X-rays vs. Cross Sections to Measure Voids
This paper validates void measurements obtained from non-destructive imaging techniques, with the physically measured void measurements of cross sectioning.
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Authored By:
Gordon O'Hara, Matthew Vandiver, Jonathan Crilly
Flextronics Corporation, Austin, Texas
Nick Brinkhoff
North Star Imaging, Rogers, Minnesota
Summary
A joint project between Flextronics Inc. and North Star Imaging Inc. is being conducted to correlate current x-ray imaging and cross-section analysis of BGA voiding with state of the art high resolution CT-Scan imaging. Our primary objective is to validate the void measurements obtained from non-destructive imaging techniques, with the physically measured void measurements of cross sectioning. A secondary goal is to characterize void properties before and after reflow.
Typical AXI inspection equipment provides one to three horizontal planes of reference for BGA void measurements. CT Scan imaging provides a full 3D volumetric representation of the BGA void, allowing for size, volume, and void position data. Information that can be used in failure analysis and process characterization projects, without physical destruction of the printed circuit board.
Five 50.0 mm FCBGA devices and five 52.5mm FCBGA devices, with known voiding, are being used in the study. The voiding for each device has been measured on a 3D AXI machine, a2D off-axis high resolution x-ray machine, and CT-Scan system. The devices will then be placed and reflowed onto printed circuit boards. After reflow, all the voiding will be measured again using each piece of equipment. In addition, select voids will be cross-sectioned, polished, and measured using a high magnification digital microscope and correlated to the other x-ray imaging tools.
Conclusions
3D AXI is necessary to screen out significant quantities of components as data points, prior to further characterization.
Combination of available void detection technologies are needed for complete characterization of process and components, especially for increased complexity (i.e. via in pad, finer pitch, etc,.
PCB and Component Design, Reflow profile parameters as well as chemistry can all affect growth and positioning of voids.
High Resolution CT Imaging allows for a complete analysis of components before and after assembly in non-destructive manner.
While IPC 7095B introduced tables for void process indicators and troubleshooting and JEDEC Std 217 has a guideline for component voids allowed (pre-reflow), a clear joint industry specification needs to be considered to create better linkage between component manufacturing and PCB Assembly & Inspection.
Initially Published in the IPC Proceedings
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