Microvia Stress Mechanics under Reflow Temperature Profiles
Substrate Z-axis thermal expansion above Tg drives low-cycle fatigue and target pad separation in HDI microvias during SAC305 lead-free reflow profiles.
Thermal Soak Duration
Soak Time
Time spans within the reflow oven allow the entire printed circuit board assembly to reach a uniform temperature before entering the liquidus zone. The length of the thermal soak duration is designed to evaporate volatile solvents from the solder paste and activate the flux to remove surface oxides. This processing step must be long enough to bring the heaviest and lightest components on the board to the same temperature, which prevents thermal gradients during the subsequent melting phase.
If this step is too short, some solder joints may not melt completely during the reflow peak.
Process Control
Thermocouples attached to the test boards monitor this time window to ensure it falls within the window of sixty to one hundred and twenty seconds. The process engineer optimizes the length of this window by adjusting the conveyor belt speed and the temperatures of the convection heaters in the soak zones of the reflow furnace. This adjustment must be done carefully because excessive exposure to heat can exhaust the solder paste flux before the actual soldering begins.
If the flux is exhausted, the solder will oxidize during the reflow phase, which leads to poor wetting and weak solder joints.
Thermal Constraint
Boards with high thermal mass require longer periods to reach thermal equilibrium than thin, double-sided boards. This material requirement means that the reflow furnace settings must be recalculated for each distinct assembly design to prevent insufficient heating. Overheating the board during this phase must be avoided as it can cause damage to the epoxy-glass laminate.