Silicon Piezoresistive Pressure Diaphragm Encapsulation Selection Criteria
Encapsulation selection balances gel potting low cost against oil-filled cell thermal stability to control long term zero drift and field recalibration expenses.
Moisture Absorption Shear Stress
Material Degradation
Polymer adhesives and encapsulation compounds undergo structural changes when exposed to humid environments over long periods. The metric known as moisture absorption shear stress measures the reduction in joint strength caused by the swelling and plasticization of the adhesive under humid conditions. It is used to qualify protective potting materials for electronic assemblies that operate in tropical or outdoor environments.
Test Protocol
Measuring this phenomenon involves subjecting bonded specimens to controlled humidity chambers before executing mechanical shear testing. The shear test is performed using a calibrated pull tester that records the force at which the joint fails. Standard reference conditions often specify a temperature of eighty-five degrees Celsius and eighty-five percent relative humidity for a duration of one hundred hours.
This environmental aging allows the moisture to diffuse fully into the polymer, which establishes a repeatable state of degradation for metrological evaluation. Calibration of the force transducer must be verified using traceably calibrated weights before the test sequence.
Diffusion Drift
Rate variations in moisture diffusion depend heavily on temperature and the chemical composition of the polymer matrix. At higher temperatures, the diffusion coefficient increases, which shortens the time required for moisture to reach the bonded interface. This accelerated absorption reduces the shear stress capacity of the bond by disrupting the hydrogen bonds at the adhesive-substrate interface.
To account for this drift, testing laboratories must monitor and record the temperature profiles of the environmental chambers with a resolution of zero point one degrees Celsius.
Reliability Limit
Reliability assessment evaluates the degradation of adhesive bonds under mechanical load. When moisture levels exceed a specific threshold, the joint fails. This transition marks the end of service life.