Test and Inspection
Voiding 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.
“A healthy supply chain is the backbone of our business. We’ll pay you in 90 days.”
This is about doing the right thing.
Doing the right thing involves admitting the right thing can be an elastic term. It depends who defines rightness. And who is most qualified to make, and enforce, that definition.
It does not follow what is right for you is necessarily right for me. Nevertheless, organizations whose business it is to codify things try to freeze ethics in amber.
Case in point: Section 7.3h of the AS9100D Standard states that in a manufacturing organization, “The organization shall ensure that persons doing work under the organization’s control are aware of … the importance of ethical behavior.”
There are more solder balls to inspect, so use oblique views.
With board area often at a premium in today’s sophisticated and high-performance electronics, using the third dimension of height to enable greater device functionality is ever more attractive. In particular, package-on-package (PoP) devices have been around for years and are popular for this purpose. From an x-ray inspection point-of-view, the most common POPs can be considered as two BGAs stacked on top of each other. Therefore, the real and “virtual” issues associated with BGAs, as mentioned in previous columns, also come into play for these devices.
However, we now have an additional dimension (sic) to consider in the features seen in the x-ray images, as well as just having a single BGA and anything within, or on the second side of, the board. On top of these, we also have the potential for overlap of solder balls from the two BGA layers (FIGURES 1 and 2). As with BGAs, all the interconnections are underneath the package, and are not visible optically, permitting x-ray the opportunity to make nondestructive inspection analysis. With twice the number of BGAs in the same place, does this mean twice the number of potential problems or more?
For BGAs it is best to start with a top-down view to look for obvious shorts and over-large voids, and then go to oblique views.
X-ray inspection, at present, requires mainly human analysis to see the flaws. This will be true perhaps unless, or until, artificial intelligence (AI) algorithms provide some alternative solution. However, with so many variables in the x-ray images of assembled boards – the shapes, sizes and density of bumps and joints, the differences in pad solder coverage, the presence of internal copper traces, the overlapping internal and bottom-side components and features, etc. – obtaining an initial training set of exemplars for a ubiquitous AI solution may be tricky.
I suggest this is particularly the case when looking for opens in BGAs. By looking only at a top-down x-ray view, you may miss the fault. In previous columns, I have recommended oblique x-ray views should also be taken to separate the pad and device interfaces from each other and thus give the (human) operator the best chance to spot the open joint or other problem(s). Consider using not only an oblique angle view, but also decide what angle, or angles, are necessary and in which direction(s) around the joint those angle views should be taken.
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