Mechanical Reliability Analysis of Ultra-Thin Chip-on-Foil Assemblies under Different Types of Recurrent Bending

We report our results on the comparative studies of the influence of chip thickness and dicing technique on the mechanical reliability of flip-chip bonded ultra-thin chip-on-foil (COF) assemblies under two different types of recurrent bending, free form bending and fixed radius bending (bending radius-5 mm). Free form bending experiments conducted on 28 μm and 250 μm COF assemblies demonstrated the improvement in fatigue reliability of the foil wiring lines of the COF assemblies with the reduction in chip thickness. Experimental results of the fixed radius bending tests revealed that COF assemblies with 12 μm chips endured the bending tests almost 2 times better than COF assemblies with 20 μm chips. Furthermore, COF assemblies with plasma diced chips showed better dynamic bending reliability than wafer sawn chips during fixed radius bending tests. Optical Microscopy and Computed Tomography analyses indicated that the dominant cause of failure occurring in COF assemblies was the rupture of wiring lines rather than the interconnect delamination or chip cracking. Besides, Atomic Force Microscopy analysis of the sidewalls of 20 μm ultra-thin chips revealed that the sidewalls of plasma diced chips were 3 times smoother than the wafer sawn chips.