Circuits Assembly - December 2007 - (Page 28)

Cover Story An A-to-Z Guide to X-Ray Inspection Dr. David Bernard Understanding x-ray inspection theory is essential when selecting a system. Ed: This is the first of a two-part article. -ray inspection systems offer PCB manufacturers a way to inspect solder joints and locations in a non-destructive manner with products that cannot be inspected optically. For example, x-ray inspection permits examination of the solder joint quality of area array packages such as BGAs and CSPs. It can also be used to investigate the wire attachment quality within semiconductor device packages and very fine pitch SMT components. Inspection applications are almost endless with a high quality x-ray machine. Without x-ray inspection, the only alternative method would be destructive removal of the part in question with subsequent optical investigation. However, the optical approach renders the product and device useless and can destroy the area that needs to be investigated, potentially eradicating vital analytical clues that can indicate the cause of process defects or failures. X-ray inspection has conventionally been used for development and field-failure analysis. More recently, x-ray’s advantages have become more important for both production quality and production process control Figure 1. Basic layout of x-ray inspection system. 28 Circuits Assembly DECEMBER 2007 X applications. Increased use of area array packages, chip scale packages and flip chips demands it. AOI is ineffective on these devices because their solder joints are hidden from view. Several different types of x-ray inspection systems include laminography and computerized tomography (CT). This article will focus on the most common type: two-dimensional x-ray inspection. 2-D x-ray inspection systems are essentially x-ray microscopes. X-rays are produced from a tube, passing through the sample and then into an x-ray image-capture device (Figure 1). The image-capture device converts the x-rays it receives into visible images typically displayed on a PC monitor. Any object, or material, within the sample that is of higher density than the surroundings will absorb more of the x-rays in that location. As a result, fewer x-rays will pass through the sample at that location, so fewer will hit the capture device at that point, thus casting a darker shadow on the detector. A typical x-ray image produced by an x-ray inspection system has solder locations, device terminations and copper tracks appearing dark compared with the laminated circuit board in a PCB. The greater the density difference between the materials, the more clearly the contrast will appear on the x-ray image. For example, voids, or air bubbles, within BGA solder balls are much less dense than the surrounding solder, and as such can be easily distinguished (Figure 1). Sample manipulation in the x, y and z directions within the x-ray inspection system is necessary to permit movement to different locations circuitsassembly.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - December 2007

Circuits Assembly - December 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing Metalization Options for COB Assembly A Test Comparison of SAC and Non-SAC Pb-Free Solders An A-to-Z Guide to X-Ray Inspection BEST: A ‘Funky Chicken’ with an EMS Niche Tech Tips Wave Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Approaches Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - December 2007

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