Use of Birefringence to Determine Redistribution Layer Stresses to Create Design Guidelines to Prevent Glass Cracking

Glass substrates have gained increasing attention for packaging due to their ideal material properties; however, the brittle nature of glass poses a challenge. This paper focuses on glass cracking due to dicing-induced defects and redistribution layer (RDL) stresses. RDL, which is composed of copper and dielectric polymer, creates stresses which become tensile at the exposed substrate glass edge due to the free edge effect and can cause crack propagation at low temperatures or when glass is exposed to water. The first part of this paper characterizes the difference in the secondary principal stresses induced in the glass by the RDL through birefringence measurements. The stress due to each material is individually calculated through finite element models by examining glass substrates with different thicknesses and copper distributions. The second part of this paper focuses on the reliability of glass substrates subjected to preconditioning and thermal cycling. In experiments, ten different glass substrate structures with thicknesses of 140-420 μm are subjected to -40 to 125 °C thermal cycling, finding that the thicker (≥120 μm total) build-ups exhibit cracking while the thinner build-ups (≤80 μm total) do not crack. Models to predict crack propagation are developed using the stresses measured through birefringence and are validated to accurately predict the occurrence of crack propagation. In the last part of this paper, these models are used to develop design guidelines to prevent glass cracking due to RDL stresses and dicing-induced defects. These guidelines specify the allowable dicing defect size and build-up thickness combinations to prevent glass cracking based on the energy release rate observed during the thermal cycling experiments. The critical levels for the energy release rate for crack propagation and no crack propagation were G = 1.05 J/m
²
and G = 0.87 J/m
²
, respectively. Allowable dicing defect size and build-up thickness are found to be inversely proportional and it is generally recommended to keep the total build-up thickness at or below 80 μm.