Circuits Assembly - January 2008 - (Page 34)

Component Inspection An A-to-Z Guide to X-Ray Inspection, Part II By Dr. David Bernard Practical recommendations for production sampling and viewing various part types. Ed: This is the second of a two-part article; part one was published in December. The complete article can be found at circuitsassembly.com/cms/content/ view/5916. V arious procedures are suggested as a standardized approach for x-ray inspection of specific component types on assembled PCBs in a production environment. It is recommended that a sample of two boards be examined from each batch for normal in-process inspection. Inspection should also occur when changing temperature profiles or when setting new product introduction profiles. This information can be valuable for future reference, particularly when using new component types. Voiding is a common fault detected by x-ray inspection. Voids are the presence of air bubbles or other nonmetallic material trapped within the solder joint. Voiding in production is usually caused by either a fault in achieving the peak reflow temperature or the board spending insufficient time above the liquidus temperature of the solder paste. The question of what level of voiding is acceptable is hotly debated. SnPb and Pb-free solder joints will almost always have some level of voiding. However, the size and quantity of voiding seen will depend on the sample and the x-ray system’s detection sensitivity. X-ray inspection should be conducted after rework of any area array devices, land grid array (LGA) or quad flatpack no-lead (QFN) components. This will quickly confirm rework quality nondestructively and help establish correct rework process profiles. BGA voiding. The presence of voids and their quantity are process quality indicators. In particular, if the voiding level over time tends to increase or decrease, this is often seen as a sensitive indicator of changes in the production process. BGA void percentage is calculated on an x-ray inspection system by totaling the lighter pixels (void pixels) within each solder ball and presenting them as a percentage of the total number of pixels within the entire solder ball area. This calculation is achieved by looking at the ball from the topdown, but recognizes that it gives values based on 2-D data for 3-D spheres. The calculation requires setting suitable greyscale thresholds to define the solder ball outline and void pixels. Two primary types of voiding can be present: process or “bulk” voids and interfacial voids. Process voids are often relatively large and positioned in the middle of the ball or associated with one of the solder ball interfaces, pad side or device side. Process voids often can be reduced in size by slightly increasing the time above liquidous for a few seconds, thereby permitting volatiles to fully escape during reflow. Interfacial voids are smaller than process voids and are typically associated with the solder ball interfaces, most often to the pad or device. The position of these voids can be confirmed by observing the solder ball at different oblique angle views within the x-ray and observing how the voids move relative to the surrounding materials. ■ Dr. David Bernard is product manager x-ray systems at Dage Precision Industries (dage-group.com); d.bernard@dage-group.com. 34 Circuits Assembly JANUARY 2008 circuitsassembly.com http://circuitsassembly.com/cms/content/view/5916 http://circuitsassembly.com/cms/content/view/5916 http://www.dage-group.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - January 2008

Circuits Assembly - January 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing Better Manufacturing Walking on Water Flux Selection for Lead-Free Wave Soldering An A-to-Z Guide to X-Ray Inspection, Part II Evolution in Action Research Priorities for the Electronics Industry Selective Soldering Process Doctor Pb-Free Lessons Learned Materials World Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - January 2008

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