oxygen barrier components. It has many important tasks to perform. The solvents dissolve the other chemicals while the gel compounds give the solder paste its rheological properties. The rheological properties are important for the first step in the PCB assembly process: Stencil printing.
Rheological properties are the thickness or viscosity properties of the paste and must be just right to perform well. A solder paste must be thixotropic, meaning that its viscosity decreases sharply as the paste is forced through the stencil apertures, but the viscosity increases dramatically when the solder paste is at rest right after it's printed. The result is a crisp, sharp solder paste deposit that maintains its shape after printing.
Next, the solder paste deposit must maintain its shape and not slump. After the components are placed on the printed circuit board, they must be held by the solder paste. This property is called tack. During the reflow process, the solder paste must dissolve any oxides on the component leads and PWB pads. This task is the main responsibility of the activators. However, the oxygen barrier chemicals must protect these newly oxide-free surfaces through the reflow process.
Of course, one wants to assure that the quality of the solder joint is acceptable, but long-term reliability is also a concern. The solder joint must pass thermal-cycling and drop-shock testing to assure mechanical reliability. In addition, an important thing to consider is that, all 15 of these features must be very good-to-excellent for a superior solder paste. If a solder paste is terrific at 14 of the 15 but was poor at, say, electrical surface installation, it would be unusable.