Interfacial Hygroscopic Swelling Stress Analysis in Surface Mount Encapsulants
Interfacial hygroscopic swelling stress arises from differential moisture expansion, compounding thermal mismatch and driving package delamination during reflow.
Reflow Soldering
Thermal Process
A thermal manufacturing method for printed circuit boards temporarily melts solder paste to form permanent electrical and mechanical connections between component leads and substrate pads. The technique of reflow soldering relies on controlled heating of the assembled board through multiple thermal zones. This process ensures that the alloy reaches its liquidus temperature without damaging the heat-sensitive semiconductor bodies.
Gradual cooling solidifies the joints to establish the structural integrity of the electronic assembly.
Thermal Profile
The heating cycle consists of distinct preheat, soak, reflow, and cooling stages. During the soak stage, the flux activates to remove oxides from the metal surfaces, which ensures proper wetting. The temperature is then raised above the melting point of the solder for a brief period known as the time above liquidus.
Precise control of this dwell time avoids excessive intermetallic compound growth, which can weaken the solder joint.
Quality Control
In-line inspection using automated optical systems verifies the geometry and alignment of the completed joints. Solder volume and wetting angles are measured to ensure compliance with international industrial standards. Deviations in the thermal profile are corrected by adjusting the conveyor speed and heater zone setpoints.
Defect Prevention
Incorrect thermal gradients can cause rapid boiling of the flux, which results in solder balls or component misalignment. Maintaining a controlled heating rate prevents these defects and ensures a reliable connection.