Solder Paste – Mixing and Metal Loading

Once the flux has been formulated and scaled up it then must be mixed with solder powder. The mixing procedure and equipment must be capable of providing a homogenous product batch after batch after batch. However, because certain aspects of the solder paste must perform in a certain manner an optimum viscosity must first be determined by finding the proper powdered solder to flux ratio, or the “metal load”. Metal load is expressed in percent by weight.

Particle size impacts viscosity. Therefore the metal loading must be adjusted accordingly. The appropriate particle size for a paste is determined by the aperture sizes for stencil printing and the gauge (inner diameter) of the needle for dispensing applications. (Simply put, smaller particles are needed to fit through smaller holes.)

Each flux vehicle, solder alloy and particle size will have its own unique optimum metal load. The metal load is also application dependant. The optimum metal load for stencil printing will be higher than the optimum metal load for dispensing.

It just so happens that the optimum metal load (for stencil printing), expressed by weight percent, often equates to about 50% metal and 50% flux by volume.

Example Solder Paste Metal Load:

*The dispensing metal load is often not as sensitive to particle size as stencil printing. It could very well be that a metal load of 85% with type III may also be appropriate with type IV powder as well. The amount of powder has already been reduced enough to offset compaction due to smaller particle size.

All solder pastes have a shelf life. As mentioned earlier the flux can begin to react with the solder powder when both are in direct contact with each other in the paste form. The flux itself, often containing organic ingredients, can degrade over time. At a point, the paste will no longer be usable. The best way to optimize the shelf life of a paste is to keep it refrigerated.