Selecting Polymer Die Attach Materials for Low Drift MEMS Accelerometers
Select low-modulus silicones or cyanate esters with uniform bondlines to decouple package stress and eliminate viscoelastic offset drift in MEMS accelerometers.
Shear-Lag Mechanics
Stress Distribution
Bonded joints transfer loads between layers through shear in the adhesive rather than through direct tension. Shear lag mechanics explains why the tensile stress in a die is not uniform.
Stress Redistribution
Force enters the bonded component at the interface and gradually builds up the internal tension. Principles of shear lag mechanics dictate that the adhesive layer undergoes deformation that lags behind the stretching of the adherends. Stress profiles resulting from this effect show maximum shear at the discontinuities of the bond line.
Interface Failure
Concentration of stress at the corners makes these regions the most likely sites for delamination. Applying shear lag mechanics allows engineers to predict the maximum die size that can be safely bonded with a specific adhesive thickness. Increasing the thickness of the bond line generally reduces the peak shear stress by providing more volume for the deformation to spread.
Measurement Drift
Mechanical strain in the silicon affects the electrical characteristics of the transistors or resistors. Because shear lag mechanics dictates a non uniform strain field, the placement of components on the die becomes a critical design factor. Centralized placement of the most sensitive circuits minimizes the impact of the edge concentrated shear forces.