Solder Powder Sizes

There are a number of variables affecting dispensing solder paste related to the size of the solder powder. These are:

The powder needs to pass through the opening in a needle. Large particles don’t pass through at all. It has been determined mathematically and in practice that a ratio of 7:1 is required to consistently dispense solder spheres (Silver flakes or other particles that are not spherical require a larger ratio.
There is a mathematical and theoretical answer then there is a real answer. Powders that is imperfect, having large size variability. The variability makes dispensing harder. These require a larger ratio between the opening and powder size.
This variability is normally a lot larger in the smallest powders where is the most important to have consistency. In other words, smaller diameter solder powders are a lot more inconsistent than larger solder powders.
Solder Pastes are normally specified in "percent metals". So a 90% means 90% solder and 10% flux by weight. But the weight is not the key for dispensing. There are 2 ways of looking at this

A better measurement is volume. How much flux covers the solder powder. The volumes are closer to 50% flux and 50% powder.

From This we can draw some general conclusions about the largest powder that can be used with different gage needles