Test and Inspection

 David Bernard

What are void calculation tools telling you?

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Robert Boguski“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.”

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David BernardThere 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?

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David BernardFor 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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David BernardThe target style of the x-ray tube impacts magnification, resolution and quantity.

In last month’s column I explained the impact a transmissive or reflective target style of x-ray tube will have on the available magnification of an x-ray system. The difference between the two target types is shown in FIGURE 1. Not just the magnification is altered by the choice of target, however. The focus, or resolution, of the tube, as well as the flux, or quantity of x-rays, that the tube produces will also be affected. This is caused by the x-ray tube settings.



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David BernardAt the highest magnifications, the differences between the two types of targets become clear.

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