3D Board Level X-Ray Inspection Via Limited Angle Computer Tomography
Authors: David Bernard, Ph.D., Dragos Golubovic, Ph.D., and Evstatin Krastev, Ph.D. Company: Nordson DAGE Date Published: 10/14/2012
Abstract: Computer Tomography (CT) is a powerful inspection technique used widely in the electronics industry, especially for the analysis of multi-layered devices and joint interconnections. As the resolution required to be able to inspect today’s devices is in the micron range, this aspect of CT is often referred to as µCT so as to differentiate it from medical and industrial CT applications where the same level of resolution is not possible or required. The µCT technique permits different layers / slices of the device to be isolated and examined individually, so practically providing an electronic, or virtual, cross-sectioning within the sample. The benefits of an ‘electronic cross-section’ compared to traditional mechanical cross-sectioning are many. These include that the electronic cross-sectioning is reversible – you cannot over polish and go too far into the sample - the cutting plane can be positioned in any orientation within the 3D space of the CT model and no additional defects are introduced or existing defects concealed compared with the process of mechanically cutting, polishing and preparing the sample for a cross-section. One of the limitations of traditional µCT is that there is a restriction to the maximum sample size that can be used to produce a µCT model with reasonable speed, quality and analytical value. Usually, the maximum practical size for a regular µCT is ~ 2” x 2” (50 x 50 mm). Thus, it is not possible to use the µCT technique on a large PCB unless you are willing to cut around the device / region of interest to be examined to make it small enough for analysis, but in so doing destroying the board. In order to overcome the sample size limitation of ‘full µCT’, a ‘limited angle’ or ‘partial’ µCT technique has been developed and used in some X-Ray systems. This permits a 3D model to be created from devices / regions of interest anywhere within a board without the need to destroy it. This paper will explain the mechanism and differences between full µCT and the various types of limited angle µCT and compare different applications where one or the other technique is applicable, backed by real life cases and examples.