Evaluation of and Inspection Metrology for Lid Attach for Advanced Thermal Packaging Materials

We have evaluated a new Ag-filled silicone thermal interface material (TIM) for its sensitivity to lid finish and impact on imaging discontinuities in the die/lid (TIM1) layer, in conjunction with two high performance lid materials, as a part of our advanced packaging technology development effort. Thermal and mechanical (shear stress and lid pull) measurements have been carried out on a number of different lid finishes to optimize thermal performance and adhesion at the TIM1/lid interface. This silicone TIM1 is found to be sensitive to the type of Ni-plating and plating bath chemistry. Nondestructive and destructive metrology has been carried out on flip chip (FC) packages using Ag-filled silicone TIM1 and either Cu or AlSiC lids. A number of silicone formulations have been investigated to assess their impact on surface acoustic microscopy (SAM) and X-ray imaging. Nondestructive evaluation (NDE) by real time X-ray and SAM has identified artifacts that make it difficult to unambiguously detect voids and delamination in the TIM1 layer. A “dark ring” or “picture frame” artifact is observed at the die perimeter in acoustic microscope images of packages with the Ag-filled TIM1. Detailed SEM cross-section and thermal mapping analyses on a number of specially constructed FC packages have been correlated with TIM1/lid delamination and voiding observed in SAM and X-ray images. Results of these studies point to changes in the TIM1 modulus during cure and post cure thermal excursions as the cause of the “dark ring” observed in the transmission SAM images rather than delamination at the TIM1/lid or TIM1/die interfaces. However, in the event that delamination is present at the edges it cannot be unambiguously deconvoluted from the “dark ring” artifact in the SAM images.