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.
Reflow Temperature Profiles
Thermal Recipe
Time-temperature relationships guide printed circuit board assemblies through the soldering furnace to melt and solidify the solder paste correctly. Appropriate reflow temperature profiles ensure that the solder paste melts fully, wets the component leads and board pads, and cools without creating excessive thermal stress or voiding. These profiles are divided into distinct zones, which include preheat, soak, reflow, and cooling, with each zone having a specific temperature rate of change and target range.
Setting the profile correctly prevents defects such as component tombstoning, solder balling, or insufficient wetting.
Profile Optimization
Thermocouples are attached to test boards and passed through the oven to record the actual temperatures experienced by various components on the board surface. This thermal data is analyzed by profiling software to ensure that the maximum temperature remains below the damage threshold of the most sensitive components while still exceeding the liquidus point of the solder. The process engineer adjusts the belt speed and zone temperatures of the furnace to align the measured curve with the solder manufacturer recommendations.
This fine-tuning is necessary because different board designs possess unique thermal masses depending on their copper distribution and component density.
Boundary Limit
Imbalanced profiles lead directly to defects on the assembly line. If the temperature rises too rapidly in the preheat zone, the solvent in the solder paste boils violently, which causes solder beads to scatter across the board. Conversely, if the cooling rate is too slow, the resulting solder joints will have a coarse grain structure that is susceptible to early fatigue failure.