A solder paste in its simplest form is a mixture of a finely divided solder powder in a flux medium. This results in a mobile paste that can easily be applied to the metal pads of a PCB, but will still retain its shape after application. The finely divided solder is spherical and comes in a range of standard particle size distributions, which are defined by IPC. These particle size ranges are common to all solder paste suppliers, yet there may be subtle variances from one supplier to the next. Just as the particle size ranges are standard across the industry, so are the main solder alloys, with all manufactures adhering to the same specification. The flux medium as it is referred to is unique to each manufacture and it is this which confers the specific properties on a given solder paste. The flux medium plays a vital role in the formulation and is what provides the materials ability to wet out to a variety of substrates, print with consistency and accuracy and allow for adequate reflow, even with the smallest of components.