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|IC Bond Pad “Ripple Effect” Investigation|
|Keywords: wirebond, bond pad ripple, bond pad cracks|
|IC bond pad “ripple effect” is observed during optical microscope inspection after destructive “crater etch”, relating directly to bond pad cracking. This paper follows up on our previous work to clarify and explain details regarding the causes and concerns of “ripple effect” in bond pads having Al – SiO2 film stacks. Thin pad Al doesn’t provide sufficient “cushion” for traditional pads, which can ripple and crack quite easily in wirebonding. A good correlation is shown between focused ion beam (FIB) cross section measurements and optical imaging of bond pads having the bond wire and pad Al films removed. Location and magnitude of sub-layer Al film deformations due to bonding stress are viewed in relation to the bond ball position and ultrasonic direction, revealing the regions of highest stress. Reduction in ripple and pad cracks is important for high reliability bond-over-active circuitry designs, especially when considered for Cu wirebond. Examples of robust pad designs are compared for ripple both optically and by FIB, showing why the sub-layer Al deformation is reduced or eliminated.|
|Stevan Hunter, Principal Engineer