Moisture Extraction
Prescriptive time and temperature profiles define the removal of absorbed moisture from moisture-sensitive devices. Adherence to a bake out schedule ensures that internal water vapor levels fall below the specific threshold where rapid expansion causes internal delamination or external cracking during the high temperatures of the reflow process. This procedure applies primarily to surface mount components that have exceeded their allowable floor life.
Process Variable
Temperature settings usually range from 40 degrees Celsius for long durations to 125 degrees Celsius for rapid drying cycles. Choosing a specific bake out schedule depends on the thickness of the component body and the sensitivity level assigned by the manufacturer. Lower temperatures protect the integrity of carrier tapes and trays while requiring more hours to reach the desired dry state.
High-temperature cycles typically occur in metal trays to prevent the melting of plastic packaging materials. Moisture migration follows a non-linear rate as it exits the epoxy mold compound.
Acceptance Condition
Excessive thermal exposure introduces risks such as oxidation of the component leads or the formation of intermetallic layers that inhibit solderability. If the bake out schedule exceeds 192 hours at high heat, the physical properties of the lead finish may degrade. Successful completion resets the shelf life of the parts when they are returned to vacuum-sealed bags.
