Is the low stress nitride really low stress? Check it by utilizing the Bowoptic Stress Measurement. Is the vendor sending the particle distribution claimed? Are there voids forming in the Cu TSVs? Check without destorying the sample in the Dage X-ray. Granted there are resolution limitations.
highest resolution and largest X-ray images for failure analysis with oblique angle views up to 70 degrees, 750nm (0.75 micron) feature recognition