Rheological Behavior
Dimensional spread describes the tendency of a printed deposit to lose its shape and expand beyond the intended stencil aperture area before the heating cycle starts. Cold solder paste slump occurs at room temperature immediately after the stencil is removed from the board surface. The movement is a function of the viscosity and the metal-to-flux ratio of the paste.
Printing Accuracy
Excessive expansion can lead to bridging between adjacent pads which causes electrical shorts after reflow. Monitoring solder paste slump is a standard part of material qualification for fine-pitch assembly processes. Paste with high stability maintains sharp edges and vertical walls even when subjected to the vibrations of a high-speed pick and place machine.
Reflow Consequence
Temperature increases during the preheating phase cause the flux to thin and the deposit to spread further. This hot solder paste slump is a primary cause of mid-chip solder balls and uneven joint fillets. Controlling the ramp rate in the oven can limit the duration the paste spends in a low-viscosity state.
Proper storage and handling prevent the material from absorbing moisture which would otherwise exacerbate the spreading effect. Flux chemistry is often adjusted to maintain structural integrity at high temperatures.
