Retention Resistance
The adhesive resistance offered by solder paste to hold surface mount components in place after placement prevents the parts from shifting during board transit. This holding capability, referred to as tack force, is essential for maintaining the alignment of parts as the conveyor moves the board into the reflow oven. If the paste lacks sufficient stickiness, components can easily migrate or detach when subjected to conveyor vibrations.
Placement Factor
Different paste formulations use specific rosin and solvent mixtures to control the stickiness of the deposit before reflow occurs. High tack force is especially critical for heavy components or for boards that experience rapid acceleration on high-speed transport rails. When the solder paste is printed, its adhesive capability begins to degrade as solvents evaporate into the factory air.
Manufacturing lines set a maximum open-time limit to ensure that the assembly is completed before the stickiness drops below the required threshold. This timing window is tightly tracked in high-reliability assembly environments to prevent component misalignment.
Measurement Standard
Quantifying this adhesive property involves using a standardized probe test to measure the force required to pull a test needle out of the paste deposit. This test ensures that incoming batches of paste meet the specified requirements for component retention. Regular monitoring helps maintain a consistent assembly process without unexpected part movement.
