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Test and Inspection

David Bernard

It’s cheaper and faster to inspect by machine over microsection.

Most of my columns have attempted to discuss the “typical,” and often more obvious, solder joint failures that can be seen using x-ray inspection. This is usually the main and most important function of this type of analysis.

Nondestructive inspection of cracks within solder joints or components is also desirous, however, but this is much more difficult to evaluate optically or by x-ray. Even for those joints that are not optically hidden, optical inspection for cracks is likely limited to the very end of the termination and requires a mostly edge-on view at a reasonable magnification (FIGURE 1) to have the best chance of seeing a crack failure. When inspecting fully populated boards, achieving this level of magnification and orientation may be difficult to do optically, and any cracks present will need to be distinct by showing a separation in the joint. If the two halves of the cracked solder are still touching, then analysis may be almost impossible to make. Furthermore, such a crack will be at the end of the termination and not necessarily extending further back into the joint – for example, into the heel fillet of a QFP, which is more crucial to joint integrity. This may mean a cosmetic issue is seen on one joint, and the actual fault may remain hidden elsewhere.

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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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