Paste Viscosity
Solder paste rheology during thermal processing relies on hot slump mechanics to prevent bridging defects between adjacent fine pitch land patterns. Elevated temperatures reduce vehicle viscosity before flux activation occurs, causing deposit spreading across masked solder masks. Stencil apertures deposit a defined volume that must resist lateral flow during preheat stages of convection reflow ovens.
Bridging occurs when molten alloy flows into spaces between closely spaced gull wing leads or ball grid array pads.
Thermal Slump
Defect generation accelerates when heating profiles exceed shear thinning thresholds specified for high density surface mount technology assemblies. Viscosity drops sharply as board temperature rises toward liquidus, permitting gravitational sag on vertical surfaces of double sided reflowed assemblies. Automated optical inspection systems measure fillet geometry deviations after solidification to quantify dimensional losses caused by excessive lateral spread.
Manufacturers adjust thixotropic index values in printing parameters to counteract heat induced deformation during the critical ramp to peak zone.
Alloy Collapse
Surface tension forces compete with gravity during solder joint formation, determining whether collapsed deposits coalesce into acceptable fillets or form discrete solder balls. Solder paste manufacturers formulate resin systems with specific polymeric additives to increase yield stress during the initial melting phase. Component weight compounds gravitational sagging when heavy integrated circuits rest on oversized deposits during bottom side reflow passes.
Proper stencil design restricts paste volume to dimensions matching pad geometries, eliminating excess material that feeds liquid bridging failures.
