Predicting Local Flow Separation and Thermal Collapse near High Mass Components in Forced Convection Ovens
Preventing thermal collapse near high-mass components requires optimizing zone convection vectors rather than reducing line speed.
Thermal Collapse
Solder Failure
Structural failure of a solder deposit occurs when the material loses its shape and spreads beyond the intended pad area before the alloy reaches its melting point. A thermal collapse is typically caused by a premature drop in the viscosity of the solder paste flux system during the ramp up phase of the reflow cycle. If the paste cannot maintain its form, the resulting solder bridges create electrical shorts between adjacent pins.
Viscosity Reduction
As the temperature inside the reflow oven rises, the resins and solvents within the solder paste soften and expand. If the heating rate is too slow or the chemistry of the flux is not matched to the profile, the paste spreads out across the solder mask. This phenomenon is distinct from the actual melting of the metal particles, occurring instead while the solder is still in a solid state.
Thermal collapse is particularly problematic for fine pitch components where the spacing between pads is minimal.
Mechanical Boundary
Controlling the ramp rate of the oven helps to keep the flux volatile components from escaping too quickly or causing the deposit to slump. Selecting a solder paste with a higher slump resistance grade is a common solution for designs with dense pad arrays. When thermal collapse occurs, the final solder joint may be thin or irregularly shaped, compromising the long term reliability of the connection.
Proper storage of the solder paste and adherence to the recommended shelf life also prevent the chemical changes that lead to this failure.