Session: 08-02 Mini-Symposium for Professor Dasgupta II

Paper Number: 98048

98048 - A Framework for Reliability Assessment of Chemical-Induced Display Delamination 

Touch-enabled displays in handheld and wearable devices are expected to meet aggressive design requirements in harsh use conditions. The use conditions include exposure to household chemicals like sweat, hand lotion, sunscreen in addition to high temperature and high humidity conditions. Typical applications include smart phones, smart watches, touch bar of a MacBook Pro, and tablet PCs. This study examines chemical induced interfacial delamination within a display module. 

 A typical display stack up consists of cover glass, optically clear adhesive, polarizer, touch panel, pressure sensitive adhesive, poly-imide based pOLED, PET film, and conductive adhesives. The lamination process, which varies from one display manufacturer to the other, typically leaves the display stack in a state of internal stress. Other than the cover glass, the edges of the display stack are susceptible to degradation due to exposure to chemicals. 

Conventional approach to address the degradation risk is to develop product design constraints (example: enclosure) that provide sufficient resistance to ingress of chemicals and moisture. However, Industrial Design requirements of lighter weight and slimmer form factors may prevent successful protection of the edge of the display stack. Hence, characterization of chemical induced delamination failures is critical for display reliability assessment.   

In this work experiments are conducted with the display edge exposed to specific chemicals. This is considered to be a limiting case of chemical exposure for display module edge. It is observed that in this test the delamination is typically initiated at a point along the edge and proceeds as an approximate semi-elliptical  crack front. It is further observed that the growth rate of the crack front diminishes exponentially with time, and the crack attains a limiting dimension. Furthermore, accelerated testing carried out at elevated temperature and humidity conditions shows that the crack growth has the same exponential behavior. 

The observed delamination can be modeled as a semi-elliptical interfacial crack front. If such delamination is driven primarily by internal stresses the energy release rate for the delamination decreases as the delamination size increases and hence such delamination would be self-limiting where the limiting size is a function of the magnitude of internal stresses and the conditions to which the display stack is subjected in the test. This is consistent with the observed delamination failures.

The observed delamination induced in this test can be modeled as a semi-elliptical interfacial crack front. Based on classical fracture mechanics, crack growth occurs when strain energy released during crack growth exceeds the energy required for the creation of the new fracture surface.  The semi-elliptical crack front suggests that the crack growth is driven by stresses that are almost isotropic. High magnification photos of the crack front show the dendritic features typical of internal stress driven delamination. Hence, internal stresses within the display stack up, induced by the lamination process, is considered to be primary driver of this interfacial crack. 

Creation of a new fracture surface (crack growth) relieves the internal stresses in the display stack, thus decreasing the incremental energy available for creation of new surfaces. This reduces the crack growth rate in an asymptotic manner till it reaches it limiting dimension.  

While fracture energy considerations can be used to understand the self-limiting nature of such delamination, temporal evolution of such defects is not easily amenable to analysis. Since a theoretical analysis is beyond the scope of this paper, an empirical equation is proposed to describe the evolution of interfacial delamination with time. The proposed relation is shown to describe the experimental data satisfactorily. Such quantification of the time-evolution of delamination enables evaluation of different display stacks in a structured manner. Finally, it is shown that this characterization framework can enable an enhanced reliability assessment of the module reliability data.

Presenting Author: joseph varghese google