Deposition Deformation
The tendency of solder paste to spread or deform outward after stencil release and before reflow affects the spacing and definition of printed deposits. This unwanted material deformation, known as print slump, reduces the gap between neighboring solder deposits on high-density circuit boards. If the slump is excessive, it increases the likelihood of adjacent deposits merging to form defects.
Viscosity Influence
Controlling the rheological properties of the paste, especially its viscosity and yield point, prevents this material movement. When the ambient temperature on the assembly floor rises, the solder paste softens and the rate of print slump accelerates. Solder paste formulations contain specialized gelling agents to maintain structure under gravity during the delay between printing and component placement.
Engineers monitor the temperature and humidity of the printing room to keep the material within its optimal operating window. This environmental control is especially critical when using ultra-fine pitch components where the spacing between pads is less than two hundred micrometres.
Assembly Risk
Excessive spreading of the deposit leads directly to solder bridging and short circuits during the reflow process. Automated optical inspection after printing catches slump before the boards enter the pick-and-place machine, allowing operators to wash and reprint boards with bad deposits. This early detection keeps defect rates low and avoids expensive rework of populated assemblies.
