SMT Solder Paste Printing Parameters and Viscosity Evaluation Methods
Optimize solder paste print volume and yield by maintaining area ratios above 0.66, controlling squeegee force, and verifying viscosity stability.
Print Speed Optimization
Machine Setpoint
Squeegee movement across the stencil must be fast enough to satisfy production throughput but slow enough to allow the solder paste to roll properly. This print speed optimization balances the forces required to push paste into the apertures with the necessity of a clean wipe. If the speed is too high, the paste might skip over small holes or create insufficient volume.
Conversely, too much dwell time on the board increases the total assembly time. Ideal settings depend on the rheological properties of the specific paste alloy.
Rheological Balance
Fluid behavior of the solder paste changes as the shear force from the blade acts upon the metal filled resin. Through print speed optimization, an engineer identifies the speed that liquefies the paste just enough for consistent filling. Slower speeds typically assist in filling large rectangular apertures.
Higher speeds might be necessary for thin foils to prevent paste leakage behind the blade. The goal is a uniform film thickness across the board.
Cycle Measurement
Determining the total output of the assembly line involves checking the time elapsed from board input to board discharge. While performing print speed optimization, the goal is often to match the printer cycle with the pick and place duration. Total idle time at any machine lowers the efficiency of the entire line.
However, shortening the print time can lower yield if bridges begin to form. Quality checks limit how far the process can be pushed toward speed.