Paste Rheology
Solder paste formulation requires specific structural control to prevent dimensional collapse during ambient storage on a printed circuit board. Slump resistance measures the ability of deposited material to retain its height and footprint edges after application through an aperture stencil. Thixotropic behavior governs this structural recovery because shear thinning during squeegee passage gives way to rapid viscosity rebound once deposition ceases.
Halide content and solvent volatility influence the drying rate which directly affects edge definition before thermal reflow occurs.
Thermal Slumping
Infrared preheat zones apply rapid heat which reduces vehicle viscosity before the alloy particles reach melting temperature. Slump resistance prevents bridging defects between adjacent fine pitch pads during this crucial temperature ramp stage. Gravitational forces pull unsupported paste volumes downward when surface tension fails to counteract the weight of suspended metal spheres during the soak interval.
Viscosity degradation at elevated temperatures accelerates lateral flow unless resin modifiers create a rigid matrix around the flux vehicle.
Acceptance Limits
Optical inspection systems measure pad encroachment percentages against a strict baseline during stencil printing audits. Slump resistance standards dictate that lateral displacement must remain below ten percent of the original aperture width to prevent electrical shorts. Board fabricators reject lots showing perimeter distortion because excessive spreading compromises solder joint volume calculations established during paste deposition design.
Post print verification methods use high magnification profile measurement to capture height loss and verify compliance with assembly specifications.
