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Test and Inspection

David BernardVoid size and location can predict future failures.

Perhaps quad flat packages are not as fashionable a package choice as they once were because of their limited I/Os compared to BGAs, but they continue to be a staple in electronics (FIGURE 1). As such, they are still a source of problems to assemblers. With their typical gull wing joint style, located on the outside of the package rather than beneath it, these devices can be inspected optically. This can identify some issues that may arise but, as with other packages, if you can’t see the problem optically, it does not mean it is not there! Therefore, x-ray inspection of QFPs can complement any optical inspection, not only by helping confirm issues raised optically, but also providing wider fault coverage through identifying and helping mitigate a range of optically invisible issues that QFPs can provide.

 

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Figure 1. X-ray image of QFP from the top down.

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Robert BoguskiAre we headed toward Singularity? Or just more PowerPoints?

Thus says IPC:

The Hermes Standard Initiative is happy and glad to announce that IPC has confirmed to recognize The Hermes Standard to be the successor to “the SMEMA Standard” IPC-SMEMA-9851, which has been the only globally accepted and broadly established standard for machine to machine communication in SMT with regard to PCB handover. Accordingly, The Hermes Standard was assigned an IPC naming code: It can now officially be referred to as IPC-HERMES-9852.

This strong acknowledgment means a lot for further enhancements of the global footprint and acceptance of The Hermes Standard. With The Hermes Standard having shown an impressively fast start from early drafts to worldwide awareness, it is now entering the second stage in global market penetration.

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David BernardUse partial CT to reveal trace discontinuities at incoming inspection.

In last month’s column, I suggested using x-ray inspection equipment more typically used for the investigation and quality control of assembled PCBs to check bare boards ahead of their use in assembly. Such equipment is often already onsite and readily available to the assembler. The benefits of enhanced magnification and resolution this equipment can usually offer toward inspection of representative bare boards ahead of assembly is, I contend, an opportunity to provide additional confidence that all is well in the bedrock of PCBA. Furthermore, the cause of any future issues, if they occur, can be more easily narrowed down to the assembly process, design or components. The example I gave related to the possible issue of poor drilling quality in the board vias and how, in x-ray images, the plating variability can be readily seen, especially if the via is buried within the board and optical inspection is not possible. Variable plating quality, however, such as voiding in the central termination of a QFN (see “QFN Inspection: Don’t Forget the Edge!” December 2018 column), can often be relatively obvious for an operator to see when it occurs in the x-ray image such that if it is not present and all looks consistent and appropriate, then other subtler issues may not be considered fully, or missed entirely.

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David BernardWhen nondestructive methods are preferred, leverage the x-ray.

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Robert BoguskiWe came. We saw. We conquered. And we completed the questionnaire.

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David BernardVoiding under edge terminations is often overlooked, to the detriment of yields.

My last column focused on voiding under QFNs. Primarily, this concentrated on voiding under the central termination. As I discussed, the potential for voiding in this area is high, owing to the limited available escape pathways to remove outgassing volatiles created during reflow from under the center of these planar objects. This can result in the often-typical voiding issues that are usually clearly seen in their x-ray images. Therefore, this can then be the natural and easy focus for an operator to concentrate on as the location of the likely fault or failure, even if the “substantial” level of voiding may be acceptable from a supplier and customer point-of-view. With voiding (when present) usually being so obvious, yet probably at an acceptable level, once the central termination has been considered and passed by the operator, then the edge terminations may not then be fully considered, or possibly even ignored completely, as the potential source of problems. Therefore, I would like to present some images of good and bad QFN edge terminations to highlight some of the features that may be seen in the x-ray images to indicate the problem could be at the edge and not in the center.

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