3D packaging technologies such as FLASH rely on die-to-die stacking of ultra-thin silicon devices with individual die thicknesses below 100 um. Because ultra-thin silicon wafers are very fragile, mechanical saw dicing of sub 100 um thick wafers tends to be more challenging, requiring slower processing and reduced throughput and/or yields. These challenges make full cut laser dicing an attractive solution. This presentation provides an investigation for machining of 50 um thick silicon wafers using a Gaussian-shaped, nanosecond pulsewidth, 355 nm UV laser. A range of machining speeds and laser fluences are compared, from single laser pulses to highly overlapped slow-velocity machining. 3D Laser Scanning Microscope and FIB/TEM cross sections are employed to characterize the state and depth of heating damage into the Si material. Implications for laser machining rates and die break strength are investigated for full cut laser dicing.