Epoxy Molding Compound Moisture Absorption Kinetic Fundamentals
Moisture kinetics in epoxy molding compounds dictate reflow vapor pressure, package strain, and MSL floor life across surface mount assembly lines.
Stress Drift
Metrological Instability
Temporal change in the output of a precision instrument is quantified by observing the shift in zero point values during periods of constant environmental conditions. This stress drift affects the accuracy of sensors by introducing a slow, non-linear shift in the zero point or sensitivity. It is often caused by the curing of adhesives and the settling of molded plastics.
Long term stability is compromised when these internal forces are not fully neutralized during the manufacturing process. These shifts are particularly noticeable in the first few hundred hours of operation as the material properties stabilize.
Mechanical Hysteresis
Residual energy stored within the structural elements of a device can lead to unpredictable behavior after a significant thermal or mechanical event. Following a rapid temperature change, stress drift may continue for several hours or days as the internal components reach a new equilibrium. This phenomenon is particularly problematic for pressure sensors and load cells that require a stable baseline.
Protective coatings and specialized mounting techniques are used to isolate the sensing element from these external influences.
Package Interaction
Encapsulation of a silicon die in a plastic resin creates a source of permanent pressure on the circuitry. Over months of operation, the physical properties of the resin change, causing the stress drift to alter the electrical characteristics of the chip. This shift is measured in parts per million and can exceed the total error budget of a high precision system.
High end instruments use ceramic or metal packages to minimize this effect.
Calibration Offset
Compensating for the movement of a sensor’s output requires periodic verification against a known reference. Because stress drift is often irreversible and non-repetitive, it cannot be easily corrected with a simple software algorithm.