Test and Inspection

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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Robert Boguski What do you do when the excrement starts flying?

I write this in California on Monday, Mar. 18, 2019. Eight days earlier, a Boeing 737 MAX 8, operating as Ethiopian Airlines Flight 302, crashed six minutes after takeoff in clear air on what should have been a routine flight from Addis Ababa to Nairobi. The equivalent route flown in Africa as a flight from San Francisco to Los Angeles. A milk run, as pilots say. Some 157 innocent souls didn’t make it.

This follows a crash five months earlier in Indonesia, under similar circumstances with no apparent weather-related impediments, of the exact same aircraft type shortly after takeoff, with similar loss of life.

It is irresponsibly premature to draw sweeping conclusions from this most recent crash, as the investigation into both disasters continues. Much remains to be learned, and early theories of causation could be proven wrong. Eerie similarities between the two incidents have emerged from the evidence reviewed thus far, however, and we already know a few things. Those few things prompt anxious questions now.

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Head-on-pillow and head-in-pillow look different under x-ray. The distinction is important.

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