Over time we can expect natural variation in transfer efficiency, especially for a thixotropic material like flux. These variations are the result of shear thinning and relaxation, adhesion to transfer pins, cohesion of the flux, and reactions with the flux and the environment.
In addition to these variables that change during the ball attach process, there are also many variables that initially impact transfer efficiency. Transfer can also be affected by reservoir depth and area, pin diameter and pitch, pin alloy, squeegee blade angle, squeegee material, machine settings, and substrate pad material.
Confused yet? Don’t be – for the most part pin transfer is a very easy process to optimize if you are using the right flux (or solder paste). Even the worst default settings on a machine will transfer flux, so getting started is easy. First, make sure the reservoir height is not too deep (to discourage flux bridging across pins). Then, pick medium parameters on your machine. Parameters like dwell and lift acceleration can be sped up later – after you have an initial set of parameters that transfer an efficient amount of flux.