Secondary Reflow

Joint Stability

Molten solder at the initial interface stays held by surface tension alone during the subsequent heating stage. Support fixtures or adhesive dots prevent heavy components from falling off the board when the board rotates to pass through the heating zones upside down. Gravity exerts a force on the component mass that opposes the wetting power of the liquid solder.

If the thermal profile runs too hot or lasts too long, the initial solder connections reach a liquidus state which leads to the formation of intermetallic layers that become thick and brittle over time. Precise control of the conveyor speed and the zone temperatures minimizes the amount of heat energy the assembly absorbs during the repeat exposure.

Process Limitation

Manufacturing yields drop when the total number of heat cycles exceeds the fatigue threshold of the board laminate or the component package. Each exposure to extreme temperatures causes microscopic expansion and contraction in the copper traces and the dielectric material. Designers specify high glass transition temperature materials to increase the resistance of the circuit to deformation during these repeated heating events.

Multiple reflow cycles accelerate the aging process of the internal circuitry and the surface finishes. Excessive thermal stress during the second cycle causes warpage that disrupts the planar alignment of the assembly.