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Test and Inspection

Robert Boguski

Needed: Solder joint forensic scientist. Common sense preferred.

Wanted: X-ray engineer. A test engineer with an interest in x-ray technology will suffice. So will a skilled and teachable technician. Hell, an intelligent person with a pulse will do in this economy. We’re open-minded. Just show us. No shrinking violets here. Honesty still matters to us (like being honest about the state of the economy and its effects on available talent). You should be honest, too, if you’d like us to hire you. Bring the aptitude; we’ll give you the qualifications.

We will train you.

 

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David Bernard

IPC-STD-001 is revising criteria for voiding and fill percentage.

In my September column, I spoke with Dave Hillman about IPC committee work on voiding guidelines for QFN central pad terminations. But he also told me the J-STD-001 task group increasingly receives requests from users for additional information and clarification of x-ray usage in other areas. This is because use of x-ray technology for analyzing solder joints has resulted in significant soldering process improvements. As with all technology introductions, however, the benefits and questions that result from the new information provided must be characterized, assessed and disseminated into practical form. One such area where x-ray technology has provided a tremendous amount of new information is plated through-hole (PTH) solder joints. The “insides” of these joints were previously “hidden” from scrutiny, unless subject to destructive methods, and the standards writers will need time to carefully revise old criteria to accommodate this new information. With this in mind, IPC formed a task group (called Team Skeleton) to discuss this and other matters, with the goal to develop additional x-ray-related guidelines and requirements for inclusion in future IPC documentation. As usual, Dave says, “All are welcome to participate and provide their comments and suggestions.”

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Robert BoguskiAs we venture out among the aerospace industry, it helps to know the lingo.

Four years have elapsed since we last provided discerning readers with a helpful field guide to the major species inhabiting trade shows. Four years is a long time. Has anything changed? Have the major species evolved? Regressed? Have some gone extinct or suffered outright obsolescence? What are the replacements?

The quest for knowledge beckons us back to the field.

Curiosity about a changing world and an evolving industry propels us to don pith helmets and binoculars and return to the source. Post-graduate work commences now.

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David BernardWill IPC accept the >50% voiding recommendation?

We have long had numerical guidelines for voiding levels below which we deem acceptable for BGA joints. Originally from IPC documentation, the limit called for less than 25% voiding of the joint area when the joint is looked at from the top-down in x-ray. More recently, and entirely because of evidential data, this has been increased to 30%.

Many other joint types also given designated qualifications in the IPC guidelines, such as through-hole joint fill levels, can be evaluated using x-ray. However, there has always been an anomaly in the level of voiding in bottom termination components (BTCs). To date, no evidence-backed, indicative values are published detailing acceptable voiding in these joints and, in particular, the large central pad under QFNs.

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A few well-placed cues make “true” design-for-test.

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David BernardIs the reflow profile the problem? X-ray can help.

Looking through some recent x-ray images of what I would call “good bad” boards (at least, that is what they are for me, as they showcase “good” examples of how certain “bad” types of failure look under x-ray inspection), I came across a number of different issues that are different from “traditional” BGA/QFN problems mentioned in this space before. To wit, I noted some images showed where solder paste had not reflowed under the devices, and there was the presence of foreign object(s), such as discrete components, trapped under the package.

FIGURES 1 and 2 show how unreflowed solder paste typically looks under QFN joints in an x-ray image. In the magnified view (Figure 2), individual grains of the solder paste are seen clearly, instead of appearing as a typical single smooth continuous joint. The cause of this is probably not an insufficient reflow profile. Rather, it is more likely the board has not been reflowed at all. As it may be desired, or necessary, to x-ray inspect (representative) boards after placement but before reflow as part of a quality control process, it is worth noting this characteristic shape of the solder under the components is different from what would be expected post-reflow.

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