Polymer Crosslinking Densities Influence Die Attach Creep Rates
Higher crosslink density suppresses polymer die attach creep rates by restricting free volume and extending rubbery plateau modulus under continuous thermal load.
Arrhenius Extrapolation
Predictive Methodology
A mathematical method for estimating the long term lifetime of electronic materials under thermal stress utilizes elevated temperature tests to accelerate chemical degradation. This predictive technique, known as Arrhenius extrapolation, relates the rate of chemical reaction to absolute temperature through an activation energy constant. Reliable calculations depend on the assumption that the underlying failure mechanism remains unchanged across the entire temperature range.
Thermal Acceleration
Kinetic models must determine the activation energy of the material through multiple high temperature exposure trials. When evaluating silicone or epoxy encapsulants, tests run at three different elevated temperatures provide the rate constants required to plot logarithm of lifetime against reciprocal temperature. The slope of this resulting linear regression establishes the activation energy, enabling the calculation of operating life at normal service limits.
Practitioners run these thermal aging procedures until a specific degradation threshold, such as a fifty percent loss in tensile strength, is reached. The time required to reach this threshold at each test temperature determines the failure point for that specific trial, which provides the raw data for plotting.
Calibration Reference
Reference conditions require precise temperature control to avoid errors in the calculated lifetime. Since temperature appears in the exponent of the equation, a drift of even one degree Celsius during thermal aging causes a large shift in the calculated rate constant. Standard calibration tests utilize high precision laboratory ovens with a verified spatial uniformity of less than half a degree.
Limit Case
The validity of this projection ceases when the polymer undergoes a phase change, such as the glass transition, within the temperature range of the test. Under these circumstances, Arrhenius extrapolation fails because the activation energy changes abruptly above the transition temperature. Measuring the modulus or transition point before setting the test temperature range protects against this physical change.