Many factors contribute to a solder paste’s overall performance for the application at hand. Some of these include: alloy composition, flux composition, alloy to flux ratio, powder size, viscosity, and slump. Solder paste slump is the likelihood for paste to spread on the PCB after it has been applied. Hypothetically, paste should remain in its original state until a component is placed on it; however, solder paste is categorized as a fluid, and thus tends to spread—or slump. This can lead to undesired bridging between pads, causing shorts, or inadequate joint stand-off height, causing joints that are less robust and more prone to cracking. To avoid solder paste slump, optimizing the contributing factors for each specific application is crucial for ideal performance and ease of use.
Solder paste slump is largely related to the viscosity (the state of being thick, sticky, and semifluid in consistency, due to internal friction) and thixotropy (the property of becoming less viscous when subjected to an applied stress) of the paste. Additionally, there are two types of solder paste slump: cold and hot. (These are literal, based on exposure to heat or not.) The degree of cold slump is determined by the height of the solder paste deposit, the viscosity/thixotropy agents in the flux vehicle, and the volatility of the binders, which impact the speed at which the paste begins to dry. Contributing factors such as taller paste deposits and lower metal load pastes increase the likelihood of experiencing cold solder slump. Hot slump occurs when applied heat increases the mobility of the flux vehicle, and it consequently becomes less able to keep the solder particles in suspension due to gravity. Water-soluble fluxes are more likely to experience hot slump than no-clean formulations.
The effect slump can have on applications is more apparent with fine pitch components—the smaller the distance between components, the more susceptible the components are to bridging due to slump. The IPC Slump Test evaluates the slump behavior after printing at both cold and hot conditions, along with two different stencil thicknesses. Ideally, it’s best to see no connection at the second closest pitch, which is a little tighter than for standard SMT. In addition to mitigating risk factors within the paste’s chemical properties, slump can be reduced by controlling the wait time between the paste being deposited on the solder pads and reflow, as more wait time gives the paste more opportunity to absorb moisture from the environment. Optimizing print speed is also important as a print speed that is too high will cause loss of viscosity that can lead to slumping.