Calculated Steam Expansion Pressures in Reflowed Plastic Integrated Circuits
Calculated steam expansion pressures in reflowed plastic integrated circuits reach 4.69 MPa at 260°C, causing catastrophic delamination if moisture exceeds critical levels.
Micro-Void Vaporization
Expansion Damage
Rapid internal expansion of trapped moisture within the structures of a plastic component leads to sudden mechanical failure during the high temperatures of soldering. Sudden phase changes from liquid to gas create intense internal pressure inside the epoxy molding compound. This micro-void vaporization occurs when the moisture concentrated at interfaces or within small cavities cannot escape the package fast enough.
The resulting pressure pushes against the internal walls and leads to the deformation or rupture of the housing.
Structural Consequence
Permanent damage often manifests as a bulging of the package bottom or a visible crack along the side of the component. This phenomenon is commonly known as popcorning because of the audible sound sometimes produced during the reflow process. Internal delamination between the die paddle and the molding compound occurs even if no external cracking is visible.
These failures compromise the long term reliability of the electronic assembly.
Prevention Mechanism
Controlling the moisture content of the plastic body through dry storage and vacuum sealing eliminates the source of the pressure. Assembly houses monitor the duration components spend outside of moisture barrier bags to ensure the concentration of water remains below the critical threshold. Preheating the boards slowly can sometimes reduce the rate of expansion.